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[ 163 ] 



VI. The Bakerian Lecture. — Experimental Researches in Electricity. — Second 
Series. By Michael Faraday, F.R.S., M.R.I., Corr. Mem. Royal Acad, of 
Sciences of Paris, Petersburgh, fyc. 8$c. 

Read January 12, 1832. 

§ 5. Terrestrial Magneto-electric Induction. 

§ 6. Force and Direction of Magneto-electric Induction generally. 

§.5. Terrestrial Magneto-electric Induction. 

140. W HEN the general facts described in the former paper were discovered, 
and the law of magneto-electric induction relative to direction was ascertained 
(114.), it was not difficult to perceive that the earth would produce the same 
effect as a magnet, and to an extent that would, perhaps, render it available in 
the construction of new electrical machines. The following are some of the 
results obtained in pursuance of this view. 

141. The hollow helix already described (6.) was connected with the galva- 
nometer by wires eight feet long ; and the soft iron cylinder (34.), after being- 
heated red hot, and slowly cooled, to remove all traces of magnetism, was 
put into the helix so as to project equally at both ends, and fixed there. The 
combined helix and bar were held in the magnetic direction or line of dip, 
and (the galvanometer needle being motionless) were then inverted, so that 
the lower end should become the upper, but the whole still correspond to the 
magnetic direction ; the needle was immediately deflected. As it returned to 
its first position, the helix and bar were again inverted ; and by doing this two 
or three times, making the inversions and vibrations to coincide, the needle 
swung through an arc of 150° or 160°. 

142. When one end of the helix, which may be called A, was uppermost at 
first (B end consequently being below), then it mattered not in which direction 
it proceeded during the inversion, whether to the right hand or left hand, or 
through any other course ; still the galvanometer needle passed in the same 
direction. Again, when B end was uppermost, the inversion of the helix and 
bar in any direction always caused the needle to be deflected the same way ; 

v2 



164 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

that way being the opposite to the course of the deflection taken in the former 
general case. 

143. When the helix in any given position was inverted, the effect was as 
if a magnet with its marked pole downwards had been introduced from above 
into the inverted helix. Thus, if the end B were upwards, such a magnet in- 
troduced from above would make the marked end of the galvanometer needle 
pass west. Or the end A being upwards, and the soft iron in its place, inver- 
sion of the whole produced the same effect. 

144. When the soft iron bar was taken out of the helix and inverted in 
various directions within four feet of the galvanometer, not the slightest effect 
upon it was produced. 

145. These phenomena are the necessary consequence of the inductive mag- 
netic power of the earth, rendering the soft iron cylinder a magnet with its 
marked pole downwards. The experiment is analogous to that in which two 
bar magnets were used to magnetize the same cylinder in the same helix (36.), 
and the inversion of position in the present experiment is equivalent to a 
change of the poles in that arrangement. But the result is not less an instance 
of the evolution of electricity by means of the magnetism of the globe. 

146. The helix alone was then permanently held in the magnetic direction, 
and the soft iron cylinder afterwards introduced; the galvanometer needle 
was instantly deflected ; by withdrawing the bar as the needle returned, and 
continuing the two actions simultaneously, the vibrations soon extended through 
an arc of 180°. The effect was precisely the same as that of using a cylinder 
magnet with its marked pole downwards ; and the direction of motion, &c. was 
perfectly in accordance with those obtained in the former experiments with 
such a magnet (39.). A magnet in that position was then used, and gave the 
same deflections, but stronger. When the helix Avas put at right angles to the 
magnetic direction or dip, then the introduction or removal of the soft iron 
cylinder produced no effect at the needle. Any inclination to the dip gave 
results of the same kind as those already described, but increasing in strength 
as the helix approximated to the line of the dip. 

147< The cylinder magnet, although it has great power of affecting the 
galvanometer when moving into or out of the helix, has no power of con- 
tinuing the deflection (39.) ; and therefore, though left in, still the magnetic 



TERRESTRIAL MAGNETO-ELECTRIC INDUCTION. 165 

needle comes to its usual place of rest. But upon repeating the experiment 
of inversion in the direction of the dip (141.), the needle was affected as power- 
fully as before ; the disturbance of the magnetism in the steel magnet, by the 
earth's inductive force upon it, being thus shown to be nearly, if not quite, 
equal in amount and rapidity to that occurring in soft iron. It is probable 
that in this way magneto-electrical arrangements may become very useful in 
indicating the disturbance of magnetic forces, where other means will not 
apply ; for it is not the whole magnetic power which produces the visible effect, 
but only the difference due to the disturbing causes. 

148. These favourable results led me to hope that the direct magneto-elec- 
tric induction of the earth might be rendered sensible ; and I ultimately suc- 
ceeded in obtaining the effect in several ways. When the helix just referred 
to (141. 6.) was placed in the magnetic dip, but without any cylinder of iron 
or steel, and was then inverted, a feeble action at the needle was observed. 
Inverting the helix ten or twelve times, and at such times that the deflecting 
forces exerted by the currents of electricity produced in it should be added 
to the momentum of the needle (39.), the latter was soon made to vibrate 
through an arc of 80° or 90°. Here, therefore, currents of electricity were 
produced by the direct inductive power of the earth's magnetism, without the 
use of any ferruginous matter, and upon a metal not capable of exhibiting 
any of the ordinary magnetic phenomena. The experiment in everything re- 
presents the effects produced by bringing the same helix to one or both poles 
of any powerful magnet (50.). 

149. Guided by the law already expressed (114.), I expected that all the 
electric phenomena of the revolving metal plate could now be produced with- 
out any other magnet than the earth. The plate so often referred to (85.) was 
therefore fixed so as to rotate in a horizontal plane. The magnetic curves of 
the earth (114. note), i. e. the dip, passes through this plane at angles of about 
70°, which it was expected would be an approximation to perpendicularity, 
quite enough to allow of magneto-electric induction sufficiently powerful to 
produce a current of electricity. 

150. Upon rotation of the plate, the currents ought, according to the law 
(114. 121.), to tend to pass in the direction of the radii, through all parts of the 
plate, either from the centre to the circumference, or from the circumference 
to the centre, as the direction of the rotation of the plate was one way or the 



166 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

other. One of the wires of the galvanometer was therefore brought in contact 
with the axis of the plate, and the other attached to a leaden collector or con- 
ductor (86.), which itself was placed against the amalgamated edge of the disc. 
On rotating the plate there was a distinct effect at the galvanometer needle ; 
on reversing the rotation, the needle went in the opposite direction ; and by 
making the action of the plate coincide with the vibrations of the needle, the 
arc through which the latter passed soon extended to half a circle. 

151. Whatever part of the edge of the plate was touched by the conductor, 
the electricity was the same, provided the direction of rotation continued 
unaltered. 

152. When the plate revolved screw-fashion, or as the hands of a watch, the 
current of electricity (150.) was from the centre to the circumference; when 
the direction of rotation was unscrew, the current was from the circumference 
to the centre. These directions are the same with those obtained when the 
unmarked pole of a magnet was placed beneath the revolving plate (99.). 

153. When the plate was in the magnetic meridian, or in any other plane 
coinciding with the magnetic dip, then its rotation produced no effect upon 
the galvanometer. When inclined to the dip but a few degrees, electricity 
began to appear upon rotation. Thus when standing upright in a plane per- 
pendicular to the magnetic meridian, and when consequently its own plane 
was inclined only 20° to the dip, revolution of the plate evolved electricity. 
As the inclination was increased, the electricity became more powerful until 
the angle formed by the plane of the plate with the dip was 90°, when the elec- 
tricity for a given velocity of the plate was a maximum. 

154. It is a striking thing to observe the revolving copper plate become 
thus a new electrical machine ; and curious results arise on comparing it with 
the common machine. In the one, the plate is of the best non-conducting sub- 
stance that can be applied ; in the other, it is the most perfect conductor : in 
the one, insulation is essential ; in the other, it is fatal. In comparison of the 
quantities of electricity produced, the metal machine does not at all fall below 
the glass one ; for it can produce a constant current capable of deflecting the 
galvanometer needle, whereas the latter cannot. It is quite true that the force 
of the current thus evolved has not as yet been increased so as to render it 
available in any of our ordinary applications of this power ; but there appears 
every reasonable expectation that this may hereafter be effected; and probably 



TERRESTRIAL MAGNETO-ELECTRIC INDUCTION. 167 

by several arrangements. Weak as the current may seem to be, it is as strong 
as, if not stronger than, any thermo-electric current; for it can pass fluids (23.), 
agitate the animal system, and in the case of an electro-magnet has produced 
sparks (32.). 

155. A disc of copper, one fifth of an inch thick and only one inch and a 
half in diameter, was amalgamated at the edge ; a square piece of sheet lead, 
(copper would have been better) of equal thickness had a circular hole cut in 
it, into which the disc loosely fitted ; a little mercury completed the metallic 
communication of the disc and its surrounding ring ; the latter was attached 
to one of the galvanometer wires, and the other wire dipped into a little me- 
tallic cup containing mercury, fixed upon the top of the copper axis of the 
small disc. Upon rotating the disc in a horizontal plane, the galvanometer 
needle could be affected, although the earth was the only magnet employed, 
and the radius of the disc but three quarters of an inch ; in which space only 
the current was excited. 

156. On putting the pole of a magnet under the revolving disc, the galva- 
nometer needle could be permanently deflected. 

157- On using copper wires one sixth of an inch in thickness instead of the 
smaller wires (86.) hitherto constantly employed, far more powerful effects 
were obtained. Perhaps if the galvanometer had consisted of fewer turns of 
thick wire instead of many convolutions of thinner, more striking effects would 
have been produced. 

158. One form of apparatus which I purpose having arranged, is to have 
several discs superposed ; the discs are to be metallically connected, alternately 
at the edges and at the centres, by means of mercury ; and are then to be re- 
volved alternately in opposite directions, i. e. the first, third, fifth, &c. to the 
right hand, and the second, fourth, sixth, &c. to the left hand ; the whole being 
placed so that the discs are perpendicular to the dip, or intersect most directly 
the magnetic curves of powerful magnets. The electricity will be from the 
centre to the circumference in one set of discs, and from the circumference to 
the centre in those on each side of them ; thus the action of the whole will 
conjoin to produce one combined and more powerful current. 

159. I have rather, however, been desirous of discovering new facts and 
new relations dependent on magneto-electric induction, than of exalting the 



168 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

force of those already obtained ; being assured that the latter would find their 
full development hereafter. 

160. I referred in my former paper to the probable influence of terrestrial 
magneto-electric induction (137.) in producing, either altogether or in part, 
the phenomena observed by Messrs. Christie and Barlow *, whilst revolving 
ferruginous bodies; and especially those observed by the latter when rapidly 
rotating an iron shell, and which were by that philosopher referred to a change 
in the ordinary disposition of the magnetism of the ball. I suggested also that 
the rotation of a copper globe would probably insulate the effects due to elec-r- 
trie currents from those due to mere derangement of magnetism, and throw 
light upon the true nature of the phenomena. 

161. Upon considering the law already referred to (114.), it appeared im- 
possible that a metallic globe could revolve under natural circumstances, with- 
out having electric currents produced within it, circulating round the revolving 
globe in a plane at right angles to the plane of revolution, provided its axis of ro- 
tation did not coincide with the dip ; and it appeared that the current would be 
most powerful when the axis of revolution was perpendicular to the dip of the 
needle : for then all those parts of the ball below a plane passing through its 
centre and perpendicular to the dip, would in moving cut the magnetic curves 
in one direction, whilst all those parts above that plane would cut them in the 
other direction : currents therefore would exist in these moving parts, pro- 
ceeding from one pole of rotation to the other; but the currents above would 
be in the reverse direction to those below, and in conjunction with them would 
produce a continued circulation of electricity. 

162. As the electric currents are nowhere interrupted in the ball, powerful 
effects were expected, and I endeavoured to obtain them with simple apparatus. 
The ball I used was of brass ; it had belonged to an old electrical machine, 
was hollow, thin (too thin), and four inches in diameter ; a brass wire was 
screwed into it, and the ball either turned in the hand by the wire, or some- 
times, to render it more steady, supported by its wire in a notched piece of 
wood, and motion again given by the hand. The ball gave no signs of mag- 
netism when at rest. 

163. A compound magnetic needle was used to detect the currents. It was 

* Chbistie, Phil. Trans. 1825. pp. 58. 347, &c. Barlow, Phil. Trans. 1825. p. 317. 



TERRESTRIAL MAGNETO-ELECTRIC INDUCTION. 169 

arranged thus : a sewing-needle had the head and point broken off, and was 
then magnetised ; being broken in half, the two magnets thus produced were 
stuck into a stem of dried grass, so as to be perpendicular to it, and about four 
inches asunder ; they were both in one plane, but their similar poles in con- 
trary directions. The grass was attached to a piece of unspun silk about six 
inches long, the latter to a stick passing through a cork in the mouth of a 
cylindrical jar ; and thus a compound arrangement was obtained, perfectly 
sheltered from the motion of the air, but little influenced by the magnetism of 
the earth, and yet highly sensible to magnetic and electric forces, when the 
latter were brought into the vicinity of the one or the other needle. 

164. Upon adjusting the needles to the plane of the magnetic meridian; 
arranging the ball on the outside of the glass jar to the west of the needles, 
and at such a height that its centre should correspond horizontally with the 
upper needle, whilst its axis was in the plane of the magnetic meridian, but 
perpendicular to the dip; and then rotating the ball, the magnet was imme- 
diately affected. Upon inverting the direction of rotation, the needle was 
again affected, but in the opposite direction. When the ball revolved from 
east over to west, the marked pole went eastward ; when the ball revolved in 
the opposite direction, the marked pole went westward or towards the ball. 
Upon placing the ball to the east of the needles, still the needle was deflected 
in the same way ; i. e. when the ball revolved from east over to west, the 
marked pole went eastward (or towards the ball) ; when the rotation was in 
the opposite direction, the marked pole went westward. 

165. By twisting the silk of the needles, the latter were brought into a 
position perpendicular to the plane of the magnetic meridian ; the ball was 
again revolved, with its axis parallel to the needle ; the needle was affected as 
before, and the deflection was such as to show that both here and in the 
former case the needle was influenced solely by currents of electricity existing 
in the brass globe. 

166. If the upper part of the revolving ball be considered as a wire moving 
from east to west, over the unmarked pole of the earth, the current of electri- 
city in it should be from north to south (99. 114. 150.) ; if the under part be 
considered as a similar wire, moving from west to east over the same pole, the 
electric current should be from south to north ; and the circulation of electri- 

mdcccxxxii. z 



170 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

city should therefore be from north above to south, and below back to north, 
in a metal ball revolving from east above to west in these latitudes. Now 
these currents are exactly those required to give the directions of the needle 
in the experiments just described ; so that the coincidence of the theory from 
which the experiments were deduced with the experiments themselves, is 
perfect. 

\&J. Upon inclining the axis of rotation considerably, the revolving ball was 
found to affect the magnetic needle ; and it was not until the angle which it 
formed with the magnetic dip was rendered small, that its effects, even upon 
this apparatxis, were lost (153.). When revolving with its axis parallel to the 
dip, it is evident that the globe becomes analogous to the copper plate ; elec- 
tricity of one kind might be collected at its equator, and of the other kind at 
its poles. 

168. A current in the ball, such as that described above (161.), although it 
ought to deflect a needle the same way whether it be to the right or the left 
of the ball and of the axis of rotation, ought to deflect it the contrary way 
when above or below the ball ; for then the needle is, or ought to be, acted 
upon in a contrary direction by the current. This expectation was fulfilled 
by revolving the ball beneath the magnetic needle, the latter being still in- 
closed in its jar. When the ball was revolved from east over to west, the 
marked pole of the needle, instead of passing eastward, went westward ; and 
when revolved from west over to east, the marked pole went eastward. 

169. The deflections of the magnetic needle thus obtained with a brass ball 
are exactly in the same direction as those observed by Mr. Barlow in the revo- 
lution of the iron shell ; and from the manner in which iron exhibits the phe- 
nomena of magneto-electric induction like any other metal, and distinct from 
its peculiar magnetic phenomena (132.), it is impossible but that electric cur- 
rents must have been excited, and become active in those experiments. What 
proportion of the whole effect obtained is due to this cause, must be decided 
by a more mature investigation of all the phenomena. 

170. These results, in conjunction with the general law before described, 
suggested an experiment of extreme simplicity, which yet, on trial, was found 
to answer perfectly. The exclusion of all extraneous circumstances and all 
complexity of arrangement, and the distinct character of the indications 





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TERRESTRIAL MAGNETO-ELECTRIC INDUCTION. 171 

afforded, render this single experiment an epitome of nearly all the facts of 
magneto-electric induction. 

171. A piece of common copper wire, about eight feet long and one twen- 
tieth of an inch in thickness, had one of its ends fastened to one of the termi- 
nations of the galvanometer wire, and the other end to the other termination ; 
thus it formed an endless continuation of the galvanometer wire : it was then 
roughly adjusted into the shape of a rectangle, or rather of a loop, the upper 
part of which could be carried to and fro over the galvanometer, whilst the 
lower part, and the galvanometer attached to it, remained steady (Plate IV. 
fig. 30.). Upon moving this loop over the galvanometer from right to left, 
the magnetic needle was immediately deflected ; upon passing the loop back 
again, the needle passed in the contrary direction to what it did before ; upon 
repeating these motions of the loop in accordance with the vibrations of the 
needle (39.), the latter soon swung through 90° or more. 

172. The relation of the current of electricity produced in the wire, to its 
motion, may be understood by supposing the convolutions at the galvanometer 
away, and the wire arranged as a rectangle, with its lower edge horizontal 
and in the plane of the magnetic meridian, and a magnetic needle suspended 
above and over the middle part of this edge, and directed by the earth (fig. 30.). 
On passing the upper part of the rectangle from west to east into the posi- 
tion represented by the dotted line, the marked pole of the magnetic needle 
went west ; the electric current was therefore from north to south in the 
part of the wire passing under the needle, and from south to north in the 
moving or upper part of the parallelogram. On passing the upper part of the 
rectangle from east to west over the galvanometer, the marked pole of the 
needle went east, and the current of electricity was therefore the reverse of the 
former. 

173. When the rectangle was arranged in a plane east and west, and the 
magnetic needle made parallel to it, either by the torsion of its suspension 
thread or the action of a magnet, still the general effects were the same. On 
moving the upper part of the rectangle from north to south, the marked pole 
of the needle went north ; when the wire was moved in the opposite direction, 
the marked pole went south. The same effect took place when the motion of 
the wire was in any other azimuth of the line of dip ; the direction of the cur- 

z2 



172 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

rent always being conformable to the law formerly expressed (114.), and also 
to the directions obtained with the rotating ball (164.). 

174. In these experiments it is not necessary to move the galvanometer or 
needle from its first position. It is quite sufficient if the wire of the rectangle 
is distorted where it leaves the instrument, and bent so as to allow the moving 
upper part to travel in the desired direction. 

175. The moveable part of the wire was then arranged below the galvano- 
meter, but so as to be carried across the dip. It affected the instrument as 
before, and in the same direction ; i. e. when carried from west to east under 
the instrument, the marked end of the needle went west, as before. This 
should, of course, be the case ; for when the wire is cutting the magnetic dip 
in a certain direction, an electric current also in a certain direction should be 
induced in it. 

176. If in fig. 31. dp be parallel to the dip, and B A be considered as the 
upper part of the rectangle (171 •), with an arrow c attached to it, both these 
being retained in a plane perpendicular to the dip,- — then, however B A with its 
attached arrow is moved upon dp as an axis, if it afterwards proceed in the 
direction of the arrow, a current of electricity will move along it from B to- 
wards A. 

177- When the moving part of the wire was carried up or down parallel to 
the dip, no effect was produced on the galvanometer. When the direction of 
motion was a little inclined to the dip, electricity manifested itself; and was 
at a maximum when the motion was perpendicular to the magnetic direction. 

178. When the wire was bent into other forms and moved, equally strong 
effects were obtained, especially when instead of a rectangle a double cate- 
narian curve was formed of it on one side of the galvanometer, and the two 
single curves or halves were swung in opposite directions at the same time; 
their action then combined to affect the galvanometer: but all the results were 
reducible to those above described. 

179. The longer the extent of the moving wire, and the greater the space 
through which it moves, the greater is the effect upon the galvanometer. 

180. The facility with which electric currents are produced in metals when 
moving under the influence of magnets, suggests that henceforth precautions 
should always be taken, in experiments upon metals and magnets, to guard 



TERRESTRIAL MAGNETO-ELECTRIC INDUCTION. 173 

against such effects. Considering the universality of the magnetic influence 
of the earth, it is a consequence which appears very extraordinary to the mind, 
that scarcely any piece of metal can be moved in contact with others, either 
at rest, or in motion with different velocities or in other directions, without an 
electric current existing within them. It is probable that amongst arrange- 
ments of steam-engines and metal machinery, some curious accidental mag- 
neto-electric combinations may be found, producing effects which have never 
been observed, or, if noticed, have never as yet been understood. 

181. Upon considering the effects of terrestrial magneto-electric induction 
which have been described, it is almost impossible to resist the impression 
that similar effects, but infinitely greater in force, may be produced by the 
action of the magnet of the globe upon its own mass, in consequence of its 
diurnal rotation. It would seem that if a bar of metal be laid in these lati- 
tudes on the surface of the earth parallel to the magnetic meridian, a current 
of electricity tends to pass through it from south to north, in consequence of 
the travelling of the bar from west to east (172.), by the rotation of the earth; 
that if another bar in the same direction be connected with the first by wires, 
it cannot discharge the current of the first, because it has an equal tendency 
to have a current in the same direction induced within itself: but that if the 
latter be carried from east to west, which is equivalent to a diminution of the 
motion communicated to it from the earth (172.), then the electric current 
from south to north is rendered evident in the first bar, in consequence of its 
discharge, at the same time, by means of the second. 

182. Upon the supposition that the rotation of the earth tended, by magneto- 
electric induction, to cause currents in its own mass, these would, according to 
the law (114.) and the experiments, be, upon the surface at least, from the parts 
in the neighbourhood of or towards the plane of the equator, in opposite 
directions to the poles; and if collectors could be applied at the equator and at 
the poles of the globe, as has been done with the revolving copper plate (150.), 
and also with magnets (220.), then negative electricity would be collected at 
the equator, and positive electricity at both poles (222.). But without the con- 
ductors, or something equivalent to them, it is evident these currents could 
not exist, as they could not be discharged. 

183. I did not think it impossible that some natural difference might occur 



174 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

between bodies, relative to the intensity of the current produced or tending to 
be produced in them by magneto-electric induction, which might be shown by 
opposing them to each other ; especially as Messrs. Arago, Babbage, Hers- 
chell, and Harris have all found great differences, not only between the 
metals and other substances, but between the metals themselves, in their power 
of receiving motion from or giving it to a magnet in trials by revolution (130.). 
I therefore took two wires, each one hundred and twenty feet long, one of iron 
and the other of copper. These were connected with each other at their ends, 
and then extended in the direction of the magnetic meridian, so as to form 
two nearly parallel lines, nowhere in contact except at the extremities. The 
copper wire was then divided in the middle, and examined by a delicate galva- 
nometer, but no evidence of an electrical current was obtained. 

184. By favour of His Royal Highness the President of the Society, I ob- 
tained the permission of His Majesty to make experiments at the lake in the 
gardens of Kensington-palace, for the purpose of comparing, in a similar man- 
ner, water and metal. The basin of this lake is artificial; the water is supplied 
by the Chelsea Company ; no springs run into it, and it presented what I re- 
quired, namely, a uniform mass of still pure water, with banks ranging nearly 
from east to west, and from north to south. 

185. Two perfectly clean bright copper plates, each exposing four square 
feet of surface, were soldered to the extremities of a copper wire ; the plates 
were immersed in the water, north and south of each other, the wire which 
connected them being arranged upon the grass of the bank. The plates were 
about four hundred and eighty feet from each other, in a right line; the wire 
was probably six hundred feet long. This wire was then divided in the mid- 
dle, and connected by two cups of mercury with a delicate galvanometer. 

186. At first, indications of electric currents were obtained; but when these 
were tested by inverting the direction of contact, and in other ways, they were 
found to be due to other causes than the one sought for. A little difference in 
temperature ; a minute portion of the nitrate of mercury used to amalgamate 
the wires, entering into the water employed to reduce the two cups of mer- 
cury to the same temperature; was sufficient to produce currents of electricity, 
which affected the galvanometer, notwithstanding they had to pass nearly five 
hundred feet of water. When these and other interfering causes were guarded 



TERRESTRIAL MAGNETO-ELECTRIC INDUCTION. 175 

against, no effect was obtained; and it appeared that even such dissimilar sub 
stances as water and copper, when cutting the magnetic curves of the earth 
With equal velocity, perfectly neutralized each other's action. 

187- Mr. Fox of Falmouth has obtained some highly important results re- 
specting the electricity of metalliferous veins in the mines of Cornwall, which 
have been published in the Philosophical Transactions*. I have examined 
the paper with a view to ascertain whether any of the effects were probably 
referable to magneto-electric induction ; but, though unable to form a very 
strong opinion, believe they are not. When parallel veins running east and 
west were compared, the general tendency of the electricity in the wires was 
from north to south ; when the comparison was made between parts towards 
the surface and at some depth, the current of electricity in the wires was from 
above downwards. If there should be any hatural difference in the force of 
the electric currents produced by magneto-electric induction in different sub- 
stances, or substances in different positions moving with the earth, and which 
might be rendered evident by increasing the masses acted upon, then the wires 
and veins experimented with by Mr. Fox might perhaps have acted as dis- 
chargers to the electricity of the mass of strata included between them, and 
the directions of the currents would be those observed as above. 

188. Although the electricity obtained by magneto-electric induction in a 
few feet of wire is of but small intensity, and has not as yet been observed ex- 
cept in metals, and carbon in a particular state, still it has power to pass 
through brine (23.) ; and, as increased length in the substance acted upon pro- 
duces increase of intensity, I hoped to obtain effects from extensive moving 
masses of water, though still water gave none. I made experiments therefore 
(by favour) at Waterloo Bridge, extending a copper wire nine hundred and 
sixty feet in length upon the parapet of the bridge, and dropping from its ex- 
tremities other wires with extensive plates of metal attached to them to com- 
plete contact with the water. The wire therefore and the water made one con- 
ducting circuit ; and as the water ebbed or flowed with the tide, I hoped to 
obtain currents analogous to those of the brass ball (161.). 

189. I constantly obtained deflections at the galvanometer, but they were 

* 1830. p. 399. 



176 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

very irregular, and were in succession referred to other causes than that sought 
for. The different condition of the water as to purity on the two sides of the 
river ; the difference in temperature ; slight differences in the plates, in the 
solder used, in the more or less perfect contact made by twisting or other- 
wise ; all produced effects in turn : and though I experimented on the water 
passing through the middle arches only ; used platina plates instead of copper ; 
and took every other precaution, I could not after three days obtain any satis- 
factory results. 

190. Theoretically, it seems a necessary consequence that where water is 
flowing, there electric currents should be formed : thus, if a line be imagined 
passing from Dover to Calais through the sea, and returning through the land 
beneath the water to Dover, it traces out a circuit of conducting matter, one 
part of which, when the water moves up or down the channel, is cutting the 
magnetic curves of the earth, whilst the other is relatively at rest. This is a 
l-epetition of the wire experiment (171 .), but with worse conductors. Still 
there is every reason to believe that electric currents do run in the general 
direction of the circuit described, either one way or the other, according as 
the passage of the waters is up or down the channel. Where the lateral ex- 
tent of the moving water is enormously increased, it does not seem improbable 
that the effect should become sensible; and the gulf stream may thus, perhaps, 
from electric currents moving across it, by magneto-electric induction from the 
earth, exert a sensible influence upon the forms of the lines of magnetic 
variation*. 

191. Though positive results have not yet been obtained by the action of the 
earth upon water and aqueous fluids, yet, as the experiments are very limited 
in their extent, and as such fluids do yield the current by artificial magnets 
(23.), (for transference of the current is proof that it may be produced (213.),) 
the supposition made, that the earth produces these induced currents within 
itself (181.) in consequence of its diurnal rotation, is still highly probable 
(222. 223.) ; and when it is considered that the moving masses extend for 

* Theoretically, even a ship or a boat when passing on the surface of the water, in northern or 
southern latitudes, should have currents of electricity running through it directly across the line of 
her motion ; or if the water is flowing past the ship at anchor, similar currents should occur. 



MAGNETO-ELECTRIC INDUCTION IN DIFFERENT SUBSTANCES. 177 

thousands of miles across the magnetic curves, cutting them in various direc- 
tions within its mass, as well as at the surface, it is possible the electricity 
may rise to considerable intensity. 

192. I hardly dare venture, even in the most hypothetical form, to ask whe- 
ther the Axirora Borealis and Australis may not be the discharge of electricity, 
thus urged towards the poles of the earth, from whence it is endeavouring to 
return by natural and appointed means above the earth to the equatorial 
regions. The non-occurrence of it in very high latitudes is not at all against 
the supposition ; and it is remarkable that Mr. Fox, who observed the deflec- 
tions of the magnetic needle at Falmouth, by the Aurora Borealis, gives that 
direction of it which perfectly agrees with the present view. He states that 
all the variations at night were towards the east*, and this is what would 
happen if electric currents were setting from south to north in the earth under 
the needle, or from north to south in space above it. 

§ 6, General remarks and illustrations of the Force and Direction of Magneto- 
electric Induction. 

193. In the repetition and variation of Arago's experiment by Messrs. Bab- 
bage, Herschel, and Harris, those philosophers directed their attention to 
the differences of force observed amongst the metals and other substances in 
their action on the magnet. These differences were very great ~\-, and led me 
to hope that by mechanical combinations of various metals important results 
might be obtained (183). The following experiments were therefore made, 
with a view to obtain, if possible, any such difference of the action of two 
metals. 

194. A piece of soft iron bonnet-wire covered with cotton was laid bare and 
cleaned at one extremity, and there fastened by metallic contact with the clean 
end of a copper wire. Both wires were then twisted together like the strands 
of a rope, for eighteen or twenty inches; and the remaining parts being made 
to diverge, their extremities were connected with the wires of the galvano- 
meter. The iron wire was about two feet long, the continuation to the galva- 
nometer being copper. 

* Philosophical Transactions, 1831, p. 202. t Ibid. 1825 ; p. 472, 1831, p. 78. 

MDCCCXXXII. 2 A 



178 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

195. The twisted copper and iron (touching each other nowhere but at the 
extremity) was then passed between the poles of a powerful magnet arranged 
horse-shoe fashion (fig. 32.) ; but not the slightest effect was observed at the 
galvanometer, although the arrangement seemed fitted to show any electrical 
difference between the two metals relative to the action of the magnet. 

196. A soft iron cylinder was then covered with paper at the middle part, 
and the twisted portion of the above compound wire coiled as a spiral around 
it, the connexion with the galvanometer still being made at the ends A and B. 
The iron cylinder was then brought in contact with the poles of a powerful 
magnet capable of raising thirty pounds ; yet no signs of electricity appeared 
at the galvanometer. Every precaution was applied in making and breaking 
contact to accumulate effect, but no indications of a current could be ob- 
tained. 

197- Copper and tin, copper and zinc, tin and zinc, tin and iron, and zinc 
and iron, were tried against each other in a similar manner (194), but not the 
slightest sign of electric currents could be procured. 

198. Two fiat spirals, one of copper and the other of iron, containing each 
eighteen inches of wire, were connected with each other and with the galva- 
nometer, and then put face to face so as to be in contrary directions. When 
brought up to the magnetic pole (53.), no electrical indications at the galva- 
nometer were observed. When one was turned round so that both were in the 
same direction, the effect at the galvanometer was very powerful. 

199. The compound helix of copper and iron wire formerly described (8.) 
was arranged as a double helix, one of the helices being all iron and contain- 
ing two hundred and fourteen feet, the other all copper and containing two 
hundred and eight feet. The two similar ends A A of the copper and iron 
helix were connected together, and the other ends B B of each helix connected 
with the galvanometer ; so that when amagnet was introduced into the centre 
of the arrangement, the induced currents in the iron and copper would tend 
to proceed in contrary directions. Yet when a magnet was inserted, or a soft 
iron bar within made a magnet by contact with poles, no effect at the needle 
was produced. 

200. A glass tube about fourteen inches long was filled with strong sul- 
phuric acid. Twelve inches of the end of a clean copper wire were bent up 



MAGNETO-ELECTRIC INDUCTION IN DIFFEEENT SUBSTANCES. 179 

into a bundle and inserted into the tube, so as to make good superficial con- 
tact with the acid, and the rest of the wire passed along the outside of the tube 
and away to the galvanometer. A wire similarly bent up at the extremity was 
immersed in the other end of the sulphuric acid, and also connected with the 
galvanometer, so that the acid and copper wire were in the same parallel rela- 
tion to each other in this experiment as iron and copper were in the first (194). 
When this arrangement was passed in a similar manner between the poles of 
the magnet, not the slightest effect at the galvanometer could be perceived. 

201. From these experiments it would appear, that when metals of different 
kinds connected in one circuit are equally subject in every circumstance to 
magneto-electric induction, they exhibit exactly equal powers with respect to 
the currents which either are formed, or tend to form, in them. The same 
even appears to be the case with regard to fluids, and probably all other 
substances. 

202. Still it seemed impossible that these results could indicate the relative 
inductive power of the magnet upon the different metals ; for that the effect 
should be in some relation to the conducting power seemed a necessary con- 
sequence (139), and the influence of rotating plates upon magnets had been 
found to bear a general relation to the conducting power of the substance 
used. 

203. In the experiments of rotation (81.), the electric current is excited and 
discharged in the same substance, be it a good or bad conductor ; but in the 
experiments just described the current excited in iron could not be transmitted 
but through the copper, and that excited in copper had to pass through iron ; 
i. e. supposing currents of dissimilar strength to be formed in the metals pro- 
portionate to their conducting power, the stronger current had to pass through 
the worst conductor, and the weaker current through the best. 

204. Experiments were therefore made in which different metals insulated 
from each other were passed between the poles of the magnet, their opposite 
ends being connected with the same end of the galvanometer wire, so that the 
currents formed and led away to the galvanometer should oppose each other ; 
and when considerable lengths of different wires were used, feeble deflections 
were obtained. 

205. To obtain perfectly satisfactory results a new galvanometer was con- 

2 a2 



180 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

structed, consisting of two independent coils, each containing eighteen feet of 
silked copper wire. These coils were exactly alike in shape and number of 
turns, and were fixed side by side with a small interval between them, in which 
a double needle could be hung by a fibre of silk exactly as in the former in- 
strument (87-). The coils may be distinguished by the letters K L, and 
when electrical currents were sent through them in the same direction, acted 
upon the needle with the sum of their powers ; when in opposite directions, 
with the difference of their powers. 

206. The compound helix (199. 8.) was now connected, the ends A and B 
of the iron with A and B ends of galvanometer coil K, and the ends A and B 
of the copper with B and A ends of galvanometer coil L, so that the currents 
excited in the two helices should pass in opposite directions through the coils 
K and L. On introducing a small cylinder magnet within the helices, the gal- 
vanometer needle was powerfully deflected. On disuniting the iron helix, the 
magnet caused with the copper helix alone still stronger deflection in the same 
direction. On reuniting the iron helix, and unconnecting the copper helix, the 
magnet caused a moderate deflection in the contrary direction. Thus it was 
evident that the electric current induced by a magnet in a copper wire was far 
more powerful than the current induced by the same magnet in an equal iron 
wire. 

207- To prevent any error that might arise from the greater influence, from 
vicinity or other circumstances, of one coil on the needle beyond that of the 
other, the iron and copper terminations were changed relative to the galva- 
nometer coils K L, so that the one which before carried the current from the 
copper now conveyed that from the iron, and vice versa. But the same striking 
superiority of the copper was manifested as before. This precaution was taken 
in the rest of the experiments with other metals to be described. 

208. I then had wires of iron, zinc, copper, tin, and lead, drawn to the same 
diameter (very nearly one twentieth of an inch), and I compared exactly equal 
lengths, namely sixteen feet, of each in pairs in the following manner : The 
ends of the copper wire were connected with the ends A and B of galvanometer 
coil K, and the ends of the zinc wire with the terminations A and B of the 
galvanometer coil L. The middle part of each wire was then coiled six times 
round a cylinder of soft iron covered with paper, long enough to connect the 



MAGNETO-ELECTRIC INDUCTION IN DIFFERENT SUBSTANCES. 181 

poles of Daniell's horse-shoe magnet (56.) (fig. 33.), so that similar helices of 
copper and zinc, each of six turns, surrounded the bar at two places equidistant 
from each other and from the poles of the magnet ; but these helices were 
purposely arranged so as to be in contrary directions, and therefore send con- 
trary currents through the galvanometer coils K and L. 

209. On making and breaking contact between the soft iron bar and the 
poles of the magnet, the galvanometer was strongly affected ; on detaching the 
zinc it was still more strongly affected in the same direction. On taking all 
the precautions before alluded to (207.), with others, it was abundantly proved 
that the current induced by the magnet in copper was far more powerful than 
in zinc. 

210. The copper was then compared in a similar manner with tin, lead, and 
iron, and surpassed them all, even more than it did zinc. The zinc was then 
compared experimentally with the tin, lead, and iron, and found to produce a 
more powerful current than any of them. Iron in the same manner proved 
superior to tin and lead. Tin came next, and lead the last. 

211. Thus the order of these metals is copper, zinc, iron, tin, and lead. It 
is exactly their order with respect to conducting power for electricity, and, with 
the exception of iron, is the order presented by the magneto-rotation experi- 
ments of Messrs. Babbage, Herschel, Harris, &c. The iron has additional 
power in the latter kind of experiments, because of its ordinary magnetic rela- 
tions, and its place relative to magneto-electric action of the kind now under 
investigation cannot be ascertained by such trials. In the manner above de- 
scribed it may be correctly ascertained *. 

212. It must still be observed that in these experiments the whole effect be- 
tween different metals is not obtained ; for of the thirty-four feet of wire in- 
cluded in each circuit, eighteen feet are copper in both, being the wire of the 
galvanometer coils ; and as the whole circuit is concerned in the resulting force 
of the current, this circumstance must tend to diminish the difference which 
would appear between the metals if the circuits were of the same substances 

* Mr. Christie, who, being appointed reporter upon this paper, had it in his hands before it was 
complete, felt the difficulty (202.) ; and to satisfy his mind, made experiments upon iron and copper 
with the large magnet (44.), and came to the same conclusions as I have arrived at. The two set of 
experiments were perfectly independent of each other, neither of us being aware of the other's pro- 
ceedings. 



182 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

throughout. In the present case the difference obtained is probably not more 
than a half of that which would be given if the whole of each circuit were of 
one metal. 

213. These results tend to prove that the currents produced by magneto- 
electric induction in bodies is proportional to their conducting power. That 
they are exactly proportional to and altogether dependent upon the con- 
ducting power, is, I think, proved by the perfect neutrality displayed when 
two metals or other substances, as acid, water, &c. &c. (201. 186.), are 
opposed to each other in their action. The feeble current which tends to be 
produced in the worse conductor, has its transmission favoured in the better 
conductor, and the stronger current which tends to form in the latter has its in- 
tensity diminished by the obstruction of the former ; and the forces of genera- 
tion and obstruction are so perfectly balanced as to neutralize each other 
exactly. Now as the obstruction is inversely as the conducting power, the ten- 
dency to generate a current must be directly as that power to produce this 
perfect equilibrium. 

214. The cause of the equality of action under the various circumstances 
described, where great extent of wire (183.) or wire and water (184.) were 
connected together, which yet produced such different effects upon the magnet, 
is now evident and simple. 

215. The effects of a rotating substance upon a needle or magnet ought, 
where ordinary magnetism has no influence, to be directly as the conducting 
power of the substance ; and I venture now to predict that such will be found 
to be the case ; and that in all those instances where non-conductors have been 
supposed to exhibit this peculiar influence, the motion has been due to some 
interfering cause of an ordinary kind ; as mechanical communication of motion 
through the parts of the apparatus, or otherwise (as in the case Mr. Harris has 
pointed out *) ; or else to ordinary magnetic attractions. To distinguish the 
effects of the latter from those of the induced electric currents, I have been 
able to devise a most perfect test, which shall be almost immediately de- 
scribed (243.). 

216. There is every reason to believe that the magnet or magnetic needle 
will become an excellent measurer of the conducting power of substances 

* Philosophical Transactions, 1831. p. 68. 



ESSENTIAL CONDITIONS OF MAGNETO-ELECTRIC INDUCTION. 183 

rotated near it ; for I have found by careful experiment, that when a constant 
current of electricity was sent successively through a series of wires of cop- 
per, platina, zinc, silver, lead, and tin, drawn to the same diameter ; the 
deflection of the needle was exactly equal by them all. It must be remem- 
bered that when bodies are rotated in a horizontal plane, the magnetism of 
the earth is active upon them. As the effect is general to the whole of the 
plate, it may not interfere in these cases ; but in some experiments and calcu- 
lations may be of important consequence. 

217. Another point which I endeavoured to ascertain, was, whether it was 
essential or not that the moving part of the wire should, in cutting the mag- 
netic curves, pass into positions of greater or lesser magnetic force ; or whe- 
ther, always intersecting curves of equal magnetic intensity, the mere motion 
was sufficient for the production of the current. That the latter is true, has 
been proved already in several of the experiments on terrestrial magneto-elec- 
tric induction. Thus the electricity evolved from the copper plate (149.), the 
currents produced in the rotating globe (161, &c), and those passing through 
the moving wire (171.), are all produced under circumstances in which the 
magnetic force could not but be the same during the whole experiment. 

218. To prove the point with an ordinary magnet, a copper disc was 
cemented upon the end of a cylinder magnet, with paper intervening ; the 
magnet and disc were rotated together, and collectors (attached to the galva- 
nometer) brought in contact with the circumference and the central part of the 
copper plate. The galvanometer needle moved as in former cases, and the 
direction of motion was the same as that which would have resulted, if the cop- 
per only had revolved, and the magnet been fixed. Neither was there any 
apparent difference in the quantity of deflection. Hence, rotating the magnet 
causes no difference in the results ; for a rotatory and a stationary magnet pro- 
duce the same effect upon the moving copper. 

219. A copper cylinder, closed at one extremity, was then put over the 
magnet, one half of which it inclosed like a cap ; it was firmly fixed, and pre- 
vented from touching the magnet anywhere by interposed paper. The ar- 
rangement was then floated in a narrow jar of mercury, so that the lower edge 
of the copper cylinder touched the fluid metal; one wire of the galvanometer 
dipped into this mercury, and the other into a little cavity in the centre of the 



184 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

end of the copper cap. Upon rotating the magnet and its attached cylinder, 
abundance of electricity passed through the galvanometer, and in the same 
direction as if the cylinder had rotated only, the magnet being still. The 
results therefore were the same as those with the disc (218.). 

220. That the metal of the magnet itself might be substituted for the moving 
cylinder, disc, or wire, seemed an inevitable consequence, and yet one which 
would exhibit the effects of magneto-electric induction in a striking form. A 
cylinder magnet had therefore a little hole made in the centre of each end to 
receive a drop of mercury, and was then floated pole upwards in the same metal 
contained in a narrow jar. One wire from the galvanometer dipped into the 
mercury of the jar, and the other into the drop contained in the hole at the 
upper extremity of the axis. The magnet was then revolved by a piece of string 
passed round it, and the galvanometer-needle immediately indicated a power- 
ful current of electricity. On reversing the order of rotation, the electrical 
current was reversed. The direction of the electricity was the same as if the 
copper cylinder (219.) or a copper wire had revolved round the fixed magnet 
in the same direction as that which the magnet itself had followed. Thus a 
singular independence of the magnetism and the bar in which it resides is 
rendered evident. 

221. In the above experiment the mercury reached about half way up the 
magnet ; but when its quantity was increased until within one eighth of an 
inch of the top, or diminished until equally near the bottom, still the same 
effects and the same direction of electrical current was obtained. But in those 
extreme proportions the effects did not appear so strong as when the surface 
of the mercury was about the middle, or between that and an inch from each 
end. The magnet was eight inches and a half long, and three quarters of an 
inch in diameter. 

222. Upon inversion of the magnet, and causing rotation in the same 
direction, i. e. always screw or always unscrew, then a contrary current of 
electricity was produced. But when the motion of the magnet was continued 
in a direction constant in relation to its own axis, then electricity of the same 
kind was collected at both poles, and the opposite electricity at the equator, 
or in its neighbourhood, or in the parts corresponding to it. If the magnet be 
held parallel to the axis of the earth, with its unmarked pole directed to the 



ESSENTIAL CONDITIONS OF MAGNETO-ELECTRIC INDUCTION. 185 

pole star, and then rotated so that its upper parts pass from west to east in 
conformity to the motion of the earth ; then positive electricity may be col- 
lected at the extremities of the magnet, and negative electricity at or about 
the middle of its mass. 

223. When the galvanometer was very sensible, the mere spinning of the 
magnet in the air, whilst one of the galvanometer wires touched the extremity, 
and the other the equatorial parts, was sufficient to evolve a current of elec- 

, tricity and deflect the needle. 

224. Experiments were then made with a similar magnet, for the purpose 
of ascertaining whether any return of the electric current could occur at the 
central or axial parts, they having the same angular velocity of rotation as 
the other parts (259.) ; the belief being that it could not. 

225. A cylinder magnet, seven inches in length, and three quarters of an 
inch in diameter, had a hole pierced in the direction of its axis from one ex- 
tremity, a quarter of an inch in diameter, and three inches deep. A copper 
cylinder, surrounded by paper and amalgamated at both extremities, was fixed 
in the hole so as to be in metallic contact at the bottom, by a little mercury, 
with the middle of the magnet ; insulated at the sides by the paper; and pro- 
jecting about a quarter of an inch above the end of the steel. A quill was put 
over the copper rod, which reached to the paper, and formed a cup to receive 
mercury for the completion of the contact. A high paper edge was also raised 
round that end of the magnet, and mercury put within it, which however had 
no metallic connexion with that in the quill, except through the magnet itself 
and the copper rod (fig. 34.). The wires A and B from the galvanometer were 
dipped into these two portions of mercury; any current through them could, 
therefore, only pass down the magnet towards its equatorial parts, and then up 
the copper rod; or vice versa. 

226. When thus arranged and rotated screw fashion, the marked end of the 
galvanometer needle went west, indicating that there was a current through 
the instrument from A to B, and consequently from B through the magnet 
and copper rod to A (fig. 34.). 

227. The magnet was then put into a jar of mercury (fig. 35.) as before 
(219.) ; the wire A left in contact with the copper axis, but the wire B dipped 
in the mercury of the jar, and therefore in metallic communication with the 

mdcccxxxii. 2 B 



186 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

equatorial parts of the magnet instead of its polar extremity. On revolving" 
the magnet screw fashion, the galvanometer needle was deflected in the same 
direction as before, but far more powerfully. Yet it is evident that the parts 
of the magnet from the equator to the pole were out of the electric circuit. 

228. Then the wire A was connected with the mercury on the extremity of 
the magnet, the wire B still remaining in contact with that in the jar (fig. 36.), 
so that the copper axis was altogether out of the circuit. The magnet was 
again revolved screw fashion, and again caused the same deflection of the 
needle, the current being as strong as it was in the last trial (227-), and much 
stronger than at first (226.). 

229. Hence it is evident that there is no discharge of the current at the 
centre of the magnet, for the current, now freely evolved, is up through the 
magnet ; but in the first experiment (226.), it was down. In fact, at that time, 
it was only the part of the moving metal equal to a little disc extending from 
the end of the wire B in the mercury to the wire A that was efficient, i. e. 
moving with a different angular velocity to the rest of the circuit (258.) ; and 
for that portion the direction of the current is consistent with the other results. 

230. In the two after experiments, the lateral parts of the magnet or of the 
copper rod are those which move relative to the other parts of the circuit, i. e. 
the galvanometer wires ; and being more extensive, intersecting more curves ; 
or moving with more velocity, produce the greater effect. For the discal part, 
the direction of the induced electric current is the same in all, namely, from 
the circumference towards the centre. 

231. The law under which the induced electric current excited in bodies 
moving relatively to magnets, is made dependent on the intersection of the 
magnetic curves by the metal (114.) being thus rendered more precise and 
definite (217- 220. 224.), seemed now even to apply to the cause in the first 
section of the former paper; and by rendering a perfect reason for the effects 
produced, take away any for supposing that peculiar condition, which I ven- 
tured to call the electro-tonic state (60.). 

232. When an electrical current is passed through a wire, that wire is sur- 
rounded at every part by magnetic curves, diminishing in intensity according 
to their distance from the wire, and which in idea may be likened to rings 
situated in planes perpendicular to the wire or rather to the electric current 



VOLTA-ELECTRIC AND MAGNETO-ELECTRIC INDUCTION. 187 

within it. These curves, although different in form, are perfectly analogous to 
those existing between two contrary magnetic poles opposed to each other ; 
and when a second wire, parallel to that which carries the current, is made to 
approach the latter (1 8.), it passes through magnetic curves exactly of the same 
kind as those it would intersect when carried between opposite magnetic poles 
(109.), in one direction ; and as it recedes from the inducing wire, it cuts the 
curves around it in the same manner that it would do those between the same 
poles if moved in the other direction. 

233. If the wire N P (fig. 40.) have an electric current passed through it in 
the direction from P to N, then the dotted ring may represent a magnetic curve 
round it, and it is in such a direction that if small magnetic needles be placed 
as tangents to it, they will become arranged as in the figure, n and * indicating 
north and south ends (44. note.). 

234. But if the current of electricity were made to cease for a while, and 
magnetic poles were used instead to give direction to the needles, and make 
them take the same position as when under the influence of the current, then 
they must be arranged as at fig. 41 ; the marked and unmarked poles a b 
above the wire, being in opposite directions to those a' b' below. In such 
a position therefore the magnetic curves between the poles a b and a' b' have 
the same general direction with the corresponding parts of the ring magnetic 
curve surrounding the wire N P carrying an electric current. 

235. If the second wire p n (fig. 40.), be now brought towards the principal 
wire, carrying a current, it will cut an infinity of magnetic curves, similar in 
direction to that figured, and consequently similar in direction to those between 
the poles a b of the magnets (fig. 41.), and it will intersect these current curves 
in the same manner as it would the magnet curves, if it passed from above 
between the poles downwards. Now, such an intersection would, with the 
magnets, induce an electric current in the wire from p to n (114.) ; and there- 
fore as the curves are alike in arrangement, the same effect ought to result 
from the intersection of the magnetic curves dependent on the current in the 
wire N P; and such is the case, for on approximation the induced current is in 
the opposite direction to the principal current (19.). 

236. If the wire p' n' be carried up from below, it will pass in the opposite 
direction between the magnetic poles; but then also the magnetic poles them- 

2b2 



188 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

selves are reversed (fig. 41.), and the induced current is therefore (114.) still 
in the same direction as before. It is also, for equally sufficient and evident 
reasons, in the same direction, if produced by the influence of the curves de- 
pendent upon the wire. 

237- When the second wire is retained at rest in the vicinity of the principal 
wire, no current is induced through it, for it is intersecting no magnetic curves. 
When it is removed from the principal wire, it intersects the curves in the 
opposite direction to what it did before (235.) ; and a current in the opposite 
direction is induced, which therefore corresponds with the direction of the 
principal current (19.). The same effect would take place if by inverting the 
direction of motion of the wire in passing between either set of poles (fig. 41.), 
it were made to intersect the curves there existing in the opposite direction to 
what it did before. 

238. In the first experiments (10. 13.), the inducing wire and that under 
induction were arranged at a fixed distance from each other, and then an 
electric current sent through the former. In such cases the magnetic curves 
themselves must be considered as moving (if I may use the expression) across 
the wire under induction, from the moment at which they begin to be de- 
veloped until the magnetic force of the current is at its utmost ; expanding as 
it were from the wire outwards, and consequently being in the same relation 
to the fixed wire under induction as if it had moved in the opposite direction 
across them, or towards the wire carrying the current. Hence the first cur- 
rent induced in such cases was in the contrary direction to the principal cur- 
rent (17. 235.). On breaking the battery contact, the magnetic curves (which 
are mere expressions for arranged magnetic forces) may be conceived as con- 
tracting upon and returning towards the failing electrical current, and there- 
fore move in the opposite direction across the wire, and cause an opposite in- 
duced current to the first. 

239. When, in experiments with ordinary magnets, the latter, in place of 
being moved past the wires, were actually made near them (27. 36.), then a 
similar progressive development of the magnetic curves may be considered as 
having taken place, producing the effects which would have occurred by motion 
of the wires in one direction ; the destruction of the magnetic power corre- 
sponds to the motion of the wire in the opposite direction. 



DISTINCTION OF MAGNETISM AND MAGNETO-ELECTRIC INDUCTION. 189 

240. If, instead of intersecting the magnetic curves of a straight wire carry- 
ing a current, by approximating or removing a second wire (235.), a revolving 
plate be used, being placed for that purpose near the wire, and, as it were, 
amongst the magnetic curves, then it ought to have continuous electric cur- 
rents induced within it ; and if a line joining the wire with the centre of the 
plate were perpendicular to both, then the induced current ought to be, accord- 
ing to the law (114.), directly across the plate, from one side to the other, and 
at right angles to the direction of the inducing current. 

241. A single metallic wire one twentieth of an inch in diameter had an 
electric current passed through it, and a small copper disc one inch and a half 
in diameter revolved near to and under, but not in actual contact with it 
(fig. 39.). Collectors were then applied at the opposite edges of the disc, and 
wires from them connected with the galvanometer. As the disc revolved in 
one direction, the needle was deflected on one side ; and when the direction of 
revolution was reversed, the needle was inclined on the other side, in accordance 
with the results anticipated. 

242. Thus the reasons which induced me to suppose a particular state in 
the wire (60.) have disappeared ; and though it still seems to me unlikely that 
a wire at rest in the neighbourhood of another carrying a powerful electric cur- 
rent is entirely indifferent to it, yet I am not aware of any distinct facts which 
authorize the conclusion that it is in a particular state. 

243. In considering the nature of the cause assigned in these papers to 
account for the mutual influence of magnets and moving metals (120.), and 
comparing it with that heretofore admitted, namely, the induction of a feeble 
magnetism like that produced in iron, it occurred to me that a most decisive 
experimental test of the two views could be applied (215.). 

244. No other known power has like direction with that exerted between 
an electric current and a magnetic pole ; it is tangential, while all other forces, 
acting at a distance, are direct. Hence, if a magnetic pole on one side of a 
revolving plate follow its course by reason of its obedience to the tangential 
force exerted upon it by the very current of electricity which it has itself caused, 
a similar pole on the opposite side of the plate should immediately set it free 
from this force ; for the currents which tend to be formed by the action of the 
two poles are in opposite directions ; or rather no current tends to be formed, 



190 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

or no magnetic curves are intersected (1 14.) ; and therefore the magnet should 
remain at rest. On the contrary, if the action of a north magnetic pole were 
to produce a southness in the nearest part of the copper plate, and a diffuse 
northness elsewhere (82.), as is really the case with iron; then the use of another 
north pole on the opposite side of the same part of the plate should double 
the effect instead of destroying it, and double the tendency of the first magnet 
to move with the plate. 

245. A thick copper plate (85.) was therefore fixed on a vertical axis, a bar 
magnet was suspended by a platted silk cord, so that its marked pole hung 
over the edge of the plate, and a sheet of paper being interposed, the plate was 
revolved ; immediately the magnetic pole obeyed its motion and passed off in 
the same direction. A second magnet of equal size and strength was then sus- 
pended to the first, so that its marked pole should hang beneath the edge of 
the copper plate in a corresponding position to that above, and at an equal 
distance (fig. 37.)- Then a paper sheath or screen being interposed as before, 
and the plate revolved, the poles were found entirely indifferent to its motion, 
although either of them alone would have followed the course of rotation. 

246. On turning one magnet round, so that opposite poles were on each 
side of the plate, then the mutual action of the poles and the moving metal was 
a maximum. 

247. On suspending one magnet so that its axis was level with the plate, 
and either pole opposite its edge, the revolution of the plate caused no motion 
of the magnet. The electrical currents dependent upon induction would now 
tend to be produced in a vertical direction across the thickness of the plate, 
but could not be so discharged, at least only to so slight a degree as to leave all 
effects insensible ; but ordinary magnetic induction, or that on an iron plate, 
would be equally if not more powerfully developed in such a position (251.). 

248. Then, with regard to the production of electricity in these cases: — when- 
ever motion was communicated by the plate to the magnets, currents existed ; 
when it was not communicated, they ceased. A marked pole of a large bar 
magnet was put under the edge of the plate; collectors (86.) applied at the axis 
and edge of the plate as on former occasions (fig. 38.), and these connected 
with the galvanometer; when the plate was revolved, abundance of electricity 
passed to the instrument. The unmarked pole of a similar magnet was then 



DISTINCTION OP MAGNETISM AND MAGNETO-ELECTRIC INDUCTION. 191 

put over the place of the former pole, so that contrary poles were above and 
below ; on revolving the plate, the electricity was more powerful than before. 
The latter magnet was then turned end for end, so that marked poles were 
both above and below the plate, and then, upon revolving it, scarcely any elec- 
tricity was procured. By adjusting the distance of the poles so as to correspond 
with their relative force, they at last were brought so perfectly to neutralize 
each other's inductive action upon the plate, that no electricity could be ob- 
tained with the most rapid motion. 

249. I now proceeded to compare the effect of similar and dissimilar poles 
upon iron and copper, adopting for the purpose Mr. Sturgeon's very useful 
form of Arago's experiment. This consists in a circular plate of metal sup- 
ported in a vertical plane by a horizontal axis, and weighted a little at one 
edge or rendered excentric so as to vibrate like a pendulum. The poles of the 
magnets are applied near the side and edges of these plates, and then the 
number of vibrations, required to reduce the vibrating arc a certain constant 
quantity, noted. In the first description of this instrument * it is said that 
opposite poles produced the greatest retarding effect, and similar poles none ; 
and yet within a page of the place the effect is considered as of the same kind 
with that produced in iron. 

250. I had two such plates mounted, one of copper, one of iron. The copper 
plate alone gave sixty vibrations, in the average of several experiments, before 
the arc of vibration was reduced from one constant mark to another. On putting 
opposite magnetic poles near to, and on each side of, the same place, the vibra- 
tions were reduced to fifteen. On putting similar poles on each side of it, they 
rose to fifty ; and on putting two pieces of wood of equal size with the poles 
equally near, they became fifty-two. So that, when similar poles were used, the 
magnetic effect was little or none, (the obstruction being due to the confine- 
ment of the air, rather,) whilst with opposite poles it was the greatest possible. 
When a pole was presented to the edge of the plate, no retardation occurred. 

251. The iron plate alone made thirty-two vibrations, whilst the arc of vibra- 
tion diminished a certain quantity. On presenting a magnetic pole to the edge 
of the plate (247.)> the vibrations were diminished to eleven ; and when the 
pole was about half an inch from the edge, to five. 

* Edin. Phil. Journal, 1825. p. 124. 



192 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

252. When the marked pole was put at the side of the iron plate at a certain 
distance, the number of vibrations was only five. When the marked pole of 
the second bar was put on the opposite side of the plate at the same distance 
(250.), the vibrations were reduced to two. But when the second pole was an 
unmarked one, yet occupying exactly the same position, the vibrations rose to 
twenty-two. By removing the stronger of these two opposite poles a little way 
from the plate, the vibrations increased to thirty-one, or nearly the original 
number. But on removing it altogether, they fell to between five and six. 

253. Nothing can be more clear, therefore, than that with iron, and bodies 
admitting of ordinary magnetic induction, opposite poles on opposite sides of 
the edge of the plate neutralize each other's effect, whilst similar poles exalt the 
action ; a single pole end on is also sufficient. But with copper, and substances 
not sensible to ordinary magnetic impressions, similar poles on opposite sides of 
the plate neutralize each other ; opposite poles exalt the action ; and a single 
pole at the edge or end on does nothing. 

254. Nothing can more completely show the thorough independence of the 
effects obtained with the metals by Arago, and those due to ordinary magnetic 
forces *, and henceforth, therefore, the application of two poles to various moving 
substances will, if they appear at all magnetically affected, afford a proof of the 
nature of that affection. If opposite poles produce more effect than one, the 
force will be due to electric currents. If similar poles produce more effect 
than one, then the power is not electrical : it will not be like that active in the 
metals and carbon when moving, and in most cases will probably be found to 
be not even magnetical, but the result of irregular causes not anticipated and 
guarded against. 

255. The result of these investigations tends to show that there are really 
but very few bodies that are magnetic in the manner of iron. I have often 
sought for indications of this power in the common metals and other sub- 
stances ; and once in illustration of Arago's objection (82.), and in hopes of 
ascertaining the existence of currents in metals by the momentary approach of 
a magnet, suspended a disc of copper by a single fibre of silk in an excellent 
vacuum, and approximated powerful magnets on the outside of the jar, making 
them approach and recede in unison with a pendulum that vibrated as the 
disc would do : but no motion could be obtained ; not merely, no indication of 



CONDITIONS OP MAGNETO-ELECTRIC INDUCTION. 193 

ordinary magnetic powers, but none of any electric current occasioned in the 
metal by the approximation and recession of the magnet. I therefore venture 
to arrange substances in three classes as regards their relation to magnets ; 
first, those which are affected when at, rest, like iron, nickel, &c. being such as 
possess ordinary magnetic properties ; then, those which are affected when in 
motion, being conductors of electricity in which are produced electric currents 
by the inductive force of the magnet ; and, lastly, those which are perfectly 
indifferent to the magnet, whether at rest or in motion. 

256. Although it will require further research, and probably close investi- 
gation, both experimental and mathematical, before the exact mode of action 
between a magnet and metal moving relatively to each other is ascertained ; 
yet many of the results appear sufficiently clear and simple to allow of expres- 
sion in a somewhat general manner. If a terminated wire move so as to cut 
a magnetic curve, a power is called into action which tends to urge an elec- 
tric current through it ; but this current cannot be brought into existence un- 
less provision be made at the ends of the wire for its discharge and renewal. 

257- If a second wire move in the same direction as the first, the same power 
is exerted upon it, and it is therefore unable to alter the condition of the first : 
for there appear to be no natural differences among substances when connected 
in a series, by which, when moving under the same circumstances relative to 
the magnet, one tends to produce a more powerful electric current in the whole 
circuit than another (201. 214.). 

258. But if the second wire move with a different velocity, or in some other 
direction, then variations in the force exerted take place ; and if connected at 
their extremities, an electric current passes through them. 

259. Taking, then, a mass of metal or an endless wire, and referring to the 
pole of the magnet as a centre of action, (which though perhaps not strictly 
correct may be allowed for facility of expression, at present,) if all parts move 
in the same direction, and with the same angular velocity, and through 
magnetic curves of constant intensity, then no electric currents are produced. 
This point is easily observed with masses subject to the earth's magnetism, and 
may be proved with regard to small magnets ; by rotating them, and leaving 
the metallic arrangements stationary, no current is produced. 

260. If one part of the wire or metal cut the magnetic curves, whilst the other 
mdcccxxxii. 2 c 



194 MR. FARADAY'S EXPERIMENTAL RESEARCHES IN ELECTRICITY. 

is stationary, then currents are produced. All the results obtained with the 
galvanometer are more or less of this nature, the galvanometer extremity being 
the fixed part. Even those with the wire, galvanometer, and earth (170.), may 
be considered so without any error in the result. 

261. If the motion of the metal be in the same direction, but the angular 
velocity of its parts relative to the pole of the magnet different, then currents 
exist. This is the case in Arago's experiment, and also in the wire subject to 
the earth's induction (172.), when it was moved from west to east. 

262. If the magnet moves not directly to or from the arrangement, but late- 
rally, then the case is similar to the last. 

263. If different parts move in opposite directions across the magnetic 
curves, then the effect is a maximum for equal velocities. 

264. All these in fact are variations of one simple condition, namely, that all 
parts of the mass shall not move in the same direction across the curves, and 
with the same angular velocity. But they are forms of expression which being 
retained in the mind, I have found useful when comparing the consistency of 
particular phenomena with general results. 

Royal Institution, 
December 21, 1831.