Albert Einstein to Stephen Brunauer - June 18, 1943

by Albert Einstein

A. EINSTEIN,
112, MERCER STREET,
PRINCETON,
NEW JERSEY, U.S.A.

June 18,1943

Lt.Stephen Brunauer
Bureau of Ordnance
Navy Dept.
Washington D.C.

Dear Mr.Brunauer:

I have already had two conversations with my colleagues Drs. White and Taub at the Institute. Both are very kind and helpful in discussing the problems with me and recommending me their special library.

According to these discussions and a remark you made yourself when you were visiting me it seems to be desirable to have a device which automatically brings to explosion a torpedo passing below a ship at the right moment. I have an idea for an electro-magnetic device for this purpose which I would like to submit to you for your judgement. My colleagues here have no knowledge in this field and advised me to write to you.

S is an electro-magnet producing an alternating magbetic field with vertical axis. s and s' are two coils with vertical axis connected in serie with opposite magnetic axis, which are located symmetrically to S. The current created by induction in those two coils may be multiplied by tubes (current i). The device is set in function only at a moment when the torpedo is far enough from the emitting ship to avoid the magnetic influence of the latter.

As long as the torpedo is on the way i will vanish for reasons of symmetry. The torpedo is supposed to travel deep enough so that it will not hit the target directly but pass below it. When the torpedo approaches the target there will be created through the nearby parts of the ship's hull an additional (induced) alternative magnetic field which at first will be stronger in s than in s' . There is now created a current i the intensity of which will change according to the following diagram:

M corresponds to the moment when the torpedo passes under the middle of the ship. At this moment i will again vanish for reasons of symmetry. This is the moment to put the explosion into action. It will be easy to construct an electrical contact device which functions if a current is first initiated and goes again to zero afterwards.

I realize that this idea is probably not new or that it might have been given up on account of technical difficulties. In that case I hope I have not taken up too much of your time.

Yours very sincerely,
A. Einstein
Professor Albert Einstein.