George Biddell Airy to Faraday   16 August 1859

Royal Observatory, Greenwich, | London, S.E. | 1859 August 16

My dear Sir

I want your assistance in a practical galvanic matter.

I am about to introduce a method of altering the rate of a clock by what is in fact a diminution or an increase of the gravity of the pendulum: thus effected. To the pendulum is to be lashed a bar-magnet pointing up-and-down. To the clock case is to be fastened an ordinary galvano-magnet bobbin without its iron core, so that one pole of the magnet will, in the vibrations of the pendulum, sweep close over the pole of the bobbin. And when we send one current through the bobbin, it attracts the magnet and accelerates the clock: with the opposite current, the clock is retarded. All this works well.

Now I want your information as to the best form of the bobbin or other convolution of wires. Ought it to be a bobbin? Ought there to be any particular relation of inside diameter to outside diameter? Ought the bobbin be long or short? &c &c &c.

I am, my dear Sir, | Yours very truly | G.B. Airy

Professor Faraday