George Biddell Airy to Faraday   19 August 1859

Royal Observatory, Greenwich, | London, S.E. | 1859 August 19

My dear Sir

Your note¹ shews clearly how difficult it is for a person who is possessed of all circumstances to think of detailing them sufficiently to any body else. I will supply my deficiencies as well as I can: and if you see the matter clearly, please to give me your advice: and if you do not see the matter clearly, do not trouble yourself to write, but wait till we can meet.

1. The pendulum carrying the magnet is to vibrate through an arc of 2¾ inches, or thereabouts.

2. Through the whole of this arc, the magnet pole is to be (a) attracted to, or (b) repelled from, the coil which is below. In the case (a), the pendulum’s vibrations will be made quicker; in the case (b) they will be made slower.

3. The dotted lines in the figure shew the extreme positions of the pendulum. The barbed lines shew the direction of force required in different parts of the vibration.

4. The case (a) is to be produced by turning on the battery in one direction, and the case (b) by turning on the battery in the opposite direction.

5. I expressly exclude iron core, because there would, when no battery is in action, be an attraction between the magnet and the iron core independent of battery, at times when I demand to have no attraction at all.

6. So the questions now are, Is the coil the most advantageous form of employment of current?

7. And if so, what shape and dimensions may be best?

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

Professor Faraday