George Biddell Airy to Faraday   14 November 1838

Royal Observatory Greenwich 1838 Nov 14

My dear Sir

I am desirous of having your assistance in a practical matter of magnetism¹. We have here one of Gauss's large bars suspended as a horizontal needle by a bundle of silk: and it seems likely to continue in vibration for ever. The method of checking the vibrations adopted by Gauss is, as I understand, by the drag (I use the old word for want of a new one) produced by a curved copper bar completely surrounding the needle in a vertical plane: but none such was furnished to me by the Göttingen artist, and it is not described in the books published by Gauss and Weber². Something of this kind I must have: and I should be glad if you would inform me of what is absolutely necessary in its shape. Must it surround the needle, either in a horizontal or vertical plane? or will a horse-shoe at each end, embracing the end of the magnet, be sufficient? Must it be solid, or laminated? What caution must be observed as to the quality of the copper? Is copper ever magnetic? Is any other metal preferable? The suspension of the needle you will understand without any diagram: the hori‑zontal plan is the following

A the needle

B the suspension-piece

C a wire-frame

D a lens acting as collimator

E, F, counterpoises

The line which must not be interrupted by the copper is CD[.]

Iam my dear Sir | Yours very truly | G.B. Airy

Michael Faraday Esq | &c &c &c