Faraday to George Biddell Airy   16 November 1838

Royal Institution | 16 Nov 1838

My dear Sir

Remember at the outset that I am writing theoretically & have not seen the magnet which you want to steady or its support¹ & do not know the velocity of the motion which you wish to stop. I do not see any need for a ring in a vertical plane. One in a horizontal plane would be more effective & is near the original form of Arago's Expt². Nor do I see any reason for extending the copper far beyond the extremes of the arc of vibration whatever that may be. The object (on the principle you refer to) is simply to bring a good conductor like copper near to the moving magnetic poles & I should have thought a sort of jacket of this

kind might have been applied. If the upper part interferes with the use of the Collimator & wire frame it would require to be suppressed more or less. The copper should at all events extend beyond the Arc of vibration[.]

I had occasion once to check the Vibration of a galvanometer & yet allow the needle to take its true place. I found I could effect this by fixing a small plane under the needle and immersing this plane in water the surface of the water wetting the filament by which it was fixed to the needle³

Would any thing of this kind be applicable to your case[.] For the plane might be used a vane with several fans or perhaps a hollow open cylinder would give rise to fewer disturbances from possible motions in the water than any other form. Instead of water Air in a vessel nearly closed might surround the plane or vane[.] I have recently found the good retarding effects of a vane in air in stopping the motions of the repelled ball of a Coulomb⁴ electrometer⁵[.]

If I could see you we might in talk say something more distinct. I must not say I will come to Greenwich but shall you be in town shortly for instance at the anniversary &c?⁶

Ever Truly Yours | M. Faraday

Professor Airy | &c &c &c