Faraday to George Biddell Airy   20 August 1859

[Royal Institution embossed letterhead] | 20 Aug 1859

My dear Sir

I should begin the experimental trials with a solid bobbin because by lowering that you would obtain the general distribution of force supplied by a hollow cylindrical helix whereas no elevation of the latter would give you the same intense central force of a near bobbin¹. The size & force of the bobbin must have relation to the magnet in the pendulum bob if that were powerful the bobbin might be small perhaps an inch in diameter & 2 inches long or if the magnet were small the bobbin might be larger[.] The magnet should be very hard & not I think nearly saturated or the continual alternations of the power to which it would be subject might leave it permanently. I think I should make the bobbin of rather large wire using a large-plated battery to give quantity.

And so I guess at a first set of conditions not knowing without trial what would be the amount of effect on a pendulum of a given weight & a given magnet or in which direction any one of these conditions would have to change to produce the desired effect.

Ever My dear Sir | Very Truly Yours | M. Faraday

G.B. Airy Esqr | &c &c &c