Faraday to George Biddell Airy   18 August 1859

The Green | Hampton Court | 18 Aug 1859.

My dear Sir

Your subject is a difficult one for a note it wants a conversation¹. A note will carry a precise answer but only conversation would permit such a development of an idea (as yet vague in its form) as to allow something like an answer at the end. Ought your controling power to be compressed as much as might be, into the line a. b.? then ought the helix to be a solid bobbin & have length:- or ought the former to be diffused and approach more towards equality over a somewhat large field? then ought the helix to be a ring:- shut & having its internal & external diameter not very different:- between which ring however & the full bobbin their proportion might vary in every degree according to your requirement. I cannot answer these points without knowing what you want: or without knowing what any given arrangement gives you & how far it falls short of your requirements.

You speak of a bobbin only, perhaps because its force will vary in constant proportion to the force of the Electric current through it. I suppose you have thought of the use of an iron core with the bobbin? I suppose there is no doubt that such a combination would be more uncertain in the proportion of its force to the current sent into the wire than the former, if the current were strong, because of the retention of magnetism by the core. But if the size of the core were so large that the iron was never much magnetized, & the iron itself very soft & well annealed, I think it likely that you would have a field of magnetic power as strong as the former, as true in its variation with the varying current, and yet with a current so much less in power as to be more regular & constant in other respects that might be of importance and advantage in practice.

But I am talking quite at random so will say no more at present[.]

Ever Very Truly Yours | M Faraday

Astronomer Royal | &c &c &c