Peter Barlow to Faraday   4 May 1825

Royl Mily Acady | May 4 1825

Dear Sir

I am much obliged to you for your hint about Arago’s experiments¹. I had heard indistinctly of them before² but after the receipt of your letter Marsh and I set to work and we made them succeed very satisfactorily[.] I have since had Herschel down and I have been spinning with him and Babbage in town³ - but at present the explanation is in the dark. The following will reach most if not all the facts at present known - viz. That a certain attraction takes place between either pole of a magnet and copper, and all other metals, but too small to be distinguished in a general manner, besides being equal on both poles it has no tendency to disturb the needle unless one pole is much nearer than the other; but by revolving the plate and consequently the point to which the pole is attracted the needle has a tendency to move with it. We obtain a similar result with a vertical plate provided one pole of the needle is brought very near. On a trial of different metals their powers appear to be in the following order Iron Copper Zinc tin lead bismuth and antimony. The former besides its property common with the other metal has the other property which I found by revolving my ball, and which as I am inclined to explain, both results differ from the latter only in this, that the effect I observed depended upon the induction of magnetism in the iron ball from the Earth, whereas the result which Arago has noticed is due to the magnetism induced in the metal by the magnet itself. We must therefore suppose copper and all metals perhaps all substances to be slightly magnetic (Partially) except when intensely excited and then existing only in a very slight degree.

It is said that when Arago’s copper plate is cut in radii the effect is diminished and by some experiments I have made it does appear to be so⁴; this perhaps is a little at variance with my explanation - but I should be glad to do without vortices if I can[.]

Compliments to Mrs Faraday

Dear Sir yours truly | P. Barlow

My paper is to be read on Thursday⁵

Address: M. Faraday | Royal Institution | Albemarle Street