V. A new suspension of the magnetic needle, intended for the discovery of minute quantities of magnetic attraction: also an air vane of great sensibility; with new experiments on the magnetism of iron filings and brass

Abstract

To manifest the various degrees of attraction between magnets and ferruginous bodies, different methods have been used. The substance to be tried has either been simply brought into contact with the magnet, or has been made to float on water or mercury. Needles are commonly made to rest horizontally on sharp-pointed wires, and as an improvement on these methods, Mr. Cavallo has suspended a needle by a chain of horse-hair, consisting of five or six links, which move very freely in each other, and allow the needle to turn more than a whole revolution round its centre. By comparing this needle with others of the best sort in use, he found it much more sensible. Others have suspended the needle by fine threads, or silk : but as these, on turning round a few times, will cause the needle to deviate from its meridian by twisting, they are certainly objectionable. After considering each of the above methods, and trying some of them in November, 1789, I suspended a small sewing needle, by means of a spider's thread, in the cylindrical glass of my gold-leaf electrometer; and having satisfactorily proved its magnetic sensibility, I now venture to propose this kind of suspension, as being well adapted to experiments requiring the needle to move with the least resistance.