322 Mr. Barlow's Experiments on 



rubbed with a little nitrate of mercury, and pure mercury 

 being poured into them, the contact is made or broken at 

 pleasure, by placing the wires in or throwing them out of the 

 cups ; and the effect thus produced is shown in the most sen- 

 sible manner by the needle within the helix ; which in very 

 delicate cases may be neutralized by the small magnets ns, ns 

 in the foot below, when the instrument is placed east and 

 west, or by inserting them in holes, for the purpose, in the 

 props AB, CD, as seen at p and q when it is in the meridian. 

 Then by applying the lamp at R, then at S, changing the 

 bars and wires, and the size of the latter, &c. all the variety 

 of experiments so judiciously arranged and combined by Pro- 

 fessor Gumming may be repeated, and the series extended 

 at pleasure. The galvanometer of Professor Gumming is un- 

 doubtedly the same in principle as the above, and may not 

 pei'haps differ much in construction; but as he has not de- 

 scribed it, the foregoing description will not, I hope, be 

 thought superfluous *. 



Apparatus for exhibiting Rotation. 

 Professor Gumming at the conclusion of his paper has 

 suggested a small combination of platinum and silver wires 

 for exhibiting a rotation about a magnet, as Mr. Faraday had 

 done in the case of a galvanic wire, in his experiments on 

 electro-magnetism, and which certainly threw more light on the 

 inquiry than any experiments before made on the subject. 



On constructing a machine precisely from the description 

 given by Professor C., it was found that it had indeed a ten- 

 dency to revolve, but so small that it was very difficult, if not 

 impossible, to exhibit the phaenomenon in a satisfactory man- 

 ner. It turned out, however, while carrying on the experi- 

 ment, that, although a magnet in the interior of the wire 

 would not produce any but a weak tendency to rotation, a 

 magnet applied exterior to it was capable of producing the 

 most decided effect, which will be seen as I proceed to describe 

 the following experiments. 



For this purpose four rectangles, figures 2, 3, 4, 5, were 

 made of platinum and silver combined, as shown in the figures, 

 where the thicker lines indicate silver wire, and the lighter 

 ones the platinum ; a ring being formed below to admit the 

 prop upon which they were to revolve ; and a fine steel point 

 brazed to the u{^pi,r side to rest in the agate on the top of the 

 prop. The stand with a rectangle suspended is shown in 

 figure C, where AB is a board, cd a brass prop with its agate 

 at top, and N S a magnet placed as nearly as possible to the 



* Professor Cunmiing has since described his instrument in the Annals 

 of I'liilosophv for October last. — Edjt. 



wire. 



