144 Mr. J. Brown on the Theory of Voltaic Action. 



described in his i Papers on Electrostatics and Magnetism/ 

 ]). 317 : — u A metal bar insulated so as to be movable about 

 an axis perpendicular to the plane of a metal ring made up 

 half of copper and half of zinc, the two halves being soldered 

 together, turns from the zinc towards the copper when vi- 

 treously electrified, and from the copper towards the zinc when 

 rcsinously electrified." Instead of a copper and zinc ring Iused 

 a copper and iron one, CI, o'l inches 

 diameter outside, with a 1 inch hole in 

 centre. It was supported on a tripod 

 inside a case with plate-glass sides, and 

 which could be connected by rubber 

 tubing with an apparatus for genera- 

 ting lrydrogen sulphide. A piece of 

 lead-paper was placed inside the case 

 to detect the first entrance of the gas. 

 From the top of the case rose a ver- 

 tical glass tube with a torsion head, 

 from which depended a platinum wire 

 •0025 in. in diameter and about 19 in. 

 long, carrying the needle or bar, n, of 

 thin sheet aluminium, 1 J in. long by 

 -j 3 ^ in. wide, a mirror M of about 4 ft. 

 focus, and a glass weight, W, which 

 dipped in a vessel of water to steady 

 it. The tripod carrying the ring 

 rested on the points of three screws 

 passing up through the bottom of the 

 case, by means of which the plane of 

 the ring could be adjusted so as to get 

 equal deflections on each side of the 

 zero-line. The needle hung at a distance of 1 or 2 millims. 

 above the ring, as nearly as possible over the junction of the 

 metals, and having its suspension-wire in the centre of the 

 ring. It was electrified by connecting it with the positive 

 or negative conductor of a Winter's plate machine. 



In a preliminary experiment with a copper-zinc ring, de- 

 flections of 5 centims. on each side of zero on the scale were 

 readily obtained. With the copper-iron ring, however, the 

 deflections were only \ to 1 centim., the iron being as zinc to 

 the copper. As the potential of the needle could not be main- 

 tained constant by the means employed, the deflection was 

 continually varying in amount ; but when the machine was 

 carefully worked these variations were slight, and did not in- 

 terfere with the result. What follows is from my notes of the 

 third time of going over the experiment. The needle being 



