on the Electric Arc. 127 



Description of Appaeatus. 



Two upright brass tubes are connected across the top by a 

 brass casting through which is bored a hole, to provide a means 

 of causing cold water to circulate through a metal electrode. 

 From the centre of the casting, extending downwards, is a 

 cylindrical piece of copper, which ends in a tip about -J- inch 

 diameter, on to which may be fitted the upper metallic 

 terminal of the arc. The brass tubes are clamped to a slate 

 base, and extend through it. Thus, by connecting the end 

 of one of them to a water supply, a steady flow is obtained, 

 and this is directed on to the very tip of the upper metallic 

 terminal holder (see fig. 1, p. 128). 



Through the centre of the slate base passes a screw of 

 four inches' length, which is capable of being turned by 

 means of a milled ebonite head, fixed at its lower end, and 

 thus being made to approach or move away from the upper 

 terminal holder. This screw likewise ends in a tip, by 

 which the lower arc terminal may be held. 



Surrounding the whole apparatus above the slate base 

 was placed a bell jar, in the neck of which was a rubber 

 cork holding a tube whose end was covered with wire gauze. 

 There was also a tube let into the slate base, by which 

 connexion could be made to a gas reservoir of any kind. 

 The gas thus admitted was expelled through the tube fixed in 

 the cork, and could, in the case of hydrogen or coal-gas, be 

 made to burn at the end. 



The two terminals of the arc were connected with the 

 source of electric supply — usually the street mains at 

 110 volts, continuous current — the upper, through one of 

 the tubes of the water circulation, and the lower through 

 the central screw. Potential leads were connected to the 

 other tube of the water circulation, and to the terminal 

 leading to the central screw. 



The electrodes used were ordinary solid carbons, 0'47, 0*4, 

 and 0*37 inch in diameter, cored carbons 047 inch diameter; 

 also metal terminals of copper, iron, and aluminium, either as 

 solid rods about 5 inches long, set into the lower holder, 

 or pieces of the shape shown in fig. 2 (p. 129), fastened to 

 the upper holder. The solid rods used were of the following 

 diameters : — copper § in., iron ^ in., aluminium -J in. 

 and | in. 



The light of the arc was passed through a double convex 

 lens, and projected onto a sheet of squared paper, at such 

 a distance that the arc length could easily be read off from 

 the image, in fractions of an inch. The arc was magnified 



