Effect of Wave Form on the Alternate Current Arc. 507 



another, with a double sliding-contact between them, as 

 shown in fig. 2. Then, in whatever position the slider may 

 be, the fundamental ratio ay = bx is always maintained, and 

 the first condition of the Kelvin bridge is mechanically ful- 

 filled. The one adjustment consists in moving the double 

 slider along the bridge until there is no deflexion of the 

 galvanometer at g ; in which case 



a _ x _ E, 



b~ y~ r 



Since writing the above, I have referred to the original 

 paper of Lord Kelvin (Proc. Roy. Soc. vol. xi. p. 313, 1861), 

 and find that he proposes the use of parallel slide-wires for 

 his auxiliary conductors ; I have no doubt he had in view 

 some such apparatus as that which I have here suggested. 

 A Kelvin bridge with a single slide-wire was used by Mat- 

 thiessen and Hockin in their differential method ; it is 

 described by Clerk-Maxwell in l Elec. and Mag.' vol. i. p. 406 

 (1873). 



LYII. The Effect of Wave Form on the Alternate Current Arc. 

 By Julius Fkith, 1851 Exhibition Scholar*. 



IN the paper by Dr. Fleming and Mr. Petavel, recently 

 read before the Physical Society, on the Alternate Cur- 

 rent Arcf, I think too little attention was paid to the wave 

 form of the alternate current used. 



It is known that if the arc is allowed to exert a prepon- 

 derating influence at all on the alternate current circuit, it 

 alters the wave form of both the current and the P. D. in a 

 very marked degree. 



As an illustration of the change produced in the wave 

 form by the character of the external circuit, I give some 

 curves for alternate current arcs taken from a paper which I 

 read before the Manchester Literary and Philosophical Society 

 in March 1894. 



Here is shown, first the E.M.F. curve of the machine on 

 open circuit, which is rather more peaked than a sine curve 

 and involves the third harmonic largely. Next is shown the 

 curve obtained under the same conditions, but with an arc 

 lamp taking 10 amperes at 40 volts joined direct to the 

 machine. 



The lag recorded is due to the self-induction of the machine, 



* Communicated by the Physical Society : read April 24, 1896. 

 t Supra, p. 315. 



