346 



Professors Ayrton and Perry on the 



appears to be much greater in steel of blue temper than in soft 

 iron,and, again, much greater in steel of very hard temper than 

 in steel of blue temper. If we call the effect in soft iron 1, the 

 effect in blue steel is perhaps 2, and that in very hard steel 4. 



L. The Resistance of the Electric Arc. 

 By Professors W. E. Ayrton, F.R.S., and John Perry, M.E* 



ONE of the results of the elaborate set of experiments on 

 the Electric Light conducted in 1878 by the late Mr. 

 Schwendler was the conclusion to which he came, that the 

 supposition that the resistance of an arc of constant length 

 was inversely proportional to the current which passed 

 through it was highly probable. His experiments, however, 

 were not sufficient to absolutely determine this point ; and it 

 has therefore appeared to us important to obtain further in- 

 formation on the subject, which, with the assistance of the 

 students working in our laboratory, we have from time to time 

 done with the following results. 



1. The method employed by us in the first instance was as 

 follows : — A number of Grove cells, G, were arranged in 



Fig. 1- 



— Ei£) ft [Wirrinnnra-^ 



series (fig. 1); and one pole 

 P was connected through an 

 ammeter A with one carbon 

 of the electric light L, the 

 other carbon of which was 

 attached to a mercury trough 

 T, which, by means of a me- 

 tallic bridge-piece, could be 

 connected with any one of 

 the mercury-cups B, C, D, 

 each of which was perma- 

 nently electrically connected 

 with the terminal of a differ- 

 ent number of cells. The two carbons were also connected 

 with the terminals of a voltmeter V T , by means of which the 

 difference of potentials between the carbons at any moment 

 could be determined. The experiment was made thus : — 

 The bridge-piece was put into D, and the carbons by means 

 of a rack adjustment separated until a good steady light was 

 obtained, when readings of the ammeter and voltmeter were 

 taken. A second bridge-piece was now put into C and that 

 in D quickly withdrawn, the effect being to suddenly increase 



* Communicated by the Physical Society of London ; read December 9, 



1882. 



