ALTERNATING-CURRENT ARCS 



maintained between iron and copper terminals either in 

 air or in a vacuum with an E.M.F. of 220 volts. 1 With 

 graphite it was very difficult to maintain an arc with such 

 a voltage. 



There appears to be no further work with such arcs where 

 the current was comparable with that of the common arc. 

 There have, however, been two investigations where very 

 small currents and high E.M.F.'s were used, but it is 

 difficult to decide whether these should be considered as 

 arcs or as sparks. 



The first of these investigations was made by Guye and 

 Monash. 2 They concluded from their experiments that 

 the difficulty of obtaining the alternating-current arc be- 

 tween metals increases as the atomic weight of the metal 

 becomes greater. In the following table there are given 

 the potential differences with the metals investigated by 

 them when the current was 0.04 ampere, the length 

 of the arc 5 mm., and the alternations 47 per second. 

 Accompanying these potential differences are the atomic 

 weights. 



Guye and Bron 3 found that by using an E.M.F. of 20,000 

 volts and by keeping the electrodes at temperatures very 

 near their melting points arcs of considerable stability were 

 obtained. Under these conditions the period of extinction 



1 Phys. Rev., 20, 374; 1905. 



2 Eel. Elec., 34, 305; 1902. Elektrot. ZS., 23, 956; 1902. 



3 Archives des Sciences, 25, 453 and 549; 1908. C. R., 146, 1090; and 

 147, 495 i98. 



