Il6 THE ELECTRIC ARC 



was negligible and the " minimum potential difference" 

 tended to a constant value whatever the nature of the 

 metal, provided it was not easily melted. This voltage 

 was in the neighborhood of 475 volts, the experiments being 

 performed in air at a pressure of 40 cm., the distance be- 

 tween the electrodes being 4 mm., the current o.i ampere 

 and the frequency 50 alternations per second. 



The space between the electrodes with an arc between 

 metals loses its conductivity more quickly than with the 

 arc between carbons. This is shown by direct measure- 

 ments of the conductivity and by the difficulty which is 

 experienced in trying to maintain an alternating-current 

 arc between metals. The experience of Edlund with the 

 arc between silver electrodes has already been mentioned. 

 Arons 1 found that the arc between metals lost its con- 

 ductivity very quickly. Duddell 2 found that it was not 

 possible to restart an arc between copper electrodes 

 1/27000 sec. after it had been broken. 



As has been stated Guye and Monash believed that the 

 difficulty is proportional to the atomic weight of the metal, 

 but it is hard to see how the atomic weight can directly 

 affect the arc and the evidence is not strong for this view. 

 Their data, for example, do not agree with the data of those 

 who have worked with larger currents. 



Granqvist believes that the difficulty of maintaining such 

 an arc is due to the rapid conduction of heat by the 

 electrodes. This, no doubt, has much to do with the 

 phenomenon, and yet it is not a complete explanation, as 

 is shown by the behavior of the alternating-current arc 

 between carbons with different pressures of the surrounding 



1 Wied. Ann., 57, 193; 1896. 

 * Lond. Elec., 46, 311; 1900. 



