ALTERNATING-CURRENT ARCS 117 



gas. At atmospheric pressure it is easy to maintain such 

 an arc on a circuit of no volts. In a vacuum where the 

 pressure is less than 0.5 mm. it is not possible to do so. 1 

 The atomic weight has not changed and the heat conduc- 

 tivity has not increased, so that neither of the above ex- 

 planations applies in this case. 



All our knowledge of the arc would indicate that we 

 must look to the cathode for an explanation of the difficulty. 

 When this is sufficiently hot a current can be started. 

 When it is not, no current will flow. In this case there 

 are two things to be considered; first, how much heat was 

 developed at the cathode during the preceding part of 

 the cycle when it was the anode, and secondly, how fast 

 it is conducted away. Granqvist called attention only 

 to the second of these considerations. 



With the carbon arc we not only have electrodes which 

 are poor thermal conductors, but we have an exceptionally 

 large anode drop with its corresponding large development 

 of heat near the anode. The electrode reaches a high 

 temperature while being the anode, and does not cool off 

 rapidly during the change in the direction of the current. 

 With the metal arc there is less heat developed and it is 

 conducted away more rapidly. 



With a carbon arc in a vacuum the heat is not conducted 

 away more rapidly than with such an arc in air, but the 

 anode drop is here very small and the carbon does not rise 

 to a sufficiently high temperature during the time that it 

 is the anode to enable it to start the arc when it becomes 

 the cathode. 



It is altogether possible that with the metal arc the ions 

 between the electrodes become more quickly loaded with 



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



