PRESSURES GREATER OR LESS THAN ONE ATMOSPHERE 8l 



The arc in a vacuum has a lower temperature than that 

 in air. 1 This has been shown by placing a platinum wire 

 0.3 mm. in diameter in the arc. At atmospheric pressures 

 this would be melted in a fraction of a second. When 

 the pressure of the surrounding gas is 4 mm., the wire 

 is not melted. With still lower pressures it gathers a 

 deposit of carbon. 2 



Metal Arc in a Vacuum. The arc between metals in a 

 vacuum has much the same appearance as that between 

 carbons in a vacuum, except that with the former the 

 striations appear to be even more peculiar than with the 

 latter. 3 The character of such an arc does not depend to 

 any great extent on the kind of metal used for the anode. 

 In fact, if the anode does not become so hot as to send 

 vapor into the tube, it appears to have no effect on the 

 discharge, so that any metal may be satisfactorily used 

 for this electrode. Quite the opposite of this is true con- 

 cerning the cathode. If the cathode is a metal which 

 melts easily, there is no difficulty in maintaining the arc 

 with very low pressures of the gas, 4 but metals with high 

 melting points can not be used for the cathode. 5 It is, 

 for example, impossible to maintain an arc with platinum, 

 iron, nickel, copper or silver for the cathode when the pres- 

 sure of the gas is less than i mm. and the impressed E.M.F. 

 is no more than 100 volts. Apparently the higher the 

 melting point of the metal used, the more difficult it is to 



1 Grove, Phil. Mag., 16, 479; 1840. 



2 Phys. Rev., 19, 123; 1904. 



3 Herwig, Pogg. Ann., 149, 523; 1873. 

 Arons, Ann. d. Phys., I, 711; 1900. 



4 Stark and Kuch, Phys. ZS., 6, 438; 1905. 

 * Phys. Rev., 20, 369; 1905. 



