66 THE ELECTRIC ARC 



duction of a carbon electrode, the vapor of the arc is changed 

 to a certain extent, so that one can not be certain that the 

 anode drop is the same as it was before this was intro- 

 duced. As a result the measurements which have been 

 made on the anode and cathode drops can not be con- 

 sidered as more than rough approximations. 



The measurements indicate, however, that the relative 

 values of the two drops are quite different in metal arcs 

 from what they are in the carbon arc. In the latter the 

 anode drop is much greater than the cathode drop. In 

 the former the two are approximately the same, as was 

 shown by Lecher 1 for platinum, iron, aluminum and copper 

 and by myself 2 for zinc, iron and copper. With graphite 

 terminals in air the anode and cathode drops are much 

 the same as they are with carbon. 3 



With a metal arc in a partial vacuum the measurement 

 of the potential within the arc can be made somewhat 

 more accurately. Banderet 4 has examined the anode and 

 cathode drops of the different forms of the copper arc 

 when in a vacuum. As has been stated, it is possible to 

 have six different forms. In one of these where the 

 appearance at the anode is somewhat similar to that at 

 the anode with the glow discharge in a Geissler tube he 

 found the anode drop to be 34 volts. In a form which 

 is similar to the carbon arc the drop is 26 volts, while in 

 an intermediate form it is 20 volts. These measurements 

 were taken with a pressure of no mm., a current of 5 

 amperes, and an arc length of 20 mm. With all of these 



1 Wied. Ann., 33, 609; 1888. 



2 Phys. Rev., 12, 149; 1901. 



3 Phys. Rev., 20, 364; 1905. 



4 Dissertation Basel, 1912. 



