1 66 THE ELECTRIC ARC 



found it to be 250 volts, to that from hot lime, where it 

 varies from 70 to 12 volts, and then to the arc, where it 

 may be as low as 6 volts. The luminous discharge from 

 hot calcium oxide is identical with the arc in a vacuum, 

 except that with the arc no independent source of energy 

 is needed to heat the cathode 1 and this is a thing which has 

 nothing to do with the manner in which the ions are pro- 

 duced. 



That there is ionization by impact at the cathode with 

 the luminous discharge from hot calcium oxide is shown 

 by the following. With hot calcium oxide it is possible 

 to have a very small non-luminous discharge and with the 

 same current heating the cathode and the same potential 

 difference between the electrodes to have a large luminous 

 discharge, providing the discharge has momentarily been 

 raised to an amount sufficiently high to start the larger dis- 

 charge. The number of electrons emitted from within 

 the calcium oxide can not be greatly different in the two 

 cases, for the temperature of the cathode has not appre- 

 ciably changed, and a study of the potential between 

 electrodes shows that the increase in current can not be 

 due to a movement of the positive ions toward the cathode. 

 The only way to explain the increase is by assuming that 

 in the second form of discharge a greater number of elec- 

 trons is produced at the cathode by the impact of a greater 

 number of positive ions. The appearance of the rays 

 streaming from the cathode confirms this view. If then 

 we have ionization by impact with hot calcium oxide, we 

 undoubtedly have the same manner of ionization in the arc. 



Causes Determining the Amount of the Cathode Drop. 

 The immediate cause of the drop in potential at the cathode 



1 Phys. Rev.,ifc 361; 1909. 



