THEORY OF THE ELECTRIC ARC 185 



with which some substances are ionized than other sub- 

 stances. Thus sodium carbonate, when at a high tem- 

 perature, appears to be much more easily ionized than air. 

 In other cases the change may be due to the greater rapidity 

 with which heat is conducted away by the gas as with an 

 arc placed in hydrogen. It is probable that a large part 

 of the decrease in the potential difference between the 

 terminals of an iron arc when it passes from the first to the 

 second stage is due to the greater ease with which the gas 

 from the vaporizing metal anode can be ionized. 



Effect Produced by Heating the Cathode. To produce 

 the high temperature of the cathode there must be an 

 expenditure of electrical energy in its neighborhood. The 

 rate at which this is done equals the cathode drop times 

 the current. It is natural to expect that if the cathode is 

 cooled artificially, it will require a greater expenditure of 

 energy to maintain the needed temperature, and to do this 

 with a given current the cathode drop must be greater. 

 Similarly, with any method of heating we should expect 

 the cathode drop to be less. 



But it was found (p. 48) that cooling the cathode pro- 

 duced but a slight effect on the cathode drop, while it had 

 a very appreciable effect on the anode drop and on the 

 fall of potential through the arc. A possible explanation for 

 this is that it is a mistake to suppose that the temperature 

 of the cathode must be kept at a certain definite value. It 

 is probable that there may be conditions where this tem- 

 perature is lower than that which is commonly found. How- 

 ever, when the temperature of the cathode is lowered, it 

 sends out fewer electrons and as a result the remaining parts 

 of the arc have a smaller conductivity, so that we get the 

 effects which have been observed. 



