1 82 THE ELECTRIC ARC 



the carbon usually loses weight as if some chemical or 

 physical change were taking place. 



Again, there is a change in the cathode drop when the 

 current changes. For example with the carbon arc in air 

 the greater the current flowing the less the cathode drop. 

 This is probably due to the increase in the amount of heat 

 developed near the cathode. Heat at this point will do 

 one of two things. It will either raise the temperature of 

 the cathode, or if the cathode is at the sublimation point, 

 it will increase the amount of vapor sent into the arc. If 

 the cathode becomes hotter, ionization at its surface will 

 occur with less vigorous impact and a smaller cathode drop 

 will be sufficient to give this impact. If on the other hand 

 the point of sublimation has been reached, and more vapor 

 is produced, we should again expect a smaller cathode drop 

 for we have already seen that the cathode drop is least 

 when the cathode is sending vapor into the arc, as it does, 

 for example, in the mercury arc. 



It is indeed more difficult to explain the fact that the 

 cathode drop of the mercury arc does not appear to change 

 when the current is changed (p. 96). It may be, how- 

 ever, that here we have so much vaporization at all times, 

 that a limited amount more or less does not produce any 

 appreciable effect. It would, however, be surprising if one 

 should find that the cathode drop did not increase some- 

 what when very small currents were used. 



The pressure of the surrounding gas also affects some- 

 what the value of the cathode drop, the drop becoming 

 larger as the pressure increases. This is no doubt due to 

 the more rapid dissipation of the heat caused by the greater 

 number of molecules and the necessity of supplying a corre- 

 spondingly greater amount of heat to the cathode. It may 



