2 THE ELECTRIC ARC 



between the momentary spark formed on opening a circuit 

 and the continuous discharge which is called an arc. By 

 gradually raising the E.M.F. and decreasing the resistance 

 of a battery and shortening the distance between the ter- 

 minals it is possible to pass continuously from a spark 

 which lasts but a small fraction of a second to an arc 

 which lasts indefinitely. Nor can we say that either the 

 potential difference or the current of the momentary flash 

 is necessarily either greater or less than that of the arc, and 

 yet whatever might be the usage in a perfectly logical 

 language, the actual usage is to make a distinction between 

 these two phenomena. About all that can be said is that 

 the discharge must last quite a while in order to be called 

 an arc. "Quite a while " would appear to be sufficiently 

 indefinite to meet the needs of this case. 



It is much easier to distinguish between the arc and the 

 continuous discharge in a vacuum tube where the current 

 is small and the E.M.F. large, such as that shown in a 

 Geissler or Crookes tube, which has also been called by 

 some a spark. The current in this case is much smaller 

 than with the arc, and the potential difference is very 

 much higher. Moreover with the arc the potential differ- 

 ence between the cathode, as the negative terminal is 

 called, and the layer of gas in its immediate neighborhood 

 is small, while with the glow discharge it is large. With 

 the former it may be as low as 5 or 6 volts, while with the 

 latter it must be as high as 300 volts. 



Difficulty is, however, experienced in making a dis- 

 tinction between the arc and the glow discharge in a 

 vacuum when the latter has for the cathode a hot oxide 

 of certain metals, as, for example, calcium oxide. In this 

 case the drop in potential at the cathode may be as small as 



