THEORY OF THE ELECTRIC ARC 171 



not happen without complicating action when cathode 

 rays strike an anti-cathode is evident, for we there have 

 secondary rays given off and also a large development of 

 heat. As far as the writer is aware there has been no work, 

 either experimental or theoretical, done on this subject in 

 the case of the arc. 



lonization of the Gas between Electrodes. With very 

 low pressures the current is carried almost entirely by 

 electrons which appear as cathode rays streaming out 

 toward the anode. These, no doubt, ionize the gas through 

 which they pass. The ability to ionize the gas is one of the 

 well-known properties of such rays and certainly ionization 

 of the gas must be produced in some way, since positive 

 ions come from the gas and heat the cathode. 



As the pressure of the gas or as the distance between the 

 electrodes is increased, a glow appears at the anode. Under 

 some conditions striations appear as was shown in Fig. 20. 

 Ordinarily with increase of pressure there is a continuous 

 glow between the electrodes. Increasing the pressure 

 further does not change the character of this glow, except 

 to make it more intense. 



With this form of the arc it is also certain that there 

 is ionization between the electrodes. In air, for example, 

 the electrons coming from the cathode can not go far 

 before being stopped by collision with the molecules of the 

 air and recombination between the ions must take place 

 in many cases. If so, there must also be fresh ionization 

 to keep up the supply of ions. 



Moreover we know that positive ions are produced in 

 the gas, since they are continually moving toward the 

 cathode, as is shown by the potential gradient between 

 the electrodes. This gradient has been found in all cases 



