THEORY OF THE ELECTRIC ARC 173 



possible for any other kind of radiation, such as Rontgen 

 rays, or "entladungstrahlen" to be the sole source of the 

 ionization. 



On the other hand there are serious objections to believing 

 that impact of ions is the sole cause of ionization. The 

 first of these is that the mean free path of the electrons is 

 through too small a difference of potential to produce 

 ionization. The work of Davis 1 indicates that at ordinary 

 temperatures the ions must move through 11.7 volts in 

 order to ionize. We may get some idea of the potential 

 difference through which the electrons in the arc pass by 

 assuming that the mean free path of the ions varies in- 

 versely as the density of the gas, that the electric force of 

 the arc is 27 volts per centimeter, 2 that the mean free path 

 of the electrons in air at a pressure of i mm. and a tempera- 

 ture of 20 C. is 0.0475 cm. as found by Townsend, 3 and 

 that the temperature of the gas of the arc is 3700 C., 

 the temperature of the anode as given by Waidner and 

 Burgess. 4 The mean free path of the electrons computed 

 from these assumptions is through a potential difference 

 of 0.025 volt, which is quite different from 11.7 volts, the 

 required amount. 



In the second place the electric force through the arc 

 does not increase in inverse proportion to the length of the 

 mean free path of the ions, as it should, if the ions are 

 caused by the impact. Thus, in experiments performed 

 by myself, it was found that the electric force in the arc 

 between two solid carbons was 23.4 volts per centimeter 

 when the pressure was 730 mm., and 13.2 volts with a 

 pressure of 2 mm. The temperature had apparently 



1 Phys. Rev., 24, 93; 1907. 3 Phil. Mag., (6), I, 215; 1901. 



2 Mrs. Ayrton's "Electric Arc," p. 231. 4 Phys. Rev., 19, 250; 1904. 



