THEORY OF THE ELECTRIC ARC 159 



negative ion must pass through 25 volts in order to ionize 

 in air, and the positive ion must pass through 70 volts. 1 

 Davis 2 gives 11.7 volts as the value needed to ionize with 

 the negative ions. 



The ions can also ionize when they hit the molecules of 

 a solid, as when the negative ions hit the surface of the 

 anode, or the positive ones hit that of the cathode. The 

 potential difference needed in this case is greater than that 

 needed with the molecules of a gas. Thus when the posi- 

 tive ions hit a cold cathode, they need to pass through at 

 least 300 volts in order to produce electrons. The poten-, 

 tial difference needed by the negative ions is not so well 

 known, but it is probably about the same as the anode 

 drop of the unstriated discharge in a vacuum tube which 

 is approximately 20 volts. 3 



lonization by Hot Solids. It has long been known that 

 substances at a high temperature discharge electrified 

 bodies, and in recent years it has been shown that the rate 

 of such discharge may be very large. This was shown by 

 Richardson in igo3 4 when a hot wire which was charged 

 negatively was placed in a vacuum. A little later Wehnelt 5 

 found that the discharge from some of the oxides, as, for 

 example, calcium oxide, was very much greater than that 

 from a metal. 



It was believed by Richardson that this discharge was 

 caused by electrons coming from within the metal and 

 passing through its surface because of the great velocity 

 which they have at high temperatures, and he derived the 



1 Phil. Mag., (6), 5, 395, and (6), 6, 613; 1903. 



2 Phys. Rev., 24, 102; 1907. 



3 Wied. Ann., 68, 752; 1899. 



4 Phil. Trans., 201 A, 497; 1903. 

 6 Ann. d. Phys., 14, 439; 1904. 



