191 1-] WHITEHEAD— HIGH VOLTAGE CORONA IN AIR. 393 



states that these new ions are formed by the impact of those already- 

 existing, and moving with higher velocity in the increased electric 

 field, with the neutral molecules of the gas. This phenomenon has 

 been called ionization by collision or secondary ionization. 



The results of the experiments which have been described above 

 are for the most part consistent with the ionization theory. The 

 various circumstances surrounding the appearance of corona all indi- 

 cate that it is an instance of secondary ionization. Formula (i) 

 indicates that near a conductor of large radius or near a plane, the 

 corona intensity approaches a value t,2 kilovolts per centimeter ; 

 secondary ionization between plane electrodes in closed vessels at 

 atmospheric pressure has been noticed by several physicists to begin 

 in the neighborhood of 30,ocx) volts per centimeter. The mass of 

 elementary negative ion or electron is approximately 5.9 X lO"-* gms. 

 and the charge it carries is 4.6 X lO'^" electrostatic units. In an 

 electric field the mechanical force acting on the electron is the 

 product of its charge and the strength of field. Hence by the laws 

 of simple mechanics it is possible to calculate the acceleration, the 

 velocity and the kinetic energy attained by an electron in moving a 

 given distance under a given electric intensity. If the mean free 

 path of the electron, about 6 X lO"' cm. at atmospheric pressure, be 

 the distance between collisions, it is thus easy to calculate the kinetic 

 energy of the electron due to the electric field, when it collides with 

 a molecule. This energy is readily seen to be equal to pVc, where p 

 is the mean free path, V the electric intensity in electrostatic units, 

 and e the charge of the electron. If now the voltage between plane 

 parallel electrodes be raised until secondary ionization begins, the 

 value of the voltage makes it possible to calculate the energy re- 

 quired to ionize a molecule of a gas. In fact the values of the 

 energy required to ionize a molecule which are now generally ac- 

 cepted are largely based on determinations of the value of electric 

 intensity at which secondary ionization begins. It has been pointed 

 out above that the values of this intensity as determined by Town- 

 send and others are in close agreement with the value 32,000 volts 

 per centimeter indicated by equation ( i ) as the lowest value at which 

 corona appears. To one skeptical as to the correctness of the theory 

 of ionization therefore (and there are many such) all that may be 



