370 A. W. Ewell — Air in an Intense Electric Field. 



Under the alternating difference of potential between the, 

 electrodes, displacement currents traversed air and glass, and 

 when a certain difference of potential was exceeded, depending 

 upon the distance between the electrodes, the air w T as ionized 

 and a conduction current was established in the air. The dis- 

 placement currents follow well known laws and it is the cur- 

 rent transported by the ions in the air and the electromotive 

 force applied to the air which is of interest. The displace- 

 ment current in the air is a quarter period in advance of the 

 electromotive force and the ionization current is in phase 

 with the electromotive force. The current in the glass will be 

 a displacement current equal to the vectorial sum of the two 

 currents in the air, but it is most convenient to resolve it into 

 two parts, each in phase with the corresponding current in the 

 air, and to resolve similarly the electromotive force applied to 

 the glass. 



In fig. 2, let A represent the total e.m.f. applied to air 

 and glass, B the total e.m.f. applied to the glass, that 

 applied to the air. B is 

 the vectorial sum of _Z), the 

 e.m.f. required to maintain 

 in the glass the equivalent 

 of the displacement current 

 in the air, and E the e.m.f. 

 required for the equivalent 

 of the ionization current in 

 the air. A, the total applied 

 e.m.f., is observed directly, 

 B, the ' e.m.f. required to 

 maintain the total current in 



the glass, can be determined with electrodes directly against 

 the glass. Since D maintains a displacement current in the 

 glass which requires an e.m.f. G in the air, their ratio must be 

 the inverse ratio of the capacities of two plate condensers of 

 equal area, one having the given thickness of air as its dielectric 

 and the other the glass. If h is the dielectric constant of the 

 glass, dj the thickness of the glass and d 2 that of the air, 



or D = rC where r is a constant. (1) 



C kd 2 



Moreover by geometry C* + 2CD = A 2 -B 2 



m . t g = ^ A * B \ = Va'-B' (l-r + jr'j (2) 



to a sufficiently close approximation, for the e.m.f. applied to 

 the air. The magnitude of the ionization current equals the 



