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



Art. XXXIY. — The Conductivity of Air in an Intense 

 Electric Field and the Siemens Ozone Generator; by 

 Arthur W. Ewell. 



In connection with a study of the electrical production of 

 ozone, the writer desired information regarding the electrical 

 conductivity of air, self-ionized in a strong electric field. Since 

 little qualitative and no quantitative data could be found, the 

 writer investigated the subject and obtained the results here 

 presented. 



For quantitative measurements the ionization should be 

 uniform over a considerable cross section.- Parallel electrodes 

 of relatively large area give the most uniform field, yet at una- 

 voidable minute projections on the electrodes the electric force 

 will be excessive and the air will be considerably ionized 

 before it is elsewhere. Owing to the increased conductivity 

 of this air, the electric force increases in the remainder of the 

 distance to the other electrode and a narrow discharge or 

 spark occurs before the potential is sufficient to uniformly 

 ionize the air. 



If an alternating electromotive force is employed a plate 

 of dielectric such as glass may be interposed in the air and any 

 current in the air will be transmitted through the glass as a 

 displacement current. There will still be excessive ionization 

 in the immediate vicinity of any minute projections on the 

 electrodes, but the dielectric prevents the appreciable extension 

 of this ionization as a spark, and when the electric force is 

 sufficient, the entire body of air is very uniformly ionized. 



The ionization current, when large, is accompanied by a 

 loud noise and the air between the electrodes is of a very 

 uniform purple color. 



The apparatus used is illustrated in fig. 1. The electrodes 

 A and B, of tinned copper, and the glass plate (7, constitute 

 essentially the simplest type of Siemens ozone generator.* F 

 and F l are equal, non-inductive resistances. E is a Thomson 

 electrostatic voltmeter. H is one of Prof. Harold B. Smith's 

 high potential transformers with a maximum ratio of trans- 

 formation of 1000:1 and a capacity of 100 kw. One side of 

 the transformer was grounded and the potential of the other 

 side was obtained from the primary voltage, read at G. From 

 readings of the actual secondary voltage with a Braun electro- 

 meter, E, for secondary 'currents of various magnitudes and 

 phases, the ratio of transformation was found to be accurately 

 500 : 1, the ratio for which the primary was connected. 



The discharge heated air, glass, and electrodes. Since the 

 dielectric constant of the glass increases with rise of tempera- 

 ture and since it was essential that the temperature of the air 

 *See article by the writer in Phys. Rev., p. 244, April, 1906. 



