388 



WHITEHEAD— HIGH VOLTAGE CORONA IN AIR. [April 21, 



breakdown was indicated, the initial discharge of the electroscope 

 occurring at the same value for both moving and stationary air. 

 The results of Table lY. are plotted in the lower line of Fig. 6 



between the values of voltage at the primary terminals of the trans- 

 former and the pressure in millimeters of mercury. This voltage 

 is directly proportional to the corresponding value of potential 

 gradient at the surface of the wire. The ratios of transformation 

 were i to 125 and i to 250, the frequency 60, and the ratio of the 

 maximum to the effective value of the alternating wave of electro- 

 motive force, as measured from an oscillogram as already described, 

 was 1.46. The temperature was 24° C. The results for a .276-cm. 

 wire are also plotted in Fig. 6. The equations of the lines as drawn 

 in Fig. 6 have no significance since they apply to a particular com- 

 bination of wire and outer cylinder. The values of surface potential 

 gradient have therefore been calculated from the expression : 



dV E 



~di~~ R' 



(2) 



