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



While both equations are linear it is seen that the slope of that 

 for the smaller wire is the steeper, that is that the variation of the 

 critical surface intensity with the pressure is greater the smaller the 

 wire. It is interesting to note that the values at 76 cm. pressure 

 66.2 and 57 correspond extremely closely with the values 66.4 and 

 57.7 observed a year before and so calculated from the equation 

 of Fig. 2. 



If Ryan's results for a .317-cm. wire be expressed in the same 

 terms used in the above formulae, the resulting equation of the 

 line is: 



dr =6.15 + . 744 />. (5) 



The slope of this line is greater than that of either the .152-cm. or 

 the .276-cm. wire as expressed in equations (3) and (4), although 

 the larger size of wire should cause the slope to be less ; also the 

 initial constant term is considerably less ; further the value of 

 critical surface intensity at 76 cm. pressure indicated by formula 

 (5) is 62.6, while that calculated from formula (i) and therefore 

 frequently observed by the writer is 55.7. Ryan used invariably 

 the visible corona for indication of initial breakdown ; some of his 

 results on wires of different size are plotted as circles in Fig. 2 

 where they are seen to be very irregularly located. Aside from the 

 uncertainty of the method of observation, the wave form and fre- 

 quency may have introduced considerable error in the results as 

 reported, although that due to frequency would have tended to a 

 lower rather than a higher value than for 60 cycles. 



Further experiments on the variation of the pressure equation 

 with the size of wire are in progress. 



Inflncnce of Temperature and Moisture. — No satisfactory inves- 

 tigation has been made of the influence of temperature on corona 

 voltage. Ryan reports a series of observations on the visible corona 

 for temperatures between 70° and 200° Fahrenheit. The size of 

 wire is not stated. The results are admittedly wanting in accuracy, 

 but indicate a linear relation between corona voltage and tempera- 

 ture ; in fact, Ryan states that the maximum value of corona voltage 

 varies inversely as the absolute temperature. 



