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



and the corrective factor for the maximum vahie is obtained from 

 the shape of the wave. 



Many of the inconsistencies among the measurements on existing 

 transmission hues and those made in laboratories arise in the difficul- 

 ties of measuring the power in high voltage circuits ; the instruments 

 must be placed in the low voltage side of transforming apparatus, the 

 losses in which, being generally greater than those to be measured, 

 introduct a troublesome source of error. The appearance of the 

 visible corona has been used by laboratory workers as an indication 

 of the beginning of loss through the air. With proper precautions 

 this method may be very reliable but its use is generally attended by 

 danger of subjective and other error. As a result of the discrepan- 

 cies among these approximate determinations of various investiga- 

 tors, there has appeared much speculative suggestion of the presence 

 of other unrecognized influences, as for example the moisture con- 

 tent of the air, the presence of " free " or natural ionization, an ab- 

 normal property of air when near a small wire, etc. 



The present problem therefore resolves itself into two parts: 

 first, a satisfactory method for the determination of the law under 

 which the air in the neighborhood of a long straight and usually 

 cylindrical conductor breaks down under electric strain ; and second, 

 the law governing the amount of loss when the voltage is carried 

 above the critical value. A year ago the writer^ described a method 

 by which it is possible to determine the voltage at which the air 

 breaks down near a round wire to a maximum inaccuracy of a few 

 tenths of one per cent. The original paper may be consulted for 

 the details, but the principle is simple and may be described briefly. 

 The wire is stretched along the axis of a metal cylinder and the 

 voltage is applied between them. Air may be passed through the 

 cylinder by means of two lateral tubes near the ends, the walls of 

 the cylinder at these points being drilled with a number of small 

 holes. Close to one set of these holes and outside the cylinder a wire 

 mesh electrode connected to a sensitive electroscope is placed. As 

 soon as the air around the wire breaks down under increasing volt- 

 age, copious ionization sets in which causes a rapid leak from the 



'J. B. Whitcliead, Proc. A.I.E.E., p. 1059, July, 1910. 



