WHITEHEAD— HIGH VOLTAGE CORONA IN AIR. 253 



curately the law of corona formation, it is necessary to be able to 

 measure accurately the value of voltage applied to its terminals and 

 to determine accurately the first appearance of corona. The instru- 

 ment, as indicated in Fig. 4, is in fact the result of a long series of 

 experimental studies of corona formation and the gradual develop- 

 ment of methods of controlling and determining all the factors 

 which enter. It has been known from the beginning of these studies 

 that corona forming voltage gradient depends on the diameter of 

 the wire and on the density of the air, no other factors entering. 

 Moisture content of the air, for example, has no influence on corona 

 forming intensity. A possible exception is the frequency of the 

 alternating voltage which appears to have a very small influence, 

 too small, however, to be of importance within the commercial range 

 of frequency. The instrument therefore provides means for ob- 

 serving the pressure and temperature at P and /, and also means 

 not indicated in Fig. 4 for removing the central rod A so that 

 another of different diameter may be substituted. 



The method of measuring the applied voltage is also indicated in 

 Fig. 4. It consists of connecting an air condenser M, of known 

 capacity, in parallel with the conora voltmeter and of measuring 

 the charging current of this capacity. This charging current is a 

 direct measure of the maximum value of the alternating voltage in 

 terms of the capacity of the condenser and the frequency of the 

 generator O. 



The alternating charging current is connected to earth by the 

 divided circuit R^K^ and R.K.^. R^ and R.. are noninductive resist- 

 ances and i^i and K^ are rectifying Fleming valves passing the posi- 

 tive and negative half waves respectively. R-^ therefore carries a 

 pulsating unidirectional current, the average value of which may be 

 measured on the calibrated d'Arsonval galvanometer G-^. The im- 

 pedence of the RJ'^i — R2R2 circuit is negligible compared with 

 that of the condenser M. In order to withstand the high voltages 

 used, and that it might have no loss, an air condenser was used at 

 M. The condenser was of cylindrical type with flaring guardrings, 

 as shown in the photograph. Fig. 6. The cylinders were made 

 from cast iron water pipes, the diameters of the inner and outer 



