364 WHITEHEAD— HIGH VOLTAGE CORONA IX AIR. [April 21, 



at the surface of the same wire when made up into a three- or 

 four-strand cable without spiral. 



Influence of Frequency and JJ'az'C Form. — By the use of a 

 cathode ray oscillograph in the high voltage circuit Ryan in 1904 

 showed that the appearance of corona was accompanied by a hump 

 or peak on the charging current wave in the neighborhood of the 

 maximum of voltage. The writer by stroboscopic methods has 

 shown that the corona is periodic, appearing every half cycle and 

 that its first appearance with rising voltage coincides accurately with 

 the maximum of the voltage wave. Also the duration of the corona, 

 with steady circuit conditions, may be reduced with lessening voltage 

 to a very small fraction of the period of the alternating electromotive 

 force. Thus a corona which was found to exist for only one 

 twentieth of a period at the crest of the voltage wave of a 60-cycle 

 circuit was plainly visible in a darkened room. It is evident, there- 

 fore, that the interval of time involved in corona formation and 

 cessation is extremely short. For these reasons it has been supposed 

 that the appearance of corona depends only on the maximum value 

 of voltage occurring in the cycle, and is therefore independent of 

 the frequency. Experience with existing lines indicates that if there 

 is an influence of frequency it is small for the range between 25 

 and 60 cycles. The closeness with which the critical voltage may 

 be read by the method described gave promise of discovering any 

 comparatively small differences due to variation of frequency. Sev- 

 eral series of tests were therefore made with different sizes of wire. 

 The observations are not recorded here as the points on the curve 

 of Fig. 5 are a sufficient indication of their accuracy. The range 

 from 15 to 90 cycles was obtained from two generators, and the 

 voltage from a lO-KW. 25-cycle 100,000-volt transformer. The 

 transformer had also a low voltage secondary coil. On the curves 

 the values of voltage are those measured at the terminals of this 

 coil ; these values are therefore proportional to the voltage in the 

 high tension winding and therefore to the electric intensity at the 

 surface of the wire. These observations were made with rods .716 

 cm. and .635 cm. in diameter placed at the center of a pipe 120 cm. 

 long and 30 cm. in diameter. The observations were taken as a 



