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



physical investigation of the nature of the process involved in the 

 electrical break down of the air. The formula indicates that the 

 value of electric intensity of a uniform field or near the surface 

 of a plane conductor, at which air would break down is 32 kilovolts 

 per centimeter. The work of von Scweidler, Townsend and others 

 indicates that at about 30 kilovolts per centimeter, secondary ioniza- 

 tion or ionization by collision sets in between parallel plates sub- 

 jected to a difference of potential. In a later paragraph several 

 other experiments are described which indicate that the start of the 

 corona in air is due to secondary ionization. So far as the writer 

 is aware however no theory has been advanced to explain the 

 influence of the curvature of the conductor. That the nature of the 

 molecular structure of the air is concerned there can be no doubt, 

 but the variation of values of critical intensity occurs within a range 

 of diameter many orders of magnitude greater than molecular 

 dimensions, and is related to the diameter in a way which offers no 

 suggestion of explanation. 



Effect of Stranding the Conductor. — It is quite obvious that if 

 the surface intensity is the determining factor in the voltage at 

 which corona occurs, then a stranded conductor should have a 

 critical voltage lower than that of a solid conductor of a diameter 

 equal to that of a circle tangent to the strands. On the other hand 

 it is not obvious that the critical voltage of a stranded conductor 

 would be less than that of a solid conductor of equivalent cross 

 section, for the diameter of the latter will always be less than 

 that of the enclosing circle of the former. Evidently also the rela- 

 tions will vary with the number of strands. The question is of 

 importance since all of the larger transmission lines consist of 

 cables or stranded conductors. 



A series of observations was made on a number of cables of 

 stranding ranging from three to nine conductors uniformly filling 

 the outer layer. The interior space was filled with a single wire or 

 several wires of suitable size, but in each conductor the wires of the 

 outer layer were all of the same size, .162 cm. diameter. The cables 

 were clean and smooth and drawn tight along the central axis of the 

 outer cylinder of the apparatus. The results are condensed in Table 

 III, in which comparison is made between the diameter of a solid 



