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



basis of comparison than the other since the interior of a multi- 

 strand cable may be made up in such manner as to cause, a con- 

 siderable variation in its cross section. In fact many transmission 

 cables have centers of hemp, or other material, the entire conducting 

 section residing in the single outer layer of strands. Thus Fig. 4 

 shows that a three-strand cable has a critical voltage which is that 

 of a single wire of seven tenths the overall diameter of the cable. 

 At nine strands the equivalent single diameter is still less than .9 

 that of the cable. 



In a stranded conductor the strands are always spiralled. The 

 pitch of the spiral for the cables described above is given in Table 

 III. The spiral arrangement of the strands tends to lessen the 

 value of the electric intensity on the outer surfaces of the strands 

 since the equipotential surfaces are rendered more nearly cylindrical 

 about the axis of the cable. The values of maximum surface electric 

 intensity for cables of various numbers of strands and in which 

 there is no spiral may be computed from an expression given by 

 Jona- and due to Levi-Civita. This expression involves a hyper- 

 geometrical series whose evaluation requires some labor. As it 

 makes no allowance for the spiralling of the strands no deduction 

 may be drawn from the present observations as to the actual inten- 

 sity at which corona occurs on the stranded conductor. Values 

 deduced from the expression should, however, be of great value in 

 the study of the nature of the breakdown of the air when taken in 

 conjunction with measurements on cables without spirals. For in 

 these cases the maximum electric intensity at the outer edge of a 

 strand would obtain over a narrow circumferential distance, while 

 the same intensity reached at the surface of a single wire obtains 

 over a wdiole circumference. A comparison of corona voltages in 

 the two cases should throw light on the distances involved in the 

 process of secondary ionization and kindred phenomena. 



At the bottom of Table III. there are given the results of obser- 

 vations on a three- and a four-strand conductor in which there was 

 no spiral. The size of the strands was the same as that of the 

 foregoing cables. The strands were carefully straightened, polished, 



'Jona, Trans. Int. Elect. Congress, St. Louis. 1904, Vol. II., p. 550. 



