191 1.] WHITEHEAD— HIGH VOLTAGE CORONA IN AIR. 395 



writers to explain the discrepancies, among different observers, in 

 the voltage at which the corona starts. The foregoing facts seem 

 fairly conclusive that this supposition is not correct. In order, 

 however, to further remove doubt on this point a simple experiment 

 was performed in which the air surrounding the conductor was 

 ionized from an independent source. A clean polished wire 15 cm. 

 in diameter was stretched vertically along the axis of a cylinder 17.5 

 cm. in diameter and about 120 cm. long, made of woven wire with a 

 I cm. mesh. The high voltage was applied between them, the wire 

 cylinder being also connected to ground. A large Rontgen ray tube 

 was enclosed in a light-tight box and placed close to the cylinder. 

 When this tube was excited a crude electroscope placed 20 or 30 

 cm. on the other side of the cylinder was immediately discharged 

 showing that the air of the neighborhood was strongly ionized. In 

 the darkened room the starting of the visible corona on the wire 

 could be located readily and the corresponding voltage determined 

 by successive trials within an error of two or three tenths of one 

 per cent. By the use of independent observers it was established 

 without doubt that the presence of the Rontgen ray tube caused 

 no variation in the value of voltage at which the corona starts. 



The general influence of a decrease in pressure or an increase in 

 temperature toward a lower critical voltage is quite consistent with 

 the ionization theory. For under the kinetic theory of gases the 

 free paths of the vibrating molecules and ions are lengthened in 

 these two conditions. During the free path or interval between 

 collisions the ions are acted on by the electric force, and the longer 

 the interval the greater the velocity acquired and the more kinetic 

 energy and ionizing power. Hence a given amount of energy 

 will be acquired at a lower voltage if the free path is lengthened. 



The lowering of the critical voltage by an increase in frequency 

 is not to be explained so simply. However if within the molecule or 

 atom there are a number of electrons in motion or free to move, 

 and there is some indirect evidence to this eft'ect, it is evident that 

 the forced vibrations set up by an external alternating field will, 

 with the increasing frequency of these vibrations, cause the mutual 

 attractions within the structure of the atom to become less and less 

 strong, and therefore more liable to be broken when in collision 



