I9II.] WHITEHEAD— HIGH VOLTAGE COROXA IN AIR. 387 



gradient at which secondary ionization sets in when electricity is. 

 passing through a gas is directly proportional to the pressure. Wat- 

 son^ investigated the spark length between spheres up to fifteen 

 atmospheres and found that the spark potential increases with the 

 pressure in an approximately linear relation. From the general 

 similarity between the corona and the brush form of spark dis- 

 charge, therefore, a linear relation between pressure and critical 

 surface intensity, or the potential gradient at which corona begins 

 is to be expected. Apparently the only study of the influence of 

 pressure on the formation of the alternating corona is a single set 

 of observations by Ryan' on a wire .t,2 cm. in diameter placed at the 

 center of a cylinder 22.2 cm. in diameter. He observed the alter- 

 nating voltage at which the visible corona appeared for the range 

 of pressure between 45 and 90 cm. of mercury; the alternating 

 frequency was 130. The resulting linear relation is given as between 

 the kilovolts K actually applied and the pressure in inches of mer- 

 cury, i^ = 2.93 + -902 b. 



In Table IV. are given the results of a typical series of observa- 

 tions on the influence of pressure on corona voltage; the values are 

 those for a wire .152 cm. in diameter. The wires were clean and 

 straight and centered accurately on the axis of the outer cylinder 

 of the apparatus which has been briefly described. This cylinder 

 has a diameter of 9.52 cm. The ends were closed with ebonite 

 caps of the same diameter and 5 cm. deep. The side tubes were 

 also closed by caps, and the leading-in wire to the discharge electrode 

 passed through a column of sulphur supported in hard rubber; no 

 troubles with either insulation or air leak were encountered with this 

 arrangement. All joints were sealed with a mixture of bees wax 

 and resin and pressures between 30 and 100 cm. of mercury were 

 reached without trouble. The discharge electrode was placed inside 

 the upper side tube and within one or two millimeters of the grating 

 formed by the holes drilled in the outer cylinder ; in the earlier work 

 it was found that a flow of air from the cylinder over the electrode 

 contributed little to the sharpness with which the condition of 



* Watson, Electrician, 62, 851, 1909. 

 'Ryan, Proc. A.I.E.E., XXIIL, loi, 1904. 



