A. W. JEwell — Air in an Intense Electric Field. 371 



total current less the air displacement current, and it is the 

 geometrical difference or the square root of the difference of 

 the squares, since the ionizations and displacement currents 

 differ in phase a quarter period. The displacement current 

 obviously equals, 



W.9-10" ° (3) 



where n is the frequency (= 60), A the area of the electrodes, 

 d the thickness of air, and C, as before, the e.m.f. applied to 

 the air. 



The following observations of a typical experiment will 

 illustrate the preceding paragraphs : 



July 5. Fair, Dry. Bar. = 75 -2, Temp, of water = 22°-5. 



Thickness of glass = -59 cm , of air = -43 cm .* 



Primary e.m.f. = 58*6f .'. e.m.f. applied to electrodes (=A) 

 = 29300. f 



Non-inductive resistance = 40000. Electrostatic voltmeter = 

 187f .*• Total current = 4*67 milliamperesf. 



From the curve, of the observations of May 1 (see below) an 

 e.m.f. of 23300 (= B) was required to maintain a current of 

 4*67 milliamperes in the glass. The effective area of the cen- 

 tral portion was 404 cm2 ,;j; and using the above values of the 

 thickness and the frequency, k comes out 8*4, in electrostatic 

 units, and r = '16. By formula (2) the e.m.f. applied to the 

 air = 15900 and correcting as described above for the displace- 

 ment current, the ionization current in the air 4*15 milliamperes. 



Observations. — Glass = Plate glass, 70 cm X 80 cm , -59 cm thick. 

 May 1. Sultry. Bar. = 743, Temp. = 20°. 

 Electrodes of tin foil pasted on glass, areas and guard ring 

 same as for copper electrodes described above. 



E.m.f. in volts, current in milliamperes. 

 E.m.f. 30700 28500 26500 24200 22200 20000 18000 



Current. 6*20 570 5*30 4 90 4'45 3'97 3'50 



E.m.f. 12200 10000 7000 5000 15300 



Current. 3'00 245 1*90 1-38 l'OO 



July 5. Dry. Bar. = 75-2, Temp. = 22°-5. 



Tinned copper electrodes as described above. Thickness of 

 air = -43. 



32000 29300 26500 24300 21500 

 5-40 4-67 4*20 3"45 2'90 

 16000 14000 13000 11300 40200 

 1-73 1-08 '77 '48 7-40 



* Total thickness of air on both sides of glass. The current was not 

 appreciably affected if the glass was not exactly midway between the elec- 

 trodes. 



f Effective, throughout. 



% Maxwell, Elect, and Mag., § 228. 



E.m.f. 



37000 



34700 



Current. 



660 



6-00 



E.m.f. 



19000 



17500 



Current. 



2-30 



2-20 



