6S6 Mr. J. A. Cunningham on the 



tubes served to lead the heating current through the grating 

 without undue heating of the blue-glass joint. The grating- 

 could thus be included in the circuit of a carefully insulated 

 secondary wound on a ring transformer. The current for 

 the primary was taken from the Cambridge town (alternating) 

 supply, and was regulated by means of a rheostat. 



The opposite electrode (A) was made of a similar grating, 

 but could not be artificially heated. The three inter mellTate 

 electrodes (E, D, & B) were made of fine platinum wires 

 whose free ends inside the discharge-tube were hammered 

 out flat, and the edges then trimmed off parallel, so that the 

 width of the blade was only about double the diameter of the 

 original wire. They were fused into side tubes perpendicular 

 to the bars of the gratings, and so that the plane of the blade 

 was parallel to the axis of the discharge-tube. Professor 

 Thomson has pointed out that, especially when working at 

 low pressures, an ordinary wire placed in front of the cathode 

 would be subjected to a bombardment of negatively charged 

 corpuscles, from which it would derive a negative charge 

 and acquire a potential which might have nothing much to 

 do with the potential of the surrounding gas. Prof. Thomson's 

 scheme of using a transverse pencil of cathode particles whose 

 deflexion would measure the electric intensity at the point 

 was not easily applicable in the present case, since when the 

 cathode was at a high temperature the illumination from it 

 would render a phosphorescent spot practically invisible. It 

 was thought that the flat-bladed electrodes here adopted would 

 expose only a very thin edge to particles moving rapidly 

 along the axis of the tube, and a maximum surface to the 

 ionized gas whose potential it was desired to ascertain. 



This tube was connected by means of a short glass tube 

 with the Topler-pump, P 2 5 bulb, and McLeod gauge, so that 

 the whole system rapidly came to one uniform pressure. A 

 three-way tap served to admit fresh supplies of air, and by 

 being closed at night prevented an excessive diffusion of 

 mercury-vapour into the discharge-tube. The discharges 

 below described always presented a rich red colour without 

 any apparent traces of the blue due to mercury. 



The current from a battery of 1000 small secondary cells, 

 used for generating the discharge, passed through two 

 variable liquid resistances, and was measured by means of a 

 low resistance d'Arsonval galvanometer of the Ayrton and 

 Mather type. A telephone in series served to check the 

 steadiness of the discharge. 



A German voltmeter of simple construction (with an 

 aluminium needle suspended about a horizontal axis a little 



