60 
MR. J. C. MCLENNAN on electrical conductivity in gases 
ebonite to the battery of storage cells E, these plates could be charged to any 
desired potential, high or low. As the electrodes and the walls of the receiver were 
earthed, this afforded a means of setting up in each chamber a field which could be 
readily modified. The fields themselves, moreover, were quite distinct, each disc 
serving as a screen to cut off any action arising from the other. 
Each of the chambers was provided with a projecting shoulder, which slid over a 
corresponding one on the anode surrounding the window opposite. By coating these 
joints with wax the chambers were then not only made airtight, but also were 
entirely separated from each other. 
In the apparatus used, the diameter of the chambers A and B was about 3 centims., 
and the distance between each of the electrodes and its corresponding plate a or 
h about 1'6 centims. The diameter of the narrow cylinders which admitted the rays 
to the chambers was 3 millims., and the distance between the aluminium windows 
and points corresponding to the centres of the electric fields was about 2 centims. 
Each of the electrodes was connected to an air condenser, whose capacity was 
about 600 electrostatic units. These condensers, G and H, were each made of two 
sets of parallel plates separated by small ebonite supports. The plates were made by 
coating both sides of a sheet of glass by a single sheet of tinfoil. In this way plates 
tolerably plane were obtained, and yet difficulties arising from electric absorption 
were avoided, the glass merely serving as a support for the foil plates. 
The measurements were made with a quadrant electrometer F, and the tube was 
excited by a 50-centims. spark-length induction coil, whose positive terminal, together 
with the anode of the tube, was kept to earth. This coil was provided with an 
Apps interrupter, and besides being very powerful was also very efficient. It 
