40 
PEOFESSOK E. EUTHERFOED AND ME. E. K. McCLUNG 
aluminium caps 1 millim. in thickness, and were made air-tight and caj)able of standing 
a pressure of 3 atmospheres. The rates of discharge due to the rays after passing 
through the tubes, were taken between two sets of parallel plates, CD and C' D'. The 
plates D and C' were of the same size and cut into three portions, of which the 
Fig. 5. 
centre plates were carefully insulated. The centre plates were thus surrounded by a 
guard ring, and the rates of discharge to the centre plates alone were measured. The 
centre plates, D and C', were connected together, and to one pair of quadrants of the 
electrometer, the other pair of quadrants being connected to earth. The plates, 
C and D', were connected to the terminals of a battery of small storage cells of 310 
volts, the middle point of winch ivas to earth. The electrometer will show no deflec¬ 
tion if the intensity of the rays between C and D is exactly equal to the intensity 
between C' and D', since the current between C and D is equal and opposite to the 
current between C' and D'. 
Lead screens, L, M and N, were placed at the positions marked in the figure, in 
order to prevent any stray radiation from reaching the testing plates. The wires 
leading to the electrometer were enclosed in metal tubes, which were connected to 
earth in order to avoid any loss of charge due to sti’ay radiation or disturbances by 
any electrostatic field. The electrometer was completely surrounded by a wire gauze 
The separation of the quadrants of the electrometer was operated from a distance by 
means of suital)le keys. Such precautions are very necessary during the very dry 
Canadian winter, when the slightest movement causes frictional electrification. The 
table and the woodwork on which the apparatus was placed was covered over with 
metal, to prevent the collection of charges either from frictional electrification or the 
action of the rays near chiirged conductors. 
