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SECTION III., 1900 [ 85 ] Trans. R. S. C. 
IX.—Electric Screening in Vacuum Tubes. 
By J. C. McLennan, B.A., Ph.D. 
Demonstrator in Physics, University of Toronto. 
(Communicated by President Loudon and read May 29, 1900.) 
In his work entitled “ Discharge of Electricity through Gases,” 
page 170, Professor J. J. Thomson describes an effect which he observed 
in a series of experiments with Faraday cylinders inserted in vacuum 
tubes for the protection of exploring electrodes. 
In these experiments tubes similar in form to that shown in Fig. I 
were used. An exploring electrode C. was sealed into the tube and sur- 
rounded by a metal cylinder D. The end of this cylinder facing the 
bulb of the discharge tube was closed by a plate of some selected metal 
such as aluminium or brass. A small opening c served to equalize the 
air pressures inside and outside the cylinder, and an ebonite plug closed 
the lower end and acted as a support to hold the cylinder in position. 
On passing an electric discharge through these tubes no unusual 
action was observed at high pressures ; but when a pressure of about one 
millimetre of mercury was reached the gas > 
within the cylinder became conducting, and 
electric charges given to the inclosed elec- 
trodes were gradually dissipated. 
With pressures low enough to allow the 
cathode rays to fall upon the end of the 
cylinder, the protected electrode gained a 
negative charge. With still higher exhaus- 
tions the same effect was obtained, but the 
rate at which the negative charge was gained 
rapidly increased as the pressure was lowered. 
The effect was obtained when the ends 
of the cylinders were closed by plates ranging 
all the way from thin aluminium foil up to 
brass 1:5 mms. in thickness, but the rate at 
which the charge was acquired by the elec- 
trode decreased with the thickness of the 
plate selected. 
In discussing this effect Professor Thom- 
son brought forward the hypothesis that 
cathode rays were in some manner produced in the gas surrounding 
the electrode C. 



