[M‘LENNAN ] ELECTRIC SCREENING IN VACUUM TUBES 93 
This effect clearly pointed to the existence of cathode rays within 
the cylinder. 
To verify this conclusion the induction coil was suddenly reversed 
while the electrode C was gaining its charge. This action at once 
stopped the effect and the electrometer needle came to rest and re- 
mained stationary. On again reversing the coil the charging immediate- 
ly recommenced. 
An electromagnet was then placed in position with the bulb of the 
tube between its pole pieces. On exciting this magnet the cathode rays 
coming from A were deflected and again the charging ceased and the 
electrode C maintained its charge. 
When the electromagnet was cut off the cathode rays once more 
impinged upon the disc d, and the charging at once began again. 
These experiments then showed that the charge gained by C was 
due to the cathode rays produced within the cylinder by the action of 
the pencil impinging on the outside. 
From a consideration of the results obtained with all the tubes, | 
it thus seems evident that a closed Faraday cylinder, placed within a 
vacuum tube in which a discharge is passing, will completely screen 
off all electrical action unless made of metal thir enough to permit the 
passage of cathode rays through it. 
4, INFLUENCE OF BAD EARTH CONNECTIONS. 
In the experiments described in Section III, special care was taken 
with the earth connections. These were made by soldering an iron wire 
to a water main and then inserting the end of the wire in the mercury 
in the vessel E. 
To test the influence of bad earth connections the cylinder was first 
capped at d with a thick dise to cut off the cathode rays, and then a 
wet string or a column of xylol was placed in the earth circuit between 
the iron wire and the mercury. | 
Under these conditions the passage of the discharge in the tube 
was almost invariably accompanied by marked induction effects. 
With very high exhaustions for example, À and B being the ter- 
minals and neither connected to earth, there was no inductive action 
when the connection was made by the iron wire or the wet string, but 
when the column of xylol was inserted in circuit the electrometer 
showed a strong positive induction. This action, however, was not ac- 
companied by any permanent charging as the insertion of the iron wire 
in the mercury was sufficient to at once bring the electrometer needle 
to rest in its initial position. 
