\ 
[M‘LENNAN ] ELECTRIC SCREENING IN VACUUM TUBES 91 
tained without loss during the passage of the discharge in the upper 
chamber. 
Now it is known from Lenard’s! experiments that when cathode 
rays impinge upon one face of an aluminium dise ‘004 millimetres in 
thickness a pencil of similar rays issues from the opposite face of the 
disc. In the experiments just cited one would then have expected at 
low pressures, especially when A was the negative terminal, that some 
indication would have been given by the electrometer of the presence 
of Lenard rays within the cylinder. 
But the lowest pressures attainable with this form of tube only 
permitted the cathode dark space to extend about one half the distance 
across the bulb from the disc A to the end of the cylinder. The cathode 
rays could, therefore, not reach the disc d; no cathode rays could then 
be produced within the cylinder, and, as a consequence, no electrical con- 
ductivity be impre sed upon the ga: surrounding the exploring electro 
While this form of tube was thus of great service in locating the 
cause of the effect observed by Professor Thomson it was not suitable for 
the production of the lowest pressures on account of the large amount of 
metal which it contained. 
A third form of tube was then 
devised in order to examine the 
screening action of Faraday cylin- 
ders at very low pressures. 
3. PENETRATING POWER OF 
CATHODE RAYS. 
In this tube, Fig. IV., the 
electric screening was partly 
brought about by the use of mer- 
cury. The tube itself consisted 
of two parts, which were united 
by a carefully ground joint, and 
the cylinder D was made from 
a steel tube held in position by 
fitting tightly to a plug of ebon- 
ite b. This plug also fitted tight- 
ly to a central glass tube, and was 
pierced, as shown in the figure, by 
a small hole which permitted the 
pressures inside and outside the 
cylinder to be equalized when the 
mercury was lowered. 







1 Lenard Wied., Aun., p. 51, 225, 1894. 
