230 



Mr. J. A. McClelland. 



cylinder was raised to a potential 6/7 of that to which it was raised 

 when the screen was away, and the cathode rays fell directly on the 

 opening c. This must not, however, be taken as meaning that the 

 now of charged particles into the cylinder was almost as great with 

 the screen as without it, because the potential to which b can be 

 raised depends, as we have seen, on the conductivity of the air 

 between a and b, and this with the screen in position is probably 

 much less than without it, so that the flow of charged particles in 

 the first case is much less than the numbers would indicate. 



But the experiment does show that when the cathode rays fall 

 upon a thin sheet of aluminium, we have a flow of particles away 

 from the further side of the aluminium, which is deflected bv a 

 magnet, and which carries a negative charge. 



The Lenard rays, therefore, like the cathode rays, carry a negative 

 charge. 



The negative potential to which the cylinder b is raised, depends 

 on the position of the screen in the tube, as shown in the table given 

 .above. The intensity of the Lenard rays at a will depend on the 

 distance of the screen from a, and also on the intensity of the cathode 

 rays falling upon the screen. 



When we consider the great amount of electricity carried by the 

 negative rays (as we have seen, a few breaks of the coil charged 

 300 C.Gr.S. units to 87 volts, and other experiments of a different 

 character on this point will be described later on) we can easily 

 understand this stream of negatively charged particles away from 

 the further side of the thin screen. This large negative charge 

 suddenly brought up to the screen and there stopped, must cause a 

 similar rush of negative charge away from the screen, especially as 

 the air in the tube is in a condition suitable for a discharge to pass 

 in it. 



The screen in this experiment was not kept connected to earth, 

 although in other similar experiments described below it was so con- 

 nected. JSTor does this appear to be of much consequence, because 

 even if the screen is well earthed, the sudden rushes of negative 

 charge up to it will cause variations in its potential so that it will 

 not shield the portion of the tube on its further side. 



In the experiments described above, the negatively charged 

 secondary rays are collected inside the tube ; it is more convincing to 

 arrange the apparatus so that they can be collected outside the tube, 

 as in this case it is easier to screen the collecting cylinder from out- 

 side electric forces. 



For this purpose a tube, as shown in fig. 2, was constructed. 



A circular brass cup, B, is made to fit closely on the end of the 

 glass tube in which are fixed the terminals A and C. The 

 brass cup has a small aperture e, 1 mm. in radius, in the centre 



