544 
Prof. Thomson , On the velocity of 
Potential difference 
in volts in 
discharge tube 
Potential difference 
required to stop 
the secondary rays 
150 
110 
90 
85 
70 
50 
42 
35 
32 
27 
42 
41 
40 
44 
40 
44 
42 
42 
39 
40 
We see from this that the energy of the secondary rays is 
independent of the energy of the primary rays. To test this 
point still further the lime covered cathode was replaced by an 
aluminium cathode, and the primary cathode rays produced by the 
discharge of an induction coil. The potential difference in this case 
being many thousand volts, the experiments were tried at several 
pressures: when the pressure was so low that the walls of the tube 
were covered with green phosphorescence, the voltage required to 
stop the secondary rays was 41 volts ; when the pressure was 
higher so that the dark space round the cathode was about 1 cm., 
the corresponding voltage was 43 ; when the pressure was higher 
and the dark space 4 mm., the corresponding voltage was 42. 
Thus the energy of the secondary rays produced by the high 
potential discharge of an induction coil is the same as that of the 
rays produced by the low potential discharge from a lime cathode. 
If we take 40 volts as the measure of the voltage required to stop 
the secondary rays, the velocity with which these are projected is 
3'7 x 10 8 cm./sec. Fiichtbauer ( Phys . Zeits. vn. p. 748, 1906) got 
3‘3 x 10 8 cm./sec. as the velocity of secondary cathode rays emitted 
when Canal-Strahleu or rapid cathode rays fall on a metal plate. 
An inspection of the results given above shows that in some cases 
the velocity of the secondary rays is greater than that of the 
primary rays producing them. Thus when the potential difference 
for the primary rays was only 27 volts, it required a potential 
difference of 40 volts to stop the secondary rays. 
This result seems strongly to support the view that the energy 
in the secondary rays is determined by the atoms emitting them 
and not by the energy of the primary rays. It is a very remarkable 
fact that the velocity with which these secondary rays are emitted 
does not seem to vary much with the nature of the atoms emitting 
them ; thus Fiichtbauer ( loc . cit.) found that the velocity of the 
secondary rays from metals is 33 x 10 8 cm./sec., while we have 
found that the velocity of those emitted by gases (air, hydrogen, 
and carbonic acid were tried and no appreciable difference found 
