[RUTHERFORD] EXISTENCE OF BODIES SMALLER THAN ATOMS 81 
to the conclusion that, if the rays were projected material particles, 
they must be so small that they were able to pass through the inter- 
stices of matter, or in other words, that to these particles matter 
behaved like a coarse sieve. 
In 1895 Perrin showed that the rays carried with them a negative 
charge. This was strong evidence in support of Crookes’ hypothesis. 
The discovery of Rontgen rays gave a great stimulus to the 
investigation of the discharge in vacuum tubes. It was found that 
Rontgen rays were able to produce charged carriers or ions from the 
gas, through which they passed, and this made it probable that car- 
riers of a similar kind existed in a vacuum tube. 
The experiments of J. J. Thomson threw a great deal of addi- 
tional light on the nature of the cathode rays and laid the foundation 
of all future work on that subject. He found that the rays were 
negatively charged particles travelling with enormous velocities and 
were probably of dimensions small compared with a molecule. 
These results were deduced from experiment in the following 
way: 
Let e = charge on the cathode ray particle, 
m == mass of the particle, 
v = velocity of the particle, 
n — number of particles in a single discharge. 
The mechanical energy of a single particle is 4 m v? and this is 
mainly transformed into heat in the impact on a metal surface. The 
energy W in a single discharge was measured by observing the rise 
of temperature when the rays fell on a specially constructed thermo- 
pile. We therefore have W — 4 mn v’.. The amount of electricity 
Q carried by a single discharge was measured by an electrometer and 
was given by Q — ne. 
Dividing the second equation by the first, we obtain 
e QE 

OA 2A 
Another relation was obtained between e, m, and v by observing 
the curvature 9 of the path of the rays when a uniform magnetic field 
H was applied perpendicular to the direction of the rays. A charge 
e, moving with a velocity v, acts as a current of strength e v. The 
force exerted on the particle by the magnetic field is H e v and is 
mutually perpendicular to the direction of the field and the rays. 
The rays are made to trace out a curved path under the action of 
this force. The force which causes the body to move in a path of 
