102 NATURE OF THE RADIATIONS WorEr 
the screen placed below it. With no magnetic field, a faint 
luminosity of the screen is observed due to the very penetrating 
y rays which readily pass through the lead. When the magnetic 
field is put on, the screen is brightly hghted up on one side over 
an area elliptical in shape (section 71). The direction of deviation 
is reversed by reversal of the field. The broad extent of the 
illumination shows the complex nature of the 8 rays. On placing 
a metallic object at various points above the screen, the trajectory 
of the rays can readily be traced by noticing the position of the 
shadow cast upon the screen. By observing the density of the 
shadow, it can readily be seen that the rays most easily deviated 
are the least penetrating. 
Comparison of the B rays with cathode rays. 
74. Means of comparison. In order to prove the identity 
of the 8 rays from active bodies with the cathode rays produced 
in a vacuum tube, it 1s necessary to show 
(1) That the rays carry with them a negative charge ; 
(2) That they are deviated by an electric as well as by a 
magnetic field ; 
(3) That the ratio e/m is the same as for the cathode rays. 
Electric charge carried by the 8 rays. The experiments 
of Perrin and J. J. Thomson have shown that the cathode rays 
carry with them a negative charge. In addition, Lenard has shown 
that the rays still carry with them a charge after traversing thin 
layers of matter. When the rays are absorbed, they give up their 
charge to the body which absorbs them. The total amount of 
charge carried by the 8 rays from even a very active preparation 
of radium is, in general, small compared with that carried by the 
whole of the cathode rays in a vacuum tube, and can be detected 
only by very delicate methods. 
Suppose that a layer of very active radium is spread on a metal 
plate connected to earth, and that the 8 rays are absorbed by a 
parallel plate connected with an electrometer. If the rays are 
negatively charged, the top plate should receive a negative charge 
increasing with the time. On account, however, of the great 
