346 
Mr Crowther, On the Fatigue of Metals 
any alteration in the amount of secondary radiation given off by 
the substance under the action of these rays. 
Section 2. 
During the progress of the above experiments Prof. L. T. 
More, working in the Cavendish Laboratory, published a paper* 
in which he described some interesting cases of fatigue in the 
secondary radiation due to Rontgen rays, under the prolonged 
action of the rays. He shewed that for metal plates which had 
been allowed to age in air for some months after polishing, an 
exposure to X-rays for from one to three hours, caused a diminu- 
tion in the secondary radiation which varied with the particular 
metal employed, but which was usually of about from 5°/ 0 to 10°/ o of 
its original value. With freshly polished surfaces there was in 
general an increase in the secondary radiation of a similar 
amount, if the plates were exposed to the continuous action of 
the rays. 
It was considered of some interest to see if an exposure to 
radium produced a similar change on the secondary radiation set 
up by X-rays, as a prolonged exposure to the latter rays. 
I was fortunately able to secure Prof. More’s apparatus, which 
therefore served for the purposes of the following experiments 
also. The apparatus is fully described in Prof. More’s paper. It 
is represented diagrammatically in figure 3. A, A' are two brass 
boxes, which serve as ionization chambers. Two plates, p 0 , p ly each 
4 cms. in diameter and 2 mm. thick are made of the metal to 
be experimented upon, and placed in A, A' respectively, and 
connected by rods which pass out of the chambers through 
sulphur plugs, to two Wilson electroscopes, fitted up in the usual 
way for the measurement of ionization currents. The X-rays 
enter the boxes through circular openings &>, w, and the ionization 
is measured between the plates p 0 , p 1 respectively, and parallel 
wire gauze electrodes e, e, kept charged to a high potential by 
means of a cabinet of small storage cells. Each plate was con- 
nected to a separate electroscope, in order that the leak in the 
two chambers might be observed simultaneously. This was the 
only alteration made in the apparatus. 
The plate g>o served as a standard, and was not exposed to the 
fatiguing agent ; p 1 was the test plate. The experiments were 
conducted as follows. The plates p 0 , p 1 were placed in position 
as shewn in the diagram, and the ionization produced by the 
X-rays in the two chambers was measured. This ionization was 
shewn by Prof. More to be almost entirely due to the secondary 
radiation from the metal plate, that due to the primary rays 
* Phil. Mag. June 1907, p. 708. 
