Metals under the Influence of Alpha Rays. 315 



which have the same retarding effect on the a-rays ; and hence 

 the emergence S-radiation from the different metals correspond 

 to different speeds of the a-rays even when the same number 

 of aluminium foils are interposed. This is plainly shown by a 

 comparison of the results for platinum with those for gold and 

 aluminium given in the previous paper. The platinum foil is 

 so much thicker than the others that the entire form of the 

 curve is distorted. Moreover, in the foregoing experiments, 

 very few points on the curve were taken, so that, indeed, it is 

 only by courtesy that it can be called a curve at all. For 

 these reasons a much more careful determination was under- 

 taken by moans of the apparatus shown in fig. 1. 



A copper plug, P, 4 mm in diameter, whose lower surface is 

 covered with polonium, is surrounded by a brass cylinder, C, 

 which limits the cone of the rays so that they fall completely 

 within the brass ring, E, which supports the sheet of metal 

 under investigation. In order to obtain more points upon the 

 ionization curves, two discs, D 1 and D 2 , are used instead of the 

 one which was used in the previous experiments. In order to 

 make the drawing clearer, the rod supporting E is shown in 

 the same plane with the axes of the discs ; in the actual appa- 

 ratus it is in the plane perpendicular to this. The discs are 

 divided into eight equal sectors. D l has a hole l-5 em in diam- 

 eter cut in each sector. One hole is left open and the others 

 are covered with 1, 2, 3, 4, 5, 6 and 7 layers of aluminium foil 

 of thickness 3*2 X 10~ 4om , having a retarding effect upon the 

 a-rays, according to Taylor's results, equivalent to that of 

 - 58 cm of air (air equivalent). The disc D 2 has one sector 

 without a hole so that the a-rays can be stopped completely ; 

 the other sectors have holes, one of which is left open, others 

 being covered with 1, 2, 3, and 4 layers of thinner aluminium 

 foil, 0-64 X 10- 4cm thick. Thus five of the thin foils are 

 equivalent to one of the thicker. The dials, S! and S 2 , outside 

 the evacuated chamber enable one to set the discs t> 1 and D 2 

 so that any combination of the thick and thin foils may be 

 interposed in the path of the a-rays, or the brass sector may 

 stop them entirely, or the two holes allow them an uninter- 

 rupted passage to the electrode. 



The metal plate attached to the ring, E, was in every case 

 chosen of sufficient thickness to absorb completely the a-rays 

 so that the 8-electrons were emitted only from the side on 

 which the a-rays were incident. The electrode is insulated 

 from the case by amber, guard-tube and ebonite, and is con- 

 nected to a sensitive gold leaf electroscope of the Hankel type.* 

 A key is connected to a potentiometer arrangement so that 



* This instrument is described in the previous paper, this Journal, xxxii, 

 405, 1911 ; Phil. Mag., xxii, 909, 1911. 



