Electrons by Metals under Influence of Alpha Rays. 46!* 



metal under investigation. In order to obtain more points 

 upon the ionization curves, two disks, D L 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 disks ; in the 

 actual apparatus it is in the plane perpendicular to this. The 

 disks are divided into eight equal sectors. D 1 has a hole 

 1*5 cm. in diameter 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~ 4 cm., having 

 a retarding effect upon the a-rays, according to Taylor's 

 results, equivalent to that of 0*58 cm. of air (air equivalent). 

 The disk D 2 has one sector without a hole so that the «-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, 0'64xl0~ 4 cm. thick. 

 Thus five of the thin foils are equivalent to one of the thicker. 

 The dials, Si and S 2 , outside the evacuated chamber enable 

 one to set the disks Di 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 uninterrupted 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 S-electrons were emitted only from the side on 

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

 from the case by ebonite earthed guard-tube and amber, and 

 is connected to a sensitive gold-leaf electroscope of the Hankel 

 type *. A key is connected to a potentiometer arrangement 

 so that the leaf can be insulated, grounded, or charged to 

 any desired potential ; the volt-sensitiveness was thus taken 

 after each reading. In the following measurements, the 

 sensitiveness was adjusted to give a deflexion of about 

 25 divisions on the scale of the microscope for 0*2 volt. The 

 tube T is connected to pump, gauge, and charcoal bulb. 



When the a-rays fall on the electrode E, they carry over 

 to it their positive charges; the 8-rays which they excite are 

 negatively charged and these, leaving the electrode, add to 

 its positive charge. In order to insure the removal of all the 

 emitted electrons from the electrode, and to prevent the 

 8- rays emitted by other parts of the apparatus from reaching 

 it, a positive potential of 40 volts is applied to the case. If 

 a sufficient magnetic field is applied, with its lines of force 

 parallel to the electrode, the electrons emitted will be turned 



* This instrument is described in the previous paper, Am. Jour. 

 Sci. xxxii. p. 405 (1911) ; Phil. Mag. xxii. p. 909 (1911). 



Phil Mag. S. 6. Vol. 24. No. 142. Oct. 1912. 2 I 



