918 Prof. H. A. Bumstead on Emission of Electrons 



3, 4, 5, and 6 layers of the thicker aluminium foil whose air- 

 equivalent is '58 cm. The disk can be rotated about its axis 

 which passes through the cover-plate with a cone bearing, 

 made tight by rubber stopcock grease. A dial, D', enables 

 one to set the disk so that the different layers of foil are 

 interposed, or the rays stopped altogether by the blank 

 sector. The electrode is insulated from the case by amber, 

 guard-tube, and ebonite, and the joints made tight by sealing- 

 wax; it was connected with a Dolezalek electrometer which, 

 with 80 volts on the needle, had a sensitiveness of about 

 650 mm. divisions per volt. It was necessary to reduce 

 somewhat the current-sensitiveness of the instrument; for 

 this purpose a small mica condenser of about 150 cm. capacity 

 was put in parallel with the electrometer. 



With a positive potential on the case, the saturation 

 current was reached with 40 volts ; and when the brass sector 

 was interposed in the path of the rays the current fell to a 

 negligible value. On the other hand, it was impossible to 

 approach saturation with a negative potential on the case, 

 even at —320 volts; the negative current when the rays were 

 stopped by the disk was alwaj^s large (20-40 per cent, of the 

 total) and increased with the negative potential on the case. 

 This part of the current was due to the electrons set free 

 between the polonium and the disk; by subtracting this from 

 the currents obtained -with the foils interposed it was possible 

 to eliminate this disturbing portion of the negative current. 

 What was left consisted of the electrons from the two foils 

 on the rings F, F', and from the surfaces of the disk-foils 

 furthest from the polonium. 



With this apparatus, practically identical results were 

 obtained whether the case was charged positively or 

 negatively ; that is to say, 1 whether the electrode emitted 

 electrons or received those given off by the other aluminium 

 foils. In the following table (p. 919), the results of various 

 series of measurements are given. In order to facilitate com- 

 parison the currents are reduced to the same scale, that ob- 

 tained with 2 foils (the smallest number used) being taken 

 as 100 in each case. 



These results leave no doubt, I think, that the increase in 

 the secondary S-radiation is real, and is not due to the charge 

 on the a-particles themselves. The descrepancy between 

 the positive and negative currents when six foils are inter- 

 posed is easily explained. With a negative charge many 

 of the elections come from the foil on the disk D ; with 

 a positive charge they all come from the electrode E, and 

 to reach this the a-rays must penetrate the foil F (air- 

 equivalent = 0*116 cm.). With 6 foils interposed we are in 



