emitted by Zinc bombarded by Alpha Rays, 497 



the alpha rays back again into that plate. This experiment, 

 therefore, showed that a field of 1000 gauss was sufficient 

 when the applied potential difference was 80 volts to entirely 

 cutoff the stream of electrons. The problem before us, then, 

 was to apply the procedure just described to the investi- 

 gation of the intensity of the electronic stream from the zinc 

 plate X when the surface of this plate was made to undergo 

 various modifications. 



Before leaving this experiment it may be pointed out that 

 the results obtained go to show that approximately three 

 electrons were emitted by the bombarded zinc plate for every 

 alpha particle which struck it. 



From the table it will be seen that the current under the 

 electric field, combined with the maximum magnetic field, 

 was approximately 44 X 10~ 5 e.s.u. This current consisted 

 of (1) alpha particles, (2) recoil atoms, and (3) the ionization 

 current. As the gas-pressure in the apparatus was ex- 

 ceedingly low the ionization current must have been negli- 

 gible. Taking it to be so the current must have been 

 carried by the alpha particles and the recoil atoms. If, 

 now, we assume that as many alpha particles were shot 

 back into the polonium plate as were projected forward from 

 it, it follows that the number of recoil atoms taking part in 

 the current was very closely equal to the number of alpha 

 particles which contributed to it. Taking the charge on the 

 alpha particles to be 2e, and that on the recoil atom to be e, 

 we have then, since the current carried by the electrons 

 emitted by the zinc plate must have been 40 x 10" 5 e.s.u., 

 the number of alpha particles striking X given by 

 44 x 10~ 5 /3^. Since the number of electrons emitted by the 

 zinc plate was 40 x 10~ r> /^ it follows that2'73 electrons were 

 emitted by the zinc plate per alpha particle which struck it. 



Experiment III. — The next experiment which was per- 

 formed served to illustrate the fatigue of the delta-ray effect. 

 In this case the apparatus was continuously evacuated for 

 two days after the measurements made in experiments I. and 

 II. were taken. At the end of this time readings were 

 taken by applying various positive potentials up to 80 volts, 

 and when this was reached the magnetic field was turned on 

 and readings were taken with fields up to 1400 gauss. These 

 are all recorded in Table III., and are shown graphically in 

 fig. 4 together with the results of experiments I. and II. 

 From the results it will be seen that while the maximum 

 current under 80 volts was 84 x 10~ 5 e.s.u. at the beginning 

 of the experiment, it was only 63xl0~ 5 e.s.u. after two 



Phil. Mag. S. 6. Vol. 30. No. 178. Oct. 1915. 2 K 



