414 //. A. Buvmtmd — Emission of Electrons by 



rent was duo to the electrons set free between the polonium 

 and the disc; by subtracting this from the currents obtained 

 with the foils interposed it was possible to eliminate this dis- 

 turbing portion of the negative current. What was left con- 

 sisted of the electrons from the two foils on the rings F, F', 

 and from the surfaces of the disc-foils farthest from the 

 polonium. 



Witli this apparatus, practically identical results were 

 obtained whether the case was charged positively or nega- 

 tively ; that is to say, whether the electrode emitted electrons 

 or received those given off by the other aluminium foils. In 

 the following table the results of various series of measure- 

 ments are given. In order to facilitate comparison, the cur- 

 rents are reduced to the same scale, that obtained with 2 foils 

 (the smallest number used) being taken as 100 in each case. 



Table I. 





Volts 

 on 



Number of Foils 



Date 















Case 



2 



3 



4 



5 



6 



May 23 



+ 160 



100 



107-2 



118-5 



107- 



39- 



M tl 



— 160 



100 



104- 



118-2 



107- 



54- 



May 27 



+ 80 



100 



104-5 



1 16-3 



106-8 



38*7 



a u 



— 80 



100 



104-6 



119- 



108-6 



55-5 



June 9 



+ 80 



100 



106' 



118- 



109- 



39-5 



a u 



— 80 



100 



104 5 



119- 



no- 



51-8 



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

 secondary 8-radiation is real, and is not due to the charge on 

 the a-particles themselves. The discrepancy between the posi- 

 tive and negative currents when six foils are interposed is easily 

 explained. With a negative charge many of the electrons 

 come from the foil on the disc, 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 the nearly horizontal "top" of the curve, 

 and a small difference in the range makes a considerable differ- 

 ence in the effect.* 



* An effect, probably due to tbis increased emission of electrons with 

 diminishing speed of the particles, was observed by Aschkinass (Ann. d. 

 Phys., xxvii, p. 377, 1908). He was measuring the charge of the a-particles 

 after passing through various thicknesses of aluminium, and had a trans- 

 verse magnetic field to curl up the 6 rays. The a-rays passed through an 

 aluminium window into a separate chamber and fell upon a copper plate. 

 Even without a magnetic field the plate acquired a positive charge ; when 



