326 Bumstead and McGougcm — Emission of Electrons by 



this very absorbable radiation. Wertenstein found that this 

 radiation could he considerably increased by depositing the 

 Ra C upon a platinum plate, and thus avoiding a film of oxide 

 which formed on other metals under the action of the radium 

 emanation. Another cause for the difference is probably to be 

 found in the fact that his experiments were made at pressures 

 of from 3 to 10'""' instead of in a high vacuum ; in the latter 

 case, fewer of the recoil atoms are charged and their ionizing 

 effect might be expected to be less. 



On the other hand, the ionizing effect of the secondary rays 

 in his experiments was small, amounting only to about 1/5 of 

 the a-ray effect, whereas our results show that it causes the 

 emission of 1-12 times as many S-electrons as the a-ravs. It is 

 true that these results depend upon the dimensions of the 

 cylinders which are used to limit the pencil of a-rays, but it 

 seems scarcely possible that so large a discrepancy can be 

 attributed to this cause. It may be that the secondary rays are 

 more efficient in causing the liberation of ^-electrons from the 

 metals than in ionizing a gas ; it may be also that the trans- 

 verse magnetic field used by Wertenstein to deflect these rays 

 from his ionization chamber was not altogether effective in the 

 presence of air which was necessary for ionization measure- 

 ments. 



The values of s in the second column of Table IV give some 

 idea of the distribution of velocities among the secondary 

 electrons, but no numerical results can be conveniently deduced 

 from them. The number of secondary (in this case tertiary) 

 electrons emitted by a metal when struck by a moving elec- 

 tron changes rapidly with the speed of the moving electron 

 especially within the range of velocities with which we have 

 to do in this case.* It will probably not be difficult, however, 

 to determine the velocity distribution by using a Faraday 

 cylinder as the recipient of the rays. 



It is evident that the range of velocities extends from less 



cm 



than 3 - 8xlO e — , which corresponds to 40 volts, to more than 



sec. 



18*8 XlO 8 , which corresponds to 1000 volts. An experiment, 

 not here reported in detail, showed that there was still a very 

 appreciable effect from the secondary radiation with an oppos- 

 ing potential of 1700 volts ; this corresponds to a velocity of 

 24*4 XlO 8 . This is not far from the maximum estimate which 

 Wertenstein obtained with the magnetic field, viz. 2 - 3Xl0 9 . 



It is plain that the existence of this radiation vitiates the 

 estimates which have hitherto been made of the velocities 

 of the slow ^-electrons. Thus, for example, an insulated source 

 of a- and S-rays, within an earthed enclosure, will lose more 



*See Gehrts, Atm. d. Phys., xxxvi, pp. 1003 et seq., 1911. 



