914 Prof. H. A. Bnmstead on Emission of Electrons 



rapidly decreased. Carve I. (fig. 2) shows the result of a 

 series of such measurements made on Jan. 12, 1911. In the 



cm. 

 10 



Fig. 2. 















*^^ 



7 











^ 













"^ 



w 





r 













.01 



.02 



03 



•04 



VOLTS PER MINUTE: 

 Curve T. Polonium. 

 Curve II. Thorium Active Deposit. 



05 



figure 



tin 



currents in volts per minute are plotted as 

 abscissae, and the number of layers of foil as ordinates ; in 

 order to facilitate comparison with the ionization curve of 

 polonium, however, the scale of ordinates indicates the air- 

 equivalents of the foils. Each plotted point is the mean of 

 from three to six separate observations, taken in various 

 orders and with frequent returns to the measurement of the 

 current with no foils, to check the constancy of the electro- 

 scope readings. Several repetitions of these measurements 

 gave entirely similar results. 



Before entering the case the a-rays passed through 0*6 cm. 

 of air and through the thick aluminium foil which covered 

 the holes in the top and whose air-equivalent was 1*47 cm., 

 so that, with this apparatus, only the upper part of the 



