Thin Films of Elements exposed to RontgejiRays. 129 



material in contact with the graphite surface, do importance 

 can therefore be attached here to the fact that the total 

 ionization observed differs widely for the various screens. 



Figs. 5 and 6 (PI III.). — For the purpose of comparing 

 the curves along their non-linear portions, each was scaled 

 up to the same maximum ordinate *. The result is shown 

 in fig. 5 (PI. III.), which might be called the cathode 

 ionization curves from the ten screens, silver as radiator, 

 and in fig. 6 (PI. III.), barium as radiator. 



In the paper previously referred to, the writer has shown 

 fully how the first derivatives of the curves shown in figs. 5 

 and 6 (PI. III.) give the density of the ionization in a 

 chamber containing air at 0° C and 76 cm. pressure at a 



distance •— x x 1'45 cm. from the screen, where p 



7b z t 5 -\-o 



is the pressure at which an ordinate such as AB is drawn, 

 1*45 cm. the thickness of the ionization chamber used, and 

 6 the mean temperature of the experiment, about 24° 0. in 

 this case. This expression has been used in order to convert 

 values of p into corresponding values of the distance from 

 the screen as measured in air at 0° C. and 76 cm. pressure. 

 The ordinate AB (==N X ) measures the total ionization by 

 /3-rays in air (at 0° C. and 76 cm. pressure) in the region 

 from the screen up to an imaginary layer 0*1 cm. from the 

 screen. 



Fig. 7 (PI. III.).— When the curves of figs. 5 and 6 

 (PL III.) were first drawn, there seemed to be little order 

 about their sequence, except that in fig. 5 (PI. III.) the 

 curve for the silver screen was lowermost and in fio-. 6 

 (PL III.) the barium curve lowermost. In order to show 

 up any sequence, fig. 7 (PL III.) was drawn. This figure 

 has for abscissae the atomic numbers of the elements of the 

 screens, whilst the ordinates are taken directly from figs. 5 

 and 6 (PL III.) as follows. The length' OB (fig. 5) 

 represents the total ionization by /3-rays in air (at 0° C. 

 and 76 cm. pressure) included within the region from an 

 imaginary plane 0*1 cm. from the screen up to the front 

 face of the chamber ; it is therefore a measure of the /3-ra v 

 energy crossing this plane, and equals (N — N x ). The line 

 CB, at the 0*1 cm. mark, is drawn right across the 10 curves, 

 and its various lengths raised above the respective elements 



* The simplest physical interpretation of such an adjustmenl is to 

 imagine the number of atoms at the surface nf each screen so packed 

 that the jS-ray emission from each screen gives vise to equal total 

 ionization in the air of the chamber for complete absorption of the 

 particles. 

 " Phil. Mag. 8. 6. Vol. 41. No. 211. Jan. 1921. K 



