402 Prof. W. H. Bragg on the Consequences of 



chamber is expressed by 



i { D * ,+ (i-?)< 1 -- vd )}- 



If kjX — k'jX' the expression becomes IDk' simply : and the 

 relation between ionization and pressure, measured by D, 

 becomes a linear equation. If k jX f is greater than k/'X the 

 curve is convex to the pressure axis, and if less it is concave. 

 So far as I know, no experiments have ever been carried out 

 with an ionization-chamber of this form in which y rays have 

 been employed to ionize air at different pressures. In the 

 experiments of Kaye and Laby * the ionization-chamber was 

 wholly made of one metal aluminium : in those of W. Wilsonj 

 it was partly of brass and partly of aluminium. If the y rays 

 have been hardened by a lead screen, k and k' are nearly 

 equal, in fact the absorption coefficients of a number of sub- 

 stances are nearly the same. Now the ft ray absorption 

 coefficients are somewhat smaller for light atoms than for 

 heavy, so that k/\ is less than k' /X' and the curve, in the 

 case I have considered, should be slightly convex to the 

 pressure axis. When the top and bottom plates are both of 

 aluminium, it should be slightly concave, as will be shown 

 presently: Kaye and Laby found this to be the case. 



In the case of 7 rays, X'D is generally small, unless the 

 pressure of the air in the chamber is very great : the 

 expression then becomes 



f M/ /V_ _ k\X /2 D 2 \ 

 I X. VV Xj 2 J' 



There is a term in this expression which is proportional to D 

 and therefore to the pressure, but it does not represent 

 exactly the action of the 7 rays on the air, as some have 

 supposed. Nor does it represent the action of the secondary 

 rays from the walls entirely. And again it has sometimes 

 been stated that a term proportional to the square of the 

 pressure will be required to represent the ionization due to 

 the ft rays made by the 7 rays in the gas. Clearly this is 

 not quite true. 



In the case of X rays k is usually so much greater than k' 

 that the latter may be neglected, and X f J) is so large that e~ vr> 

 is negligible also. The exponential term is only to be retained 

 when the pressure of the gas is so low that the cathode rays 

 originating in the walls of the chamber can get across it in 



* Phil Mag-. Dec. 1908. 

 t Phil. Mag. Jan. 1909. 



