446 Ml' Groivther, On the Transmission of /3-rays. 



sufficiently low for all practical purposes. In experiments for 

 which a parallel emergent beam was not necessary, the radium 

 was placed at A. If, however, a parallel beam was desired (as in 

 the experiments on the absorption of the rays to be described 

 later) the apparatus was reversed and the radium placed at B. 



Two such chambers were made, and placed so that the window 

 A of the one came directly opposite the window B of the other. 

 The two magnetic fields were arranged so that rays of the proper 

 velocity would be deflected round the two systems and emerge 

 finall}'^ into an ionization chamber of the usual pattern. 



Fig. 2. 



The measurements of the ionization produced were made by 

 means of the compensating method devised for experiments on the 

 scattering of the /3-rays from uranium, and fully described in 

 a previous paper*. In brief it consists in compensating the 

 ionization current through the first chamber by an opposing 

 ionization current from a second chamber connected to the 

 same electroscope, in which the ionization can be varied, in a 



* J. A. Crowther, Proc. Roy. Soc. A, Vol. lxxx. p. 186. 



