452 Mr Growther, On the Transmission of ^-rays. 



The chief points in the method are therefore, briefly, as 

 follows : 



(1) The homogeneous rays are produced by magnetic de- 

 flection through a suitable system of apertures in a vacuum. 



(2) They fall upon the absorbing sheet normally, in a 

 narrow and nearly parallel pencil. 



(3) The intensity is measured in an ionization chamber of 

 such a form that, at whatever angle the rays emerge from the 

 absorbing sheet, their length of path in the chamber is practically 

 the same. 



Table II. 



It may be permissible to note in passing the advantages pos- 

 sessed by the compensator method in experiments of this kind 

 over the ordinary methods of timing the leaf over a given number 

 of divisions. These are, in brief, 



(1) The large 7-ray leak through the ionization chamber is 

 balanced by a similar leak through the compensator. In this way 

 the effect due to the yQ-rays is measured directly, and not as 

 a difference between two other large quantities. 



(2) The results are not affected by defects in the insu- 

 lation, as is the case when a timing method is used. As a defect 

 in the insulation only comes into play when the gold leaf system 

 begins to charge up, it cannot affect the direction in which the 

 leaf begins to move, and hence cannot affect the final balance. 

 In a timing method, however, defective insulation will alter the 



