766 Mr. A. L. McAulay on an Electrical Method 



The potentials on the various parts are as follows : — 

 The sides of the compensating chamber are charged to 

 — 120 volts, the aluminium ionization chamber to +40, the 

 leaf through the charger, (fig. 2) , and the electrode to —40, 

 and the case of the electroscope is connected to earth. When 

 in use the whole apparatus is of course flooded with y ray 

 ionization from the source, and the current which this 

 carries in the compensating chamber gives the leaf a higher 

 negative charge, while the current in the ionization chamber 

 tends to discharge it. The volume of the former can be 

 adjusted to give any strength of current, and thus any rate 

 of movement to the leaf. 



The principal objects in view in the design were : — 

 (i.) To keep the capacity as low as possible while 

 ensuring approximate saturation. 



(ii.) To use an ionization chamber as small as possible. 

 This is necessary to give definiteness to the absorption 

 curve. Dimensions at right angles to the radiation must be 

 small in order that hydrogens driven in the direction of the 

 a particles shall not enter the chamber too obliquely, and 

 dimensions parallel to the radiation must be small in order 

 that the depth of air in the chamber shall not represent more 

 than a small part of the absorbing material necessary to 

 bring the hydrogen particles to rest. A lower limit is set 

 to the size of the ionization chamber by the necessity of a 

 measurable ionization, 



(iii.) To make the volume of the electroscope as small as 

 possible. This is important because the ionization due to 

 the recoil radiation must be as large as possible compared 

 with that due to <y rays, in order to reduce fluctuations, and 

 while the former is only produced in the ionization chamber, 

 the latter is produced in the electroscope as well. 



Method. 



The method employed is briefly as follows : — The brass 

 box is placed between the poles of a powerful electro- 

 magnet which prevents /3 rays from the source entering the 

 ionization chamber. The a rays are alternately cut off and 

 allowed to fall on the hydrogen by rotating the cylinder, D. 

 In the latter case, though the a particles are absorbed in the 

 window, F, the recoil atoms penetrate it and enter the 

 ionization chamber, causing a slight increase in the ionization. 

 The walls of the compensating chamber, being certainly 

 equivalent to more than 50 cm. of air absorption, are not 

 penetrated. With the « particles cut off, the volume of the 



