1040 Mr. Shenstone and Prof. Schlundt on the Numher 



held centrally in a glass tube which could be exhausted by 

 means of an aspirator. The end of the tube facing the 

 source was closed by a brass plate having a central bevelled 

 hole of 3 mm. diameter. This hole was closed by a thin 

 sheet of mica of stopping-power equivalent to 4*3 cm. of air. 

 The zinc-sulphide counting-screen was mounted on a bracket. 

 8 mm. from the end of the tube. The total stopping-power 

 in the path of the a-particles was therefore 4 , 3 + , 8 = 5 , l cm. 

 of air *, which is just sufficient to prevent the 5 cm. a-particles 

 from thorium C from reaching the screen. 



The efficiency of a zinc-sulphide screen varies considerably 

 over its surface. To ensure permanence of experimental 

 conditions, it was therefore necessary to be certain that the 

 same portion of the screen was always used. This was 

 accomplished by mounting the screen with its crystal face 

 against a piece of thin brass having a hole of diameter 

 slightly less than the field of the microscope. The micro- 

 scope could then always be focussed on the identical portion 

 of the screen. The microscope used gave very bright 

 images of the scintillations. It was fitted with Messrs. 

 Watson's " Holos " x 5 eyepiece and " Holos " objective of 

 *45 n.a. and 16 mm. focal length. The usual "pea" lamp 

 was used to slightly illuminate the screen. 



The " Wheel" Method of Counting. 



To enable large counts to be made with ease, a method of 

 counting devised by Sir Ernest Rutherford was adopted. 

 The essential feature of this method is the use of a disk of 

 about 20 cm. diameter mounted with its edge intercepting 

 the beam of a-particles. The disk is provided near its edge 

 with a slit of suitable width which was 1*6 mm. in our case. 

 The disk must be mounted on good bearings and must be 

 fitted with a driving device capable of rotating it at a very 

 steady rate of from 50 to 200 revolutions per minute.. 

 When the disk is rotating, the particles can reach the screen 

 only when the slit is passing through the beam of particles. 

 Moreover, the number of a-particles which can pass per 

 minute is not a function of the speed of the wheel, but 

 merely of the ratio of the width of slit to the circumference 

 of the disk. The method of counting and recording also 

 differs from the ordinary method. The observer does not 

 mentally add the number of scintillations over a minute, but 

 calls out the number which appears at each revolution of the 



* To this should be added about 0-2 cm. for the air left in the tube 

 between the source and the mica disk. 



