740 Mr. J. Chad wick on the Charge on the 



10 cm. radius the reduction factor will be about 620 ; 

 that is, i£ without the wheel the number of scintillations 

 is 20,000 per minute, the number when the wheel is rotated 

 will be 30. 



This method has the further advantage that by varying 

 the speed of the wheel the time-distribution of the scintil- 

 lations can be changed. For example, suppose that, rotating 

 the wheel, the number of scintillations is 120 per minute. 

 If the wheel is rotated once a second, then on the average 

 2 scintillations will be observed every time the slit comes 

 round. Owing to the probability fluctuations the number 

 at each rotation will vary — sometimes 0, sometimes as many 

 as 4 or 5 will occur, — but the observer has a second in 

 which to collect and register his impressions. As the speed 

 of rotation is increased, the number per rotation decreases 

 and the interval between successive flashes gets shorter, 

 until the ordinary time- distribution is approached. It was 

 found that with a little practice a flash of about 6 or 7 

 scintillations was readily identified, provided that an interval 

 of about half a second was allowed before the next flash. 

 This enables counting to proceed at a far greater rate than 

 is possible by the usual method ; in fact, with the wheel 

 100 scintillations per minute is quite a moderate rate, and 

 after some practice 200 per minute can be counted with 

 accuracy. Under the usual conditions 30 to 40 scintillations 

 per minute is the highest rate for accurate counting. 



The advantages of counting both with the direct and 

 scattered particles on the same screen are obvious ; the 

 chief source of error in the determination of the nuclear 

 charge is surmounted. Provided that the scattering foil is 

 thin and uniform, so that the correction for change of 

 velocity of the a. particle in passing through the scattering 

 foil is small and accurately known, the accuracy of the 

 measurement will now depend chiefly on the number of 

 a particles counted in the experiments. 



§ 3. Apparatus. 



The experimental arrangement is clear from fig. 2. The 

 diaphragm D defining the beam of a particles was made 

 by suspending a circular disk 11*22 mm. in diameter in the 

 centre of a circular opening 19*22 mm. diameter. The 

 scattering foil was held on the support A, of similar 

 construction to D. The circular opening had a diameter 

 of 40 mm. and the central disk 15 mm., sufficient to prevent 

 any particles scattered by the edges of the diaphragm from 



