﻿420 Sir E. Rutherford and Dr. J. Chadwick on the 



edge o£ the beam of scintillations appeared a little above a 

 horizontal cross-wire in the eyepiece of the microscope, 

 marking the centre of the field of view. 



When the magnetic field was applied in such a way as to 

 bend the a particles upwards (called the positive direction 

 of the field), the edge of the beam is deflected downwards in 

 the field of the microscope and the scintillations appear only 

 in the lower half. When the field was applied in the opposite 

 direction (negative field), the edge of the beam moved 

 upwards in the field of view. The strength of the magnetic 

 fields used in the experiments was always such that the 

 whole field of view was covered with scintillations when 

 the negative magnetic field was applied. In the experiments 

 on the magnetic deflexion of the long-range particles, the 

 number of particles is far too small to give a band of scintil- 

 lations with a definite edge. It is clear, however, that if 

 the particles are positively charged, the number of scintil- 

 lations observed with the negative magnetic field will be 

 greater than the number observed with the positive field, 

 and that the ratio of these numbers will give a measure 

 of the amount of deflexion of the particles. By determining 

 this ratio for the long-range particles and comparing it with 

 that for projected H particles of known velocity, we can 

 obtain an approximate value for the magnetic deflexion 

 of the long-range particles. The general method of the 

 reduction of the observations is perhaps best shown by an 

 account of the experiments on the particles from aluminium. 



Experiments on Particles from Aluminium. 



After fixing the position of the microscope in the way 

 described above, an aluminium foil of 3*37 cm. stopping- 

 power was placed over the source. Dry oxygen was passed 

 through the box, and a mica sheet of 10 cm. stopping-power 

 was inserted in front of the ZnS screen. The total absorption 

 between the source and screen was then equivalent to 30 cm. 

 of air. The scintillations observed were consequently due to 

 long-range particles from the bombarded aluminium ; the 

 ranges of the particles under observation varied from 30 cm, 

 to 90 cm._, the average range being about 45 cm. 



Counts of the numbers of scintillations observed with 

 positive and negative fields due to an exciting current of 

 6 amps, were then made. The mean ratio of the numbers 

 with a — field to those with a + field obtained from several 

 experiments was 3*7. The observations were repeated with 



