202 24 



When the apparatus is to be used to measure radioactivily, it must be 

 remembered that the activity is generally proportionate to the leakage from the 

 inner cylinder divided by the time. Therefore in these measurements it is necessary 

 to record the time, as well as to measure the leakage. 



It is evident from equation (4) that measurements made by means of the 

 testing apparatus may be executed in two different ways. In the first place by 

 allowing the potential difference between the electroscope vessel and the inner 

 cylinder to remain constant, when the amount of the leakage is obtained by 

 determining the potential difference between the electroscope vessel and the ionisa- 

 tion chamber at fixed intervals. Secondly, the rate of leakage is obtained by 

 determining the time that elapses before the potential difference between the inner 

 system of conductors and the electroscope vessel has acquired a fixed value, after 

 having made a given alteration in the potential difference between the electroscope 

 vessel and the ionisation chamber. For reasons of a practical nature I preferred 

 to use the latter method of observation. 



We have a good criterion as to the identity of the potential differences between 

 the electroscope vessel and the inner system of conductors, when the deflection of 

 the aluminium leaf is constant. 



The measurements were made in the following manner. One's first care is 

 to see that the potential difference between the electroscope vessel and the ionisa- 

 tion chamber remains constant, e. g. by connecting them with the poles of a con- 

 stant battery, or at any rate by connecting them with a thin copper wire, so that 

 the potential difference becomes zero. Then the inner system of conductors is 

 charged until the aluminium leaf makes a suitable deflection. 



The potential difference between the aluminium leaf and the electroscope 

 vessel was about 300 volts in my experiments. The microscope is then adjusted 

 so that a clear outline of the leaf is seen in the neighbourhood of a mark in the 

 eye piece, — two threads crossways will serve this purpose. 



The deflection of the leaf decreases gradually on account of the leakage from 

 the inner cylinder, and by adjusting the microscope one ensures that the edge of 

 the leaf can be seen moving towards the fixed mark in the eye piece. 



Note is taken of the time at which the image of the edge coincides with the 

 mark. The potential difference between the electroscope vessel and the ionisation 

 chamber is altered, so that the deflection of the leaf becomes greater, and the 

 time at which the next coincidence of the edge of the leaf and the mark in the 

 eye piece takes place is also noted. Let t represent the number of seconds between 

 the two periods of coincidence; then the rate of leakage per second is given by 



c,. 



t 

 or 





E =-- -5 



