Bvonson — Radio-active Measurements. 185 



Art. XII. — Radio-active Measurements by a Constant Deflec- 

 tion Method / by Howard L. Bronson. 



The ordinary method of using an electrometer for compar- 

 ing ionization currents through gases has been to measure the 

 rates of movement of the needle. The ionization currents are 

 then proportional to these rates, if the following conditions 

 are fulfilled : (1) that the capacity of the system remains con- 

 stant, (2) that the deflection is proportional to the potential of 

 the quadrants, (3) that the lag of the needle behind the poten- 

 tial is the same for different rates. In some cases these condi- 

 tions are closely fulfilled, but in others, especially where the 

 needle is moving rapidly, the last condition and probably the 

 first are not. The introduction of additional capacity into the 

 system, while it reduces the rate of motion of the needle, creates 

 the added difficulty of comparing capacities, which not only 

 takes considerable time but is never entirely satisfactory. 

 Another difficulty with this method is that it is practically use- 

 less in the case of rapidly changing ionization currents. The 

 desirability, therefore, of a more direct and rapid method of 

 measurement is evident. 



If, as usual, one pair of quadrants of the electrometer is con- 

 nected to earth, and the other pair is not only connected to the 

 testing vessel but also to earth through a very high resistance, 

 it is easily seen that an ionization current in the testing vessel 

 will charge the quadrants until the discharge current through 

 the high resistance is equal to the ionization current. In this 

 case the current will be proportional to the potential of the 

 quadrants, that is, to the deflection of the needle. 



Resistances of the order 10 11 ohms made of pure amyl alcohol 

 and of carbon on glass were tried, and gave results which com- 

 pared favorably with those obtained by the " rate " method. 

 The results, however, were not satisfactory, because in the 

 case of the liquid resistance there seemed to be a variable polar- 

 ization, whereas the resistance of the carbon was not perfectly 

 steady. Professor Rutherford then suggested the possibility 

 of using an ionization current in place of the conduction cur- 

 rent through a high resistance. For this purpose he furnished 

 a very radio-active bismuth plate from Dr. Sthamer of Hamburg. 

 The activity of this plate was due to a deposit of the so-called 

 radio-tellurium of Marckwald, which has such a long period of 

 decay that it remains practically constant during any short 

 experiment. 



The bismuth plate was earthed and covered with a very thin 

 sheet of aluminium to avoid contact potential difference. 

 Another aluminium plate was placed parallel to this and con- 

 nected to the same quadrants as the testing vessel, and the 



