A Null Method of Measuring Small Ionisations. By Norman 

 R. Campbell, B.A., Trinity College. 



[Bead 1 May 1905.] 



For the measurement of spontaneous ionisations a method 

 has been tried and found successful, which consists in balancing 

 the current through the ionised gas against that through a con- 

 stant volume of air, of which the pressure can be varied, ionised 

 by a constant amount of uranium. 



The vessel (A) in which the unknown ionisation is to be 

 measured is provided with an electrode surrounded by a guard 

 ring of the usual type, and connected to a Wilson gold-leaf 

 electroscope. To the same electroscope is connected the electrode 

 of the " compensator " (B). This is an air-tight cylinder of brass 

 with a central wire electrode insulated from its guard ring by 

 amber ; in the cylinder is placed a packet containing a suitable 

 amount of uranium nitrate wrapped in thin aluminium foil. 

 A side tube leads to a three-way tap, the other branches of which 

 lead to a water-pump and inlet tube for regulating the pressure. 



The quantity of uranium is first adjusted so that when the 

 exterior of the pot is raised to a potential equal and opposite to 

 that of the walls of the vessel (A) and great enough to send 

 a saturation current through the air, the current is slightly 

 greater than the largest current which has to be measured 

 through the vessel (A). 



A is then disconnected from the electroscope and the compen- 

 sator standardised by observing the rate of rise of potential of the 

 electrode for different pressures in the vessel. The capacity of 

 B is measured by repeating one of the same observations with 

 a condenser of known capacity connected in parallel with the 

 electrode. Thus the relation between the pressure in the vessel 

 and the current flowing through it is obtained. 



The unknown ionisation is then measured by observing the 

 pressure in the compensator when the sum of the currents 

 flowing through A and B is zero as indicated by the absence 

 of movement in the leaf of the electroscope. 



The arrangement has the following advantages over the 

 ordinary methods : — 



1. After standardisation little care is needed that the insula- 

 tion is good, for the potential of the electrodes never rises appre- 

 ciably above zero. 



2. Only one capacity, that of the compensator, need be 

 measured in order that all the currents may be expressed in 

 absolute units. 



3. Advantage is taken of the great sensitiveness of the 

 Wilson electroscope near its zero. Experience shows that obser- 

 vations may be made more rapidly, as well as more accurately, 

 than by the usual method. 



4. All the benefits of null methods in detecting small changes 

 are obtained. 



