662 • 



Measuring method of ionisation currents. 



A quantity of radio-active substance is placed on the bottom of 

 tlie ionisation cylinder ƒ ; the system is charged to the state : -J- 20 V., 

 v., — 3 V. ; ƒ is then brought to a potential value, dependent on 

 the strength of current to be measured. While a, c, and /maintain 

 their potential values, b is insulated by breaking the contact with /; 

 the ions formed, let us say the positive ones, will then charge b to 

 a constantly increasing potential, with the consequence that the 

 number of lines of force between a and b will decrease, and a 

 couple will be formed, which will cause the needles to slowly recede 

 from each other, and that the quicker as the current is the stronger 

 (to return later on to particulars of the motion). 



Consideration. 



It will be seen from the arrangement of the electrometer, how the 

 before mentioned advantages of the WiLSON-electroscope are realized in 

 it ; in the space ƒ namely the ionisation current is directly carried to 

 the needle b -f- d, this needle being perfectly insulated by a single 

 piece of amber. The separation of ionisation space and ineasuring 

 space has, moreover, this advantage that the measuring system is 

 not contaminated with radio-active impurities, while the ionisation 

 space and the rod d, which can be removed, as regards the part 

 that lies in f, can be easily cleaned. 



As to the measuring sj'Stem proper, the principle, of it differs from 

 that of the quadrant electrometer ; it has been thus cliosen on purpose 

 that the lines of force formed by the ionisation current contribute 

 as much as possible and as favourably as possible to the movement 

 of the needle a. 



This is not the case in the quadrant electrometer; there namely 

 the movement is caused by the lines of force which run between 

 the quadrants and the rims of the~ needle, whereas the vertical lines 

 of force between needle and quadrants do not contribute anything to 

 the moving couple. 



In my opinion it would not be possible to modify the quadrant 

 electrometer in such a way that, while maintaining the principle of 

 the quadrants, many lines of force are not retained at the same time 

 which in a measurement either give no movement, or will even 

 counteract the movement. The latter might be possible, if the flat 

 needle should be replaced by a horizontal wire, in which way a 

 large horizontal surface is, indeed, avoided, but on the other hand 

 the formed lines of force would act on the two sides of the needle, 

 when the latter is rotated. The advantage of the described apparatus 



