67J 



capacit}' can even all but vanish. Tliis circunislance can also be 

 expressed thus, that then even a supply of negative charge will no 

 longer make the potential of b go down, because the approach of 

 a to b brought about by this supply just compensates the expected 

 potential decrease. 



- The angle, for wliiv'i this consideration holds, will still be found 

 outside the region of turning and may be realized by means of a 

 storage battery. 



If more and more negative charge is added at this angle to b. e.g. 

 through an ionisation current, the potential value of b will even 

 continually rise in consequence of the preponderating influence of the 

 approach of a to b. F'inally the state becomes this, and it is then 

 that the turning takes place, that for a further approach of a to b a 

 supply of charge to b is not even required any longer. For the mei-e 

 induced charge on b called forth by the approach will be more 

 than sufficient to give rise to an electrostatic couple, which can be 

 in equilibrium with the formed torsion couple. 



That, however, in case of such a turning the parallel state is not 

 entirely reached, which was already pointed out, maj^ be accounted 

 for in this way that the lines of force between a and b at decreas- 

 ing angle will also act on the back of a in appreciable quantity, 

 and this more and more as the angle becomes smaller, so that through 

 this circumstance the electrostatic couple, which tends to make the 

 angle between a and b smaller, is counteracted. It will follow from 

 this, that after the turning, the two needles will always continue to 

 form a (generally small) angle with each other. 



It follows therefore from this explanation of the angle of turning, 

 as was already pointed out on p. 547, that, when the measure- 

 ment is made in the neighbourhood of this angle, the capacity which 

 must then be taken into account, will chiefly consist of useful capacity, 

 by which the sensitivity of the state is to be explained. 



In conclusion a single example of a measurement of air-ionisation 

 and of Uubidium-ionisation. 



In these measurements the needle a was brought to potential zero ; 

 the state of charge was :0 V.,— 26 V., — 32 V. 



The ionisation space contained only air ; volume 1 litre ; the 

 number of seconds successively fouiid for the passage through 10 mm. 

 was: 43, 24, 31, 32, 55, 55. 



Then a quantity of Rubidium salt was placed in a dish with an 

 area of 50 cm^ on the bottom of/; it was found that successively : 

 9, 10, 10, II seconds were required for the passage of 20 mm. In 

 this case ƒ was at -j- 80 V. 



Physical Laboratory of the University at Groningen. 



