666 



2. So far the lines of force starting from the lower rim. of a, ot* 

 from the bach of a, ending on b, were not taken into account; 

 they represent injurious capacity. This iniluence will make itself 

 slightly felt in the middle of the needle, but will have little effect 

 there on the motion. 



3. It is difficult to say anything definite about the value of the 

 injurious capacity of b with respect to c. 



At any rate it w ill also appear from what follows, how for very 

 sensitive states the total influence of the injurious capacity may 

 almost be disregarded. 



In the case of the quadrant-electrometer, on the other hand, the 

 injurious capacity is that of large surfaces with respect to a metal 

 needle lying close by. 



Before confirming what has been said above about this increased 

 sensibility of charge for greater potential differences between a and 

 b by the communication of some experimental results, a few par- 

 ticulars may be added about the mode of motion of the needle 

 during the current measurement. 



Mode of motion of the needle during the current measurement : 



When the needle b is charged starting from potential V by 

 means of an ionisation current, when therefore the potential 

 difference between a and b decreases, a will l)egin to move away 

 from b ; consequently a motion of the scale division under the 

 crosswire will take place through the reflection of the mirror, which, 

 however, will not be uniform. For the different positions occupied 

 by a both the potential sensibility and the value of the capacity of 

 b will be different ; for the smaller the angle with b, the greater is 

 the potential sensibility, as well as the capacity. 



The causes are the following: 1. With a smaller angle the distance 

 between a and b is smaller and therefore the diminution of the 

 induced charge for a definite change of angle greater. 



2) With a smaller angle the potential difference itself is also 

 greater, and this again causes a greater decrease of induced charge 

 for a definite angle. 



For both reasons greater potential sensibility, but at the same 

 time greater capacity is to be expected at a smaller angle, but here 

 too for the same reason as for conditions of charge with greater 

 potential difference between a and b, the result will be a greater 

 sensibility of charge 



In agreement with this the curves I show, how for e\ery state of 

 charge the potential sensibility decreases with greater angle between 

 a and b. 



