Junctional Electricity, 459 



battery of about 12 storage-cells in some of the experiments, 

 in others by the 200-volt mains. The electrometer was cali- 

 brated every day during the course of the experiments by 

 connecting its terminals to various points in a volt box 

 joined up to two storage-cells. The deflexions (observed by 

 a telescope and illuminated scale) due to various fractions 

 of a volt were thus determined. In some cases the electro- 

 meter deflexion was proportional to the difference of potential 

 of the quadrants. When this was not so, a calibration curve 

 was drawn and the voltage corresponding to any deflexion 

 read off from this. In measuring the charge on the rubbed 

 specimen the zero of the electrometer was first observed, the 

 earth connexion of the terminal connected to the jar then 

 removed, and the specimen lowered into the jar. The 

 deflexion produced depends on the charge on the specimen, and 

 on the capacity of the system consisting of the outer surface of 

 the inner jar, the electrometer and its connecting wires, and 

 also the parallel plate condenser if this was connected. The 

 capacity of the parallel plate condenser, whose plates were 

 circular, was calculated from the formula 



d =r 2 /4d + t/Att | log ^— y - 1 + - log — j 



where r — radius of each plate, 



d= distance between the plates, 

 §= thickness of each plate. 



The capacity C 2 of the rest of the system was measured by 

 communicating to it a charge and observing the deflexions 

 before and after this charge was shared with the condenser. 



If the rubbed specimen, on being placed inside the jar, 

 produces a deflexion which indicates a potential V as read 

 off from the calibration curve, then the charge on the speci- 

 men is (d 4- C 2 ) V. 



The insulation of all parts of the apparatus was frequently 

 tested, and it was also verified that the specimen lost no 

 appreciable charge during the process of removing it from 

 the lever to ihe jar. It was in fact possible to take the 

 specimen from the jar, fix it in position on the lever, and 

 then bring it back to the jar, without any appreciable change 

 of deflexion. 



Before each rub the specimen was completely discharged 

 by radiating to its surface for a few seconds with radium. 



* Kohlrausch, Leitfad. d. prakt. Phys. 8 Aufl. p. 409. 



