﻿264 Mr. W. Morris Jones : Frictional 



that of the insulated quadrants, without unduly increasing 

 the deflexion. 



The scale was calibrated by the difference of potential due 

 to a known current passing through a known variable re- 

 sistance. The capacity of the measuring system was deter- 

 mined by the method of mixtures, using a standard condenser. 

 Knowing the capacity of the electrometer and accessories, 

 and also knowing the potential of the insulated quadrants 

 corresponding to any deflexion of the needle, the charge 

 producing that deflexion was obtained. 



The Insulators. 



In rubbing the specimens, care was taken to bring the 

 middle part of the specimen evenly into contact with the rim 

 of the rubbing wheel. If this was not done, the deflexion 

 obtained with insulators was always too small owing to the 

 smaller area ol contact. This precaution also prevented the 

 sliding contact from breaking into a series of impacts. It 

 was also found necessary to give the insulators a rest after 

 each rub, for if an insulator was rubbed a number of times 

 in succession the deflexions rose to the maximum, though the 

 work done was the same for each rub and less than that 

 required to give the maximum for a first rub. Owen also 

 observed this effect, and found that he had to give his 

 specimens a rest of about three hours after each rub with 

 slate or copper. In the experiments described here, the rest 

 needed was far shorter, this being apparently due to the fact 

 that the rubbers used were softer. To get the rubber as soft 

 as possible a few layers of the material were wound round 

 the wheel. 



The insulators rubbed were fused quartz disks, quartz 

 crystal, glass, polished ebonite, amber, sealing-wax, and 

 crystals of fluorspar, Iceland spar, and heavy spar. The 

 rubbers were bands of flannel, silk, and chamois leather 

 wound round the rim of the wheel, the axle of which was 

 earthed, and the normal thrust between specimens and 

 rubbers was in all cases 178 grams weight. 



The following tables give the charges in electrostatic 

 units produced by various quantities of frictional work re- 

 presented in joules, when four typical specimens were rubbed 

 with flannel, silk, and leather. 



