Frictional Electricity. 461 



the wheel was turning rapidly. In the results given below 

 only the results of good rubs, as indicated by a uniform 

 sliding sound, are included. 



It was also found very important to ensure that there was 

 even contact over the whole width of the specimen ; if the 

 wheel touched it only at one side a small deflexion would 

 result, owing to the small area rubbed. 



Another precaution which it was found necessary to take, 

 was to give the specimen a few hours' rest after each rub. 

 If the specimen was rubbed a number of times in succession 

 at short intervals, the work in each case being the same, and 

 less than the amount required to give the maximum deflexion 

 at the first rub, the deflexion invariably increased with the 

 successive rubs up to a maximum value, though the specimen 

 was completely discharged by radium before each rub. This 

 maximum was the same as could be obtained at the first rub 

 with a sufficient amount of work. After an interval of three 

 or four hours, the specimen appeared to have come back to 

 its original state and the same series of increasing deflexions 

 could be obtained with successive rubs. In the results given 

 below the charges produced correspond to the first rub after 

 an interval of three or four hours' rest. 



The results were much influenced by the state of the 

 atmosphere ; no deflexion was sometimes obtained during 

 wet weather. 



(4) Results. 



In the experiments the results of which are here given the 

 wheel was of slate and its axle was earthed. In some of the 

 experiments the rim of the Avheel was covered with a band of 

 copper. The specimens rubbed were of ebonite and glass. 

 Curves were obtained for ten specimens of ebonite of the 

 same size cut from the same sheet, the friction taking place 

 under three different pressures. Seven of the specimens 

 gave very concordant results ; the other three gave irregular 

 readings, probably owing to unsatisfactory mounting. Ten 

 specimens of glass were also used with equally concordant 

 results. 



In the following tables the first column shows the fric- 

 tional work in millions of ergs, the second, third, and fourth 

 columns the quantities of electricity Q l9 Q 2 , Q 3 , in electro- 

 static units, produced on the specimen of ebonite or glass 

 when the pressure between the wheel and the specimen was 

 306*8, 1326*8, and 2579*2 grammes weight respectively. 



The method was not suitable for determining the surface- 

 density of the charge produced on the specimen, owino- to 



