M'Clelland and Power — Electrification by Friction. 45 



charge or else a negative charge after the use of the emery paper. It is 

 probable, we think, that when sufficient care is taken to keep the nature of 

 the surface constant during the experiment, the curves for metals taking a 

 negative charge will be of the same type as those given in fig. 2 when the 

 metal is charging positively. 



Variation of production of charge with temperature, speed of rubbing and 

 pressure remaining constant. 



We have tried a few preliminary experiments on how the rate of produc- 

 tion of charge depended on the temperature of the metal surface. For this 

 purpose we used a hollow metal cap with two tubes attached so that a stream 

 of water could be passed through it. The vessels from and to which the 

 water flowed were insulated, so that the rate of charging could be measured 

 with or without the flow of water. Care was, of course, taken that the stream 

 of water did not of itself produce any electrical effect. Small junctions 

 soldered to the metal enabled the temperature of the rubbing surface to be 

 estimated. By placing junctions close to and at greater distances from the 

 rubbing surface we decided that the temperature of the surface in contact 

 with the silk certainly did not reach 100 c C. at the highest speeds used. We 

 could assume, therefoi-e, that the metal surface in contact with the silk was 

 raised in temperature by passing steam through the cap, and lowered in 

 temperature by passing a stream of water at the temperature of the room. 



The results were similar in all the cases we examined. When the metal 

 was charging positively, the stream of cold water increased the rate of 

 charging, while the effect of steam was to decrease the rate or even to change 

 the sign to negative. When the metal was charging negatively, the steam 

 increased the rate of charging and the cold water decreased it or changed the 

 sign to positive. 



It is impoi'tant to note that an increase of temperature produces the same 

 effect as a slight roughening of the metal surface. When the metal surface is 

 very slightly rough, the actual points of contact may be considerably raised 

 in temperature, although the average temperature is not appreciably altered. 

 The effects of changes of temperature will obviously he worth full inves- 

 tigation. 



Variation of rate of production of chargi with amount ofmoistwi in the air. 



To test how the rate of production of charge varied with the humidity of 

 the surrounding air, the apparatus was slightly modified so that the humidity 

 could be altered as desired. An extension pf the axis of the motor, working 



