31'GlellaND A.NU PoWER — Electrification by Friction. 41 



The apparatus was quite simple. A circular disc of wood was hollowed out 

 on one face to a depth of about a quarter of an inch, leaving a circular rim 

 around the edge. Silk— or linen — was stretched over the hollowed out face 

 and bound firmly in position by wires fitting into shallow grooves on the 

 circumference of the disc. The disc was fixed to an axis that could be 

 revolved by a motor so that the silk rotated in its own plane anil formed one 

 of the nibbing surfaces. Discs of different sizes up to 14 inches diameter 

 were used at different times, and a wide range of speed of rotation was avail- 

 able. The other rubbing surface was a metal, and different metals have been 

 used. Small metal spherical caps of large radius of curvature were used. 

 They could readily be attached to a simple lever arrangement that allowed 

 them to be pressed against the silk surface with any desired pressure. The 

 stem carrying the metal surface was insulated and joined to one pair of 

 quadrants of an electrometer. The rate of production of charge on the metal 

 surface was measured, either by observing the rate of charging of the electro- 

 meter, with a suitable capacity attached, when the motion of the silk surface 

 was steady, or more often by connecting the metal to earth through a high 

 resistance, and using the electrometer to measure the steady potential 

 difference between the ends of the resistance. 



This apparatus was modified for some of the experiments aud especially 

 for the work in gases at reduced pressures, but the general method remained 

 the same. 



It will be observed that in our method of working the metal cap which 

 forms one of the rubbing surfaces is always either at zero potential or differs 

 from it by only a fraction of a volt. On the other hand, the silk or linen 

 which forms the other rubbing surface retains its charge to some extent, and 

 we do not know the potential which it reaches. After some revolutions of the 

 disc, that is in a very short time, the steady condition is no doubt reached 

 when the leakage from the silk balances the rate of production, and it is the 

 rate of charging of the metal surface under these conditions which we measure. 

 No doubt there is uncertainty as to the extent to which the charging of the 

 metal surface is influenced by coming in contact with silk electrified during 

 the previous revolution of the disc, but we have used this method- at any 

 rate up to the present — because of the steadiness of the results obtained by 

 it. Besides, the difficulty referred to cannot be avoided completely by anj 

 method of experimenting. When two surfaces are electrified by friction, 

 recombination of the charges will always lake place at a rate depending on 

 the conductivity of the bodies and on the relative velocity at the point of 

 contact. 



[6*] 



