ivith the Brush Discharge. 43 



stated. The appearance of the discharge obtained from 

 concave ends is also shown. Concave terminals behave like 

 angular ones. 



2. Force of Wind from Points. 



The mechanical force exerted by the strongly electrified air- 

 particles which are repelled from the points is well known, 

 and the experiment of blowing out a candle is one of the 

 commonest shown as illustrating the action of points. The 

 magnitude of the force was roughly measured by causing 

 the discharge to play upon one pan of a delicate Robervahl 

 balance and measuring the weight necessary to restore the 

 equilibrium. Care was taken to use the same point and to 

 change the polarity by reversing the machine. This was the 

 only way in which comparative experiments were possible. 

 If the attempt be made to measure from two different points 

 it will be found that the minute differences in the points, 

 notwithstanding every care to make them precisely similar, 

 will show themselves by entirely altering the appearance of 

 the brush, and great differences in magnitude will be observed, 

 even with the same kind of electricity, from the different 

 brushes. 



In order that vibration might be avoided as far as possible, 

 the measurements were made on a stone slab, built up from 

 the foundations of the laboratory. The following results 

 were obtained : — 



Wimshurst machine 15 in. plate. 



No. of sectors on each plate . . 16. 

 Size of sectors 8 sq. cm. 



(1) Speed of revolution 450 per minute. 



Potential-difference corresponding to 



this speed, about 43,000 volts. 



Force from positive brush equal to 



a weight of 0*29 gramme. 

 Force from negative brush equal to 



a weight of 0*24 gramme. 



(2) Speed of revolution 182 per minute. 



Potential-difference 35,000 volts. 



Force from positive brush equal to a 



weight of 0-08 gramme. 

 Force from negative brush equal to 



a weight of 0*066 gramme. 

 The best distance of brush from pan 



of balance 0*04 metre. 



