with the Brush Discharge. 45 



3. Electrical Action at a Distance. 



If an electroscope or a leyden-jar be placed at some distance 

 from a point from which a brush-discharge is taking place, 

 it will become charged. If the brush be a positive one the 

 electroscope or jar will be charged positively ; if the brush be 

 a negative one it will be charged negatively. 



The distances at which the effects make themselves evideut 

 vary with the potential and the atmospheric conditions. 

 Statements of lengths therefore are only valuable as allowing 

 of comparison being made between experiments which are 

 performed at the same time and in the game laboratorj^. The 

 same relation will, of course, hold, but the actual measure- 

 ments will be different. With these reservations the following 

 are given : — 



With the plates of the machine revolving at the rate of 

 450 revolutions, and giving a difference of potential of about 

 43,000 volts, an electroscope was affected at a distance of 

 1'8 metres from the point. No difference was made by altering 

 the polarity. 



When the plates were revolving at the rate of 105 revolu- 

 tions, and the potential-difference was 25,000 volts, the 

 same electroscope was affected at a distance of 1/0 metre. 



The size of the collecting-plate of the electroscope makes a 

 considerable difference in the ability of the instrument to 

 become charged. The larger this plate the greater the 

 distance at which it can be charged. A point on the plate 

 reduces the distance somewhat, probably because it allows 

 the electricity to escape as fast as it can be collected. 



No increased effect could be obtained by increasing the 

 condensation. 



The shape of the point which gives the best results is 

 acute. This is different from the effect in producing lumi- 

 nosity. In that case it will be seen that a larger luminous 

 brush is obtained with a greater angle up to 90°. 



Notwithstanding the enormously greater difference of 

 potential produced, the brushes from the coil used were quite 

 unable to influence the electroscope at distances equal to those 

 obtained from the machine. Thus, when the coil was giving 

 sparks of 0*07 metre in length and thus causing a potential- 

 difference of about 63,000 volts, the positive brush was only 

 capable of charging the electroscope at a distance of 0*62 metre 

 and the negative at the same distance. 



The same relative results were obtained when the brushes 

 were used for charging a leyden-jar. Thus, with a difference 

 of potential of about 40,000 volts, a jar was charged by the 



