46 Dr. E. H. Cook on Experiments 



positive brush from the machine at a distance of 0*55 metre. 

 The negative brush charged it negatively at the same distance. 

 With a concave terminal this distance was reduced to 

 0*50 metre. 



With the coil giving a spark of about 0*7 metre (63,000 

 volts) the jar was charged at a distance of only 0*14 metre. 



But, as has been already stated, these distances vary greatly. 

 With favourable atmospheric conditions, and a plate on the 

 electroscope 16 by 23 centims., it was found that the electro- 

 scope could be readily affected at a distance of 4 metres from 

 the point attached to the 15-inch Wimshurst machine. On 

 comparing these distances with those obtained when measuring 

 the mechanical force of the wind from the point, it is seen 

 that the electrical effects are felt at a much greater distance, 

 amounting, if we take the maximum distances, to more than 

 six times. 



It is not at all necessary that the conductor ending in the 

 point should be directed towards the electroscope. The 

 instrument will be found to be influenced in almost any 

 position ; in fact, when experimenting in frosty weather, I 

 have obtained results at equal distances all round the point, 

 even in the exactly opposite direction. It is therefore clear 

 that the point is the centre of a disturbance which radiates 

 from it in all directions. 



The interposition of objects between the point and the 

 electroscope gives interesting results. A wire- cage, if com- 

 pletely covering the instrument and earth-connected, will 

 prevent it from being charged, but if it is not completely 

 covered, even if only one side of the base is tilted up, the 

 instrument will become charged. The same wire-cage held 

 between the point and electroscope has no effect. Plates of 

 metal, wood, or other material placed between, do not alter 

 the effect unless they are either close up to the point or to the 

 electroscope. In both cases a diminished effect is observed. 



Experiments were tried with the view of discovering if the 

 effects could be produced through substances ; i. e. if like the 

 Rontgen rays the brush-discharges possessed any penetrative 

 power. Although by this mode of experimenting definite 

 proof could not be obtained, several of the results are instruc- 

 tive. Thus, if the electroscope be placed at from 2 to 3 

 metres away from thepoint, and a board 56 by 78 by 15 centims. 

 be interposed midway in the path of the discharge, the leaves 

 are easily affected, almost as easily as if the board were absent. 

 The same thing happens if sheets of metals are interposed. 

 The direction of the point does not affect the result. If the 

 sheet be placed within 30 centims. of the point, the effect on 



