by a Charged Condenser moving through Space. 163 



-silvered glass ball, 0*5 mm. in diameter, at each end, which 

 was suspended by an insulating suspension, and provided 

 with a mirror, for obtaining the deflexions. Another silvered 

 glass ball was fastened to a piece of thin platinum wire, which 

 was insulated by sulphur. When this fixed ball was charged 

 it attracted one of the movable balls, and the sensitiveness 

 of the arrangement could be adjusted by moving the fixed 

 ball by a tangent screw. 



The proof plane whose potential was required was caused 

 to touch the wire connected to the fixed ball, and the de- 

 flexion a was observed. The electrometer was standardized 

 bv taking a series of readings in which the fixed ball was 

 touched with the proof plane after the latter had been raised 

 to known potentials by allowing it to touch a thin wire 

 connected to the central condenser plate of the apparatus. 

 The potential of this plate, which was of course reduced to 

 potentials varying from 400 to 1000 volts, was measured by 

 the electrostatic voltmeter *. 



Tables V. and VI. give for the two blocks the deflexions 

 referred to as a above, corresponding to the potential E ot 

 the central charged plate given in the first column. The 

 third column gives the potentials corresponding to these de- 

 flexions, and the last columns the distances y of the points 

 at which the potentials were measured, from one of the 

 points of intersection of the coil with the paraffin wax. 



The curves (fig. 4) represent the observations plotted 



Fig. 4. 

 Curves showing the variation of potential with the distance y. 







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* It will be seen that the error due to neglect of the part of the 

 potential of the proof plane contributed by the charge on the wire, just 

 cancels the error referred to in the note on page 162. 



M 2 



