610 Miss P. M. Borthwick on Potential Measurements in 

 Fig. 1 shows the arrangement of the apparatus. 



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Electrolier 



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In experiments (1), the discharge point P was earthed 

 through a high resistance galvanometer, the plate p, insu- 

 lated from P, being earthed directly. The point N, of much 

 smaller diameter than P, supplied external ions to P of 

 opposite sign to that of P, and was connected, together with 

 the plate n, to a wimshurst. N could be drawn down below 

 plate n if desired, in which case ordinary point-plane dis- 

 charge occurred. The distance between the electrodes (x) 

 could be varied at will. In experiments (2) N was removed, 

 P was connected to the wimshurst, and the opposite plate 

 (of polished brass and diameter 15 cm.) was earthed through 

 the galvanometer. 



The exploring point E was of finest platinum wire (50 G) 

 and protruded about 1 mm. from a fine quartz sheath made, 

 however, sufficiently thick to be rigid under the existing 

 electrostatic forces. It was connected to a previously cali- 

 brated Braun electrometer by a wire sheathed in insulation, the 

 outer surface of which was earthed. The explorer was movab'e 

 along the vertical line a b. A second Braun electrometer was 

 used to measure the difference of potential between the 

 electrodes when required. 



Results. 



Potential readings near the point. — Owing to the disturbing- 

 influence of E it was impossible to obtain accurate readings 

 within about 3 mm. of P, since it was found that in this 

 region the potential difference between the point P and 



