194 



Prof. J. A. Cunningham on the 



discharge-tube with side-tubes and movable electrodes is 

 drawn to scale in fig. 1. The potential at any point along 



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1. 







1 







t 







ii 









0. 







V 



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V 



K 









c l 



the axis o£ the discharge was obtained by means of two thin 

 platinum- wire electrodes (B and C) 1 mm. apart, which were 

 carried by a sliding-piece consisting of two cylindrical portions 

 2'1 cms. in external diameter, joined together by a narrow 

 glass tube 22 cms. long, which lay along the bottom of the 

 discharge-tube. This was regarded as the arrangement least 

 likely to produce a disturbance of the discharge as it was slid 

 along from place to place. This anticipation was fully justified 

 during the course of the experiments. Not the slightest dis- 

 turbance of the discharge was noticeable except with very 

 small currents, when the discharge was often found to wander 

 about on the cathode even when the movable electrodes were at 

 rest. All such results have been rejected. 



The platinum wires from the electrodes B and C passed 

 along the centre of the narrow connecting tube of the movable 

 portion (S S), and were welded into copper wires which were 

 twisted into spirals to allow of free movement, and were again 

 fused into platinum wires passing through the ends of the 

 discharge-tube. The whole structure S S carrying the elec- 

 trodes B and C was moved along to any desired place by 

 means of a horseshoe electromagnet acting on the iron ring 

 M sprung into one of the expanded ends. A millimetre- 

 scale was attached to the side of the discharge-tube at D, 

 and served to fix the position of the intermediate electrodes. 

 The use of the two moving electrodes was found distinctly 

 advantageous. The one (0) nearest to the cathode was con- 

 nected to one terminal of a Kelvin multicellular voltmeter 

 whose other terminal was connected with the cathode. 

 Similarly the potential-difference between the anode A and 

 the electrode B nearest it was obtained by means of an 

 Ayrton & Mather electrostatic voltmeter reading from 100 





