506 



Mr. H. A. Wilson on the Variation of th 



The interest attaching to observations of this kind has been 

 considerably increased since Prof. J. J. Thomson * has shown 

 how from the curves for the electric intensity to determine 

 where the ionization and recombination principally take place, 

 so that it was thought worth while to make a series of obser- 

 vations with the object of confirming and extending Graham's 

 results. 



The method employed was to measure the difference of 

 potential between two exploring- wires, kept at a fixed distance 

 apart in the discharge, which could be brought into any 

 desired portion of the discharge by moving the electrodes 

 between which the discharge took place. 



The discharge-tube employed is shown in fig. 1. The 

 glass tube A B in which the discharge took place between 

 the electrodes E, E' was 50 cms. long 

 and 2*5 cms. in diameter, and was sup- 

 ported vertically. The electrodes E, E' A 

 were thin disks of aluminium which fitted 

 the bore of the tube loosely, and were 

 made slightly convex towards each other. 

 The electrodes were connected together 

 by three parallel glass rods each 1 millim. 

 in diameter, which passed through holes 

 close to their circumferences. These 

 glass rods were fused to glass tubes to 

 which the electrodes were fixed, as shown 

 in the diagrams. In this way the elec- 

 trodes were kept at a constant distance 

 apart, but were free to slide up and 

 down in the tube A B. 



The tube carrying the lower electrode 

 E' was floated in mercury in the tube 

 C ? to winch a barometer-column was 

 attached by means of which the level 

 of the mercury in C could be varied, 

 and so the electrodes E, E' raised and 

 lowered. The upper electrode E was 

 connected by means of a spiral of fine 

 brass wire to an electrode at A, and the lower electrode E' 

 was connected with the mercury in C by a wire sealed through 

 the lower end of the tube floating in the mercury. 



The electrode A and the mercury in C were connected with 

 the battery of 600 small secondary cells which was used to 

 produce the discharge. 



* Phil. Mag. March 1899, p. 253. 



