between the Terminals of a Vacuum Tube. 161 



begins, being determined by the distance the aperture of the disc 

 was from the cathode, fig. 3. 



As pointed out at the beginning the potential difference was 

 connected with the area of the aperture, and a comparison was 

 made between the rise in potential caused by a disc with no 

 aperture, and with discs having different apertures. 



An apparatus, somewhat similar to fig. 2, was used, but both 

 tubes had ground stoppers and aluminium electrodes. This was 

 first exhausted without any discs being present and the potentials 

 of each tube observed. The ratio was found to be 1 : 1. 



Then a solid copper disc was inserted in one tube and a per- 

 forated copper disc in the other. 



When the diameter of the hole was about *25 cm. the ratio 

 of the potential of the tube with the solid disc to that of the tube 

 with the perforated one was about 5:1. With smaller apertures 

 the ratio was smaller, and with larger apertures greater. Approxi- 

 mately, between 01 and 0'5 cm. the potential was inversely as the 

 size of the hole but exact figures could not be obtained owing to 

 the difficulty of securing the same degree of exhaustion, after 

 changing the discs in the tube. 



As it seemed likely that the disc acted as a secondary 

 cathode*, the effect of having a second disc in the tube was 

 next tried. The electrodes were of aluminium and the discs 

 of iron. 



The tube was exhausted and one disc moved up close to the 

 cathode, and the potential compared with that of a standard tube 

 exhausted at the same time. The second disc was then moved up 

 close to the first, and the potential was then found to be increased 

 to nearly twice as much as with the one disc. 



The rise in potential caused by the approach of the second 

 disc to the first was of the same character as that seen when using 

 one disc only, viz., a maximum occurred just a little way from the 

 first disc, i.e. the first disc behaved as though it were a cathode 

 itself f. 



The greater part of the observations were made when the tube 

 had been filled with air before exhaustion, but a number of ob- 

 servations were also taken after filling the tube with hydrogen, or 

 carbonic acid, or nitrogen. The results were practically the same 

 as with air. 



* Goldstein has shown (Phil. Mag. 10, PL 173) that when a discharge passes 

 through a constriction in a vacuum tube, the constriction acts as a secondary 

 cathode, and also that " the cathode itself may be replaced by a system of small 

 holes or closely packed pores congruent with the cathode." 



f Goldstein, working with movable cathodes and secondary cathodes with 

 variable apertures (Phil. Mag. 14, PI. 366) showed that the number of stratifications 

 depended upon the aperture area, the smaller the aperture the fewer the number of 

 stratifications. 



