Potential at the Electrode* in Vacuum-tube Discharge. 497 



sheathed in glass tubing. An inserted wire W was used to 

 obtain the cathode drop. A guttapercha screen S parallel 

 to and slightly larger than the cathode was mounted on a 

 glass tube entering through a barometer column. This screen 

 could be shifted to any desired distance from the cathode 

 and its effect on the drop noted. It was found in this case, 

 however, that the negative glow was simply reflected by the 

 screen without any change in the cathode drop. Had the 

 screen been replaced by an anode of the same form the drop 

 would have increased greatly, as previously shown. Though 

 this form of tube was evidently useless for making the 

 desired study, the above experiment is important in that it 

 shows that the increased drop at the cathode when the anode 

 is brought near * is not, as might be supposed, due to an 

 obstruction. 



Tubes as shown in fig. 5 were then tried. To vary the 



conditions three of different diameters (11, 7, and 5*5 mm. 

 resp.) were used. In each a steel cathode (diam. 2 mm., 

 exposed length b* mm.) passed through a guttapercha cylin- 

 der, and was capped by a piece of the same material F. The 

 anode was a steel disk (fitting the tube) placed practically in 

 the negative glow, where its drop is a vanishing quantity. 

 The difference of potential between the terminals of the tube 

 gave, therefore, the cathode drop. The results obtained were 

 puzzling, and it was thought that the three tubes were not 

 operating under similar conditions. They were taken apart, 

 cleaned, and reconstructed with some slight modification in 

 design, and were tried again with similar results, which are 

 given in Table II. and plotted in fig. 6 (p. 199) . The curves 

 show at various gas pressures the variation of cathode drop 

 with current. 



At a pressure 3"6 mm. the drop in the two larger tubes 



was practically the same, while the smaller gave a higher 



value. This was to be expected, for the walls of the smaller 



tube were already confining the negative glow. At 1*2 mm. 



* See first paper. 



