624 Prof. Skinner on the Drop of Potential 



must, then, be constant ; and therefore, in consequence, the 

 number in unit volume proportional to the current. 



The drop at the anode increases with gas-pressure, that is, 

 it increases with the potential gradient in the positive column. 

 In other words, the drop at the anode increases with the force 

 with which the negative ion is driven against the anode. 

 This increase is not as rapid as that of the gradient*, due 

 partly at least to; the decreased mobility of the ion with in- 

 creased gas-pressure. 



Two casual observations made by the writer add strength 

 to the conclusion that the drop increases due to the increased 

 force with which the ion is driven up to the anode ; namely, 

 with increased current the drop at the anode decreases by a 

 very slight quantity, simultaneous (and comparable in magni- 

 tude) with the decrease in the potential gradient in the 

 positive column observed by Herz f; also, as the luminous 

 discharge changes to the dark, accompanied by an increased 

 gradient near the anode J, the drop at the anode increases 

 (from a drop of about 25 to one of 40 volts, for instance). 



At the Cathode.— War burg § and others have studied the 

 drop of potential at a cathode in form of a wire placed in the 

 axis of the discharge- tube. They have found the drop to be 

 constant for all gas-pressures so long as the discharge is not 

 restricted to a definite area; it increases with the current, 

 however, when restricted. G. C. Schmidt || has observed 

 with a similar cathode that the drop for a restricted discharge 

 is a linear function, of the current.. 



If the drop be due to the velocity of impact of the positive 

 ions on the electrodes, we should expect somewhat different 

 results with a disk electrode discharging along the axis of 

 the tube. 



There is a distinction between the phenomena designated 

 as " anode " and as " cathode " drop, which must first be 

 noted. The " anode " drop is an immediate drop of 

 potential between anode and gas, so far at least as experiment 

 can determine. The "cathode drop," as defined and measured 

 by Warburg, is the difference of potential between the 'cathode 

 and the highly conductive negative glow. It is the drop 

 between metal and gas pins the drop in the cathode dark 

 space. 



'* Compare potential gradient at different gas-pressures (Phil. Mag. 

 Dec. 1900) with drop at anode at different gas-pressures ( Wied. Ann. lviii.). 

 t A. Herz,, Wied. Ann* liv. p. 244 (1895). 

 X Phil. Mag. /. c. Tables II. and III. 



$ E. Warburg, Wied. Ann. xxxi. p. 545 (1887) ; xl. p. 1 (1890). 

 II G. C. Schmidt, Ann. d. Phys. i. p. 625 (1900). 



