592 



SCIENCE. 



[N. S. Vol. XIV. No. 355. 



The results led to a more general investiga- 

 tion of the drop of potential at the elec- 

 trodes. 



A cylindrical vacuum-tube, with disk 

 electrodes (perpendicular to its axis) in- 

 sulated so as to force the discharge directly 

 across the space between them, and nitrogen 

 were used. The potential of the gas at any 

 position was obtained by a movable wire. 



As the electrodes approach each other the 

 drop at the anode remains constant so long 

 as there is an extended positive column. 

 With the anode passing through the Fara- 

 day dark space its ' drop ' rises to a maxi- 

 mum and then falls to zero in the negative 

 glow, but rises again rapidly as the anode 

 moves into and through the cathode dark 

 space. The cathode drop remaining con- 

 stant until the anode reaches the cathode 

 dark space begins then to increase with 

 great rapidity. During these changes the 

 conductivity of the gas does not vary. 



These results are explained on the hy- 

 pothesis that the impact velocity of the 

 discharging ion tends to prevent charge 

 from being transmitted to the electrode ; 

 that to give up its charge the ion must 

 first come to rest. 



Other Observations. — At the cathode the 

 drop may be expressed by the equation 



where i is the current density ; p, the gas 

 pressure; C, a constant approximating in 

 value the cathode drop, as measured by 

 Warburg ; c, a constant depending on the 

 nature of the gas, but independent of the 

 metal used as cathode ; and a, a constant. 

 At the anode the drop appears to vary ac- 

 cording to a similar law, where c is about 

 one thirty-fifth its magnitude at the ca- 

 thode. 



The great difference in magnitude of the 

 drops at the two electrodes may be ex- 

 plained oa the hypothesis that the positive 

 ion is of much greater mass than the nega- 



tive and hence (for the same impact veloc- 

 ity), its elastic reaction baing greater, the 

 greater resistance to its discharge. 



The difference in the drop occasioned by 

 the use of different metals may be ex- 

 plained by their contact- potentials. Of two 

 metals, that possessing the greater attrac- 

 tion for negative electricity is found to pos- 

 sess the lower drop as anode, apparently by 

 its attraction for the charge on the ion aid- 

 ing in overcoming the elastic reaction of 

 the impinging ion. The same applies to 

 the cathode. (To be printed in the Phil. 

 Mag.) 



13. ' The Influence of Temperature upon 

 the Photo-electric Effect ' : John Zeleny, 

 University of Minnesota. 



Ultra-violet light impinging upon nega- 

 tively electrified bodies dissipates their 

 charge. The electricity is supposed to be 

 carried away by ions which are formed at 

 the surface of the body by the rapid ab- 

 sorption of certain of the waves of the in- 

 cident light. A study of the influence of 

 temperature upon the effect was made to 

 see if it would throw any light upon the 

 question as to the relative parts played in 

 the phenomenon by the material of the 

 body and by the occluded gases. The 

 charged body which was experimented upon 

 was a platinum wire that could be heated 

 by sending an electric current through it. 

 The light used was obtained from an elec- 

 tric spark produced between two zinc rods 

 by an induction coil. The results obtained 

 are somewhat complicated. As the tem- 

 perature was gradually increased to about 

 200° C, it was found that the rate at which 

 the negative electricity was discharged di- 

 minished, but for still higher temperatures 

 it increased again, so that at about 700° C, 

 it was nearly three times its value at room 

 temperature. The rate of discharge at cer- 

 tain temperatures depends considerably 

 upon the immediate previous history of the 

 wire, being much larger for a given temper- 



