October 18, 1901.] 



SCIENCE. 



591 



current. The platinum plate was charged 

 to a known potential by means of a battery 

 and the current between the platinum plate 

 and a parallel copper plate observed by 

 means of a sensitive galvanometer. The 

 rate of discharge of positive and negative 

 electrification was examined under varying 

 conditions of temperature, distauce and 

 P. D. between the plates, and compared 

 with the theoretical results deduced from 

 the ionization theory of gases. 



At a temperature of a dull red heat posi- 

 tive ions are produced at surface of the 

 platinum plate, which travel to the opposite 

 plate through the gas, if the platinum is 

 positively charged. The movement of the 

 ions disturbs potential gradient between 

 plates and at high temperatures the slope 

 of potential near hot plate is extremely 

 small. 



Let i ~ current per sq. cm. through the gas. 



n = number of ions per cc. at any point distant 



X from platinum plate. 

 F=P. D. between the plates. 

 K= velocity of positive ion for unit potential 

 gradient. 

 e= charge on ion. 

 d ^= distance between the plates. 



Assuming the potential gradient zero at 

 the platinum surface, it can readily be 

 shown that 



* " 3277^3 



The current is thus independent of num- 

 ber of ions produced, i. e., the temperature 

 of the plate, providing it exceeds a certain 

 value. Experimentally it was found : 



1. For a distance of 2 cm. and under, 

 the current at first increased rapidly with 

 rise of temperature up to a certain point, 

 and then more slowly. For distances of 

 from 3 to 8 cm. the current rose to a 

 maximum, and then steadily decreased with 

 rise of temperature. 



2 . The maximum current varied approxi- 

 mately as square of the P. D. 



3. Current diminished more rapidly than 

 1 



The differences observed in (1) and (3) 

 between experiment and theory are due to 

 (a) decrease of velocity of ions with distance 

 from heated platinum plate, (6) presence of 

 some very slow-moving carriers produced 

 at platinum surface at high temperatures. 



The velocity of the ions was measured 

 by a direct method depending on the use 

 of an alternating P. D. of known frequency. 

 The method was similar to that used pre- 

 viously for determination of velocity of 

 ions produced by ultra-violet light (Proc. 

 Roy. Soc, 1898). It was found that the 

 velocity of ions was not a constant, but 

 varied between wide limits. 



This is probably due to difference in size 

 of ions in consequence of their varying 

 power of forming clusters of molecules 

 round them. 



The following table gives results obtained 

 for different distances between plates. Un- 

 der heading ' calculated velocity ' are given 

 values of K calculated from equation (1) 

 by measuring maximum current i for given 

 distance and voltage. 



12. ' On Conditions controlling the Drop 

 of Potential at the Electrodes in Vacuum- 

 Tube Discharge ' : Clarence A. Skinner, 

 University of Nebraska. 



With the passage of electricity through 

 rarefied gases, it requires, relatively, a very 

 high P. D. to force the discharge directly 

 across the space between the electrodes 

 when these are brought within a certain 

 small distance apart. To locate at what 

 part of the path the potential increases'was 

 the primary object of this investigation. 



