May 21, 1897.] 



SCIENCE. 



793 



The rate of discharge was found by noting 

 the fall in potential when the rays were 

 allowed to strike the charged body for a 

 few seconds, usually four, and also by noting 

 the fall in potential when the zinc plate 

 was connected through a very high resist- 

 ance to a constant source of potential and 

 the rays were allowed to strike the plate 

 ■continuously. The high resistance was 

 secured by winding two wires about each 

 other which were covered with cotton insu- 

 lation. The resistance through the insula- 

 tion was about 5,000 megohms. The re- 

 sults from these two methods were in as 



2J. The former obeys fairly well the law 

 of Benoist and Hurmusezcu, that the rate 

 of discharge is proportional to the square 

 root of the density. This agreement is 

 shown by plotting the rates of discharge as 

 given by them. The curve thus found is 

 practically the same as that for 40 volts. 

 The curve for 2\ volts obeys an entirely dif- 

 ferent law, having, in fact, a maximum for 

 about 200 mm. pressure. This eiiect seems 

 less improbable when we remember that 

 the discharging effect for the ultra-violet 

 reaches a maximum at about 200 mm. 

 It is known that a metal tends to assume a 



Potentials i; 



Fig. 2. 



i^olti 



fully complete concordance as could be ex- 

 pected from the nature of the work. 



That which had the most effect upon the 

 relation between the rate of discharge and 

 the density of the surrounding gas was the 

 potential at which the discharge took place. 

 Curves are given in Fig. 1 for the rates of 

 discharge when the initial potentials 

 were 2 J, 5, 10, etc., up to 40 volts. The 

 ordinates indicate the number of volts that 

 the charged plate fell per sec, and 

 the abscissas indicate pressure in mm. 

 of mercury. The difference in the essential 

 character of these curves can be seen by 

 comparing that for 40 volts with that for 



final potential different from zero when 

 X-rays strike upon it. It was thought at 

 first that this effect might be complicating 

 the phenomena that were being studied, but 

 care was taken that the plate from which 

 the discharge took place was surrounded by 

 a grounded plate of the same metal, and 

 the final potential when the plate was dis- 

 connected from the source of the E. M. F. 

 and the X-rays were allowed to strike upon 

 it continuously varied so little from zero as 

 to be entirely negligible. 



It is evident that another set of curves 

 could be plotted showing the relation be- 

 tween the rate of discharge and the E. M. F. 



