Mr. J. J. E. Durack on Lenard Rays. 37 



When the Lenard rays leave the window they strike the 

 plate and give it a negative charge ; on the way, however, 

 they ionize the gas between the plates. By putting on a 

 saturating e.m.f. we may collect all the ions formed. 



Let p be the pressure of the gas in the bell-jar ; d the 

 distance from the plate to the window ; a v the number of 

 pairs of ions produced by one Lenard-ray ion in travelling 

 1 cm. at the pressure p ; N the number of ions leaving the 

 window per second ; e the charge on an ion. 



The current due to the Lenard rays is 



-No* 

 (calling a motion of positive ions from the window to 

 the plate the positive direction of current). 



If a saturating e.m.f. is applied, the plate being charged 

 positively, then all the negative ions formed move to the plate 

 so that the current is 



7i=— NoS-NoV* 



=-n hi +«;</), 



assuming that the Lenard rays travel straight across from the 

 window to the plate. 



Now if the plate be charged negatively we have, since we 

 now collect the positive ions, 



hence 



7l + y 2 =-2N ', 



which gives the Lenard-ray current. 

 Also 



hence 



7i-72=— 2« P £NV, 



71 + 72 



which gives <x p . 



We may, if we choose, pump the air in the bell-jar down 

 to a very low pressure, so that a v is small compared with 1, 

 and then we should obtain the current carried by the Lenard 

 rays no matter what is the direction of the field or its value. 



Curve II. (p. 38) has been drawn in this way: the abscissae 

 represent the pressure in the discharge-tube, and the ordinates 

 the Lenard-ray current. The points shown by the circles 

 and © have been plotted from readings taken with no e.m.f. 

 applied to the gas, so that the plate pp was originally at 

 the same potential as the zinc plate, and gradually charged 

 up negatively as the Lenard rays fell on it. If the air is 

 appreciably ionized between the plates the charging plate will 



