38 



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



collect positive ions from the gas ; also if negative ions are 

 given off by the plate, as when ultra-violet light falls on a 

 zinc plate, these negative ions will leave the charging plate 

 with a positive charge. 



Curve II. 



Discharge-tube Pressure, mm. X '00018. 



Both these possible causes of error will make the Lenard- 

 ray current too small. 



Now let the leaking plate be charged positively : the nega- 

 tive ions are now collected from the gas, and this will make 

 the Lenard-ray current too large ; but the negative ions 

 which leave the plate, as in the case of ultra-violet light, 

 will be attracted back to it if the electric intensity is 

 sufficiently great, so that they will be without effect in this 

 case. The curve II. proves that neither of these possible sources 

 of error exists to any appreciable extent ; for, as has been 

 said, the two circles show the current as measured when the 

 plate is charging up negatively, the crosses X the current 

 when the plate was charged positively, a field of 630 volts 

 per cm. being applied to the gas, and all three series fit the 

 curve about equally well. r I he pressure in the bell-jar was 

 always less than '06 mm. 



We can, then, state that the current carried by the Lenard 

 rays at first increases as the pressure in the discharge-tube 

 decreases, and that then there is a sudden falling off in the 

 current, as we were led to suspect from the photographs. 



Having measured the current carried by the Lenard rays 



