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



The curves have only been carried up to the point where 

 the Lenard rays reach a maximum, past this the presence of 

 Eontgen rays prevents one from giving a proper interpreta- 

 tion to the values of the saturation-currents. 



We can now find a p for any type of radiation (indicated 

 by the pressure in the discharge-tube). 1 have selected the 

 type given out when the pressure was 105 x -0002 mm., as 

 this is well removed from the point where Eontgen rays are 

 given off, and is considered likely to be most accurate. 



Calculating a v in this way we get the following numbers: — 



ssure mm. 



10-7 



ap. 

 51 



a p Jp. 



•48 



9-4 



4-0 



•43 



5-2 



2-0 



•39 



The mean of these three values of a P /p is '43, that is the 

 number of pairs of ions one Lenard- ray ion makes in 

 travelling 1 cm. of air at a pressure of 1 mm. of mercury 

 is -43. 



Professor Townsend* in his experiments found 21 for the 

 number of ionizing collisions one ion (produced in air by 

 Eontgen rays) makes under the same conditions, or about 

 50 times the number obtained here. 



This difference I was unable to account for by any experi- 

 mental errors, but before seeking an explanation it was 

 thought desirable to confirm the result. I therefore carried 

 out further experiments, using a D'Arsonval galvanometer 

 instead of an electrometer to measure the currents. 



The arrangement was very simple; the cells, plates, and 

 galvanometer were connected in series and the window 

 always to earth. 



Measurements of the Lenard-ray current were first made 

 by pumping the bell-jar down to a pressure of about "01 mm. 

 It was found to make no difference whether a field + 10' 5, 

 — 10*5, or volts per cm. was applied to the gas; the 

 variation of the current was also in agreement with the 

 electrometer observations. 



a p was next determined from-, the same formula as before 

 for two pressures, as shown in the table : — 



Bell-jar Field. , 



pressure, mm. Volts per cm. ap - a p/P- 



10-1 500 7-5 -74 



27-6 . 625 155 -5(3 



* Townsend, Phil. Mag. Feb. 1901. 



