36 



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



and the cylindrical anode omitted, the window serving as 

 anode. With the new tube the intensity of the Lenard rays 

 was increased and also the amount o£ gas given off by the 

 tube during a given time of discharge. The amount of gas 

 given off was found to increase with the intensity of the rays. 

 Hence when it is desirable to keep the pressure pretty constant 

 and very intense Lenard rays are not required, the cylindrical 

 anode will be found useful. 



The brass tube and water-jacket were soldered to a zinc 

 plate about 9 cm. square and 5 mm. thick, a bevelled hole 

 was bored in the zinc plate opposite the cathode, and the 

 aluminium window stuck on with soft wax as before. 



A bel'-jar was fastened to the zinc plate and supported 

 an aluminium phite pp which was parallel to the zinc plate 

 and 2 mm. from it ; aluminium was used for this plate to make 

 the secondary ionization, if any exists, as small as possible. 

 The plate, 7 cm. in diameter and 2 mm. thick, was supported 

 by a rod passing through the ebonite stopper (shown by 

 the shading in fig. 3) in the mouth of the jar ; the rod was 

 then connected to an electrometer and screened from electro- 

 static effects, as in former experiments. The jar was connected 

 to a second pump and McLeod gauge *. 



* The McLeod gauge used lias worked so satisfactorily that it is 

 perhaps worth while describing it. 



In order to measure small pressures with the McLeod gauge it is 

 necessary to have 



(i.) a large ratio of volumes. 



To avoid capillary errors one must have 



(ii.) tubing of the same bore inside at the zero and outside where 

 differences in level are measured. 



To avoid having to wait a long time for the pres- 

 sure to equalize in the gauge and apparatus one must 

 have 



(iii.) a tube of large bore connecting the bulb of 

 the gauge with the rest of the apparatus. 



These three conditions can be realized by having 

 the bulb of the gauge and the connecting tubes large, 

 but this is objectionable, since it adds so much capacity 

 to the apparatus and consequently requires so much 

 more work in pumping. 



By using capillary tubes inside the gauge and out- 

 side, we can keep the bulb small and at the same time 

 have conditions (i. x . and (ii.) fulfilled ; condition (iii.) 

 is then very easily fulfilled by having a wide tube 

 lor the air to pass through in parallel with the 

 capillary as shown in tig. 4 ; by blowing a second 

 small bulb above the first the range of pressures that 

 can be measured by the gauge is greatly increased. 

 With the gauge used in these experiments, pressures 

 from 30 mm. to - 001 mm. can be easily measured, 

 assuming, of course, Boyle's Law to be true through- 

 out this range. 



Fiff. 4. 



