Mr. J. J. E. Durack on Lenard Rays. 31 



deflexion of the rays, the electromagnet M M being used for 

 that purpose. 



The camera consisted of two brass cylinders about 1 cm. 

 long and 3 cm. diameter, and fitted nicely in the observation- 

 tube. One of the two brass cylinders fitted tightly into the 

 other so as to shut out all stray light ; the photographic 

 plate pp was stuck on to the flat end of one of these tubes, 

 while the other had a hole 2*2 mm. bored in the centre of the 

 end and had a neck 1/5 cm. long attached, the end of the 

 neck being pushed right up to the window. 



An induction-coil with an Apps interrupter was used to 

 work the discharge-tube, the tube and coil being inclosed in a 

 metal box ; the anode, water-jacket, and box were connected to 

 earth. The observation-tube projected through a hole in the 

 side of the box, so that the camera could be easily placed in 

 position for taking photographs ; sheets of lead screened off 

 stray light and Rontgen rays coining from the discharge-tube 

 directly. 



The terminals of the secondary coil were provided with 

 spheres of 1 cm. diameter, so that a rough measurement of the 

 potential-difference required to send the discharge could be 

 made, the distance between the spheres being adjusted till the 

 coil seemed to spark at the terminals and discharge through 

 the tube about equally. 



In making the observations, the discharge-tube was pumped 

 down to a pressure of about 60 x *0002 mm., the furthest 

 limit at which the Lenard rays produced an image on the 

 plate *. The equivalent spark-length (measured between the 

 terminals of the coil) was about 4*5 cm. for this pressure. 



The camera was placed in position, and the observation-tube 

 exhausted to a pressure of about 5 mm. of mercury ; the coil 

 was then turned on, and an exposure varying from 5 to 

 15 seconds given to the plate with the magnet MM "on"; 

 the camera was then removed and the centre marked to 

 obtain the position of an undeflected spot. After developing, 

 the deflexion of the ions could be measured. 



The discharge always increased the pressure in the discharge- 

 tube, gas being given oft' probably by the brass struck by the 

 cathode-rays, the potential required to produce the discharge 

 was then less, and the Lenard rays " softer," than before. 



The cmiera was then replaced and another photograph 

 taken. In this way a series was obtained throughout the 

 range at which Lenard rays were produced, this range being 

 from spark-lengths of about 4*5 cm. to 3 cm. 



* This limit depends on the form of discharge-tube and the working 

 of the coil. 



