612 Dr. F. W. Aston on the 



The Discharge Tube. 



Fig. 1 is a rough diagram of the present arrangement. 

 The discharge-tube B is an ordinary X-ray bulb 20 em. 

 in diameter. The anode A is of aluminium wire 3 mm. 



=3 



thick surrounded concentrically by an insulated aluminium 

 tube 7 mm. wide to protect the glass walls, as in the Lodge 

 valve. 



The aluminium cathode C, 2'5 cm. wide, is concave, 

 about 8 cm. radius of curvature, and is placed just in 

 the neck of the bulb — this shape and position having 

 been adopted after a short preliminary research *. In 

 order to protect the opposite end of the bulb, which would 

 be immediately melted by the very concentrated beam of 

 cathode rays, a silica bulb D about 12 mm. diameter is 

 mounted as indicated. The use of silica as an anticathode 

 was suggested by Prof. Lindemann, and has the great 

 advantage of cutting down the production of undesirable 

 X rays to a minimum. 



The discharge is maintained by means of a large induction- 

 coil actuated by a mercury coal-gas break ; about 100 to 

 150 watts are passed through the primary, and the bulb is 

 arranged to take from - 5 to 1 milliampere at potentials 

 ranging from 20,000 to 50,000 volts. Owing to the par- 

 ticular shape and position of the electrodes, especially 

 those of the anode, the bulb acts perfectly as its own 

 rectifier. 



The method of mounting the cathode will be readily 

 seen from fig. 2, which shows part of the apparatus in 

 greater detail. The neck of the bulb is ground off short 

 and cemented with wax to the flat brass collar E, which 

 forms the mouth of an annular space between a wide 

 outer tube F and the inner tube carrying the cathode. 



* F. W. A., Proc. Carnb. Phil. Soc. xix. p. 317. 



