230 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



10 centimeters long. When turned, the paraffin is easily removed by 

 heat and solvents. 



THE DISCHARGE TUBE. 



Figure 2 is a rough diagram of the present arrangement. The dis- 

 charge tube B is an ordinary X-ray bulb 20 centimeters in diameter. 

 The anode A is of aluminium wire 3 millimeters thick surrounded 

 concentrically by an insulated aluminium tube 7 millimeters wide to 

 protect the glass walls, as in the Lodge valve. 



Fig. 2. — Diagram of the mass-spec troscope. 



The aluminium cathode C, 2.5 centimeters wide, is concave, about 

 8 centimeters radius of curvature, and is placed just in the neck of the 

 bulb, this shape and position having been adopted after a short pre- 

 liminary 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 millimeters diameter, is 

 mounted as indicated. The use of silica as an anticathode was sug- 



Fig. 3. — The discharge tube. 



gested by Professor 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 

 0.5 to 1 milliampere at potentials ranging from 20,000 to 50,0000 

 volts. Owing to the particular shape and position of the electrodes, 

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

 rectifier. 



8 Extracted from " The mass spectra of chemical elements," Philosophical Magazine and 

 Journal of Science, May, 1920, pp. 611-625. 



