Mass-Spectra of Chemical Elements. 



613 



The concentric position of the neck is assured by three 

 small ears of brass not shown. The wax joint is kept cool 

 by circulating water through the copper pipe shown in 

 section at G. 



Fig. 2. 



The gas to be analysed is admitted from the customary 

 fine leak into the annular space and so to the discbarge 

 by means of the side-tube attached to F shown in dotted 

 section at Q. Exhaustion is perlormed by a Gaede 

 mercury-pump through a similar tube on the opposite side. 

 The reason for this arrangement is that the space behind 

 the cathode is the only part of the discharge bulb in 

 which the gas is not raised to an extremely high potential. 

 If the inlet or outlet is anywhere in front of the cathode, 

 failing special guards, the discharge is certain to strike 

 to the pump or the gas reservoir. Such special guards 

 have been made in the past by means of dummy cathodes 

 in the bore of the tubes, but, notwithstanding the fact 

 that the gas can only reach the bulb by diffusion, the 

 present arrangement is far more satisfactory and has the 

 additional advantage of enabling the bulb to be dismounted 

 by breaking one joint only. • 



The Slit System. 



The centre of the cathode is pierced with a 3 mm. hole, 

 the back of which is coned out to fit one of the standard 

 slits Si*. The back of the cathode is turned a gas-tight fit 

 in the brass tube 2 cm. diameter carrying it, the other end 

 of which bears the brass plug H which is also coned and 

 fitted with the second slit S 2 . The two slits, which are 

 •05 mm. wide by 2 mm. long, can be accurately adjusted 

 parallel by means of their diffraction patterns. The space 

 between the slits, which are about 10 cm. apart, is kept 

 exhausted to the highest degree by the charcoal tube I x . 



* F. W. A.., Phil. Mag. xxxviii. Dec. 1919, p. 714, 



