Spectra of Helium, Sfc. in the Ultra-Violet. 289 



that it can, i£ necessary, be readily cut off, repaired, and 

 sealed on again. At A± is a copper plate anode *5 cm. wide 

 and 2 cm. long, at A 2 a copper wire anode 1 cm. long. The 

 anode A 1 was placed in the position which it occupies in 

 order that the radiation which might come from the copper 

 itself under high velocity impacts could be investigated. 

 The extra anode is in any case a valuable adjunct since, 

 when charged to a small negative potential, it permits the 

 highest vacua to be measured and continuously followed if 

 changing by the fluctuations through a galvanometer of the 

 positive ion current * collected by it. These parts are con- 

 tained in a vertical cylinder 8 cm. high, the cross-section of 

 which is shown in the figure. At P are two copper plates 

 '8 cm. by *5 cm., fitted into a rectangular neck connecting 

 this first cylinder with the rounded quartz box at the left. 

 These plates are used to prevent the passage of any ions or 

 electrons from the discharge-chamber into the quartz box. In 

 this box is a copper cylinder 2 cm. high by 2 cm. in diameter, 

 provided with a bottom and lid and divided internally into 

 four compartments as shown. At the front, facing the aper- 

 ture between the plates P, is a slit *8 cm. high by *2 cm. 

 across. At T is a vertical copper target 1*2 cm. high by 

 •2 cm. wide. At S x and S 2 are bevelled slits in copper 

 partitions. These slits are *7 cm. high by *16 cm. wide and 

 are arranged to lie exactly on the circumference of a circle 

 the radius of which, taken at the centres of target and slits, 

 is exactly *5 cm. Beyond Si is a curved copper plate or cup 

 contained in an insulator, I, moulded of one piece of quartz 

 and provided with a leading-out tube which projects through 

 a hole in the side of the copper cylinder. B is a quartz prop 

 or strut to keep the cylinder in place. G is a leading-out 

 wire connected to the cylinder. M is the outlet tube for 

 attaching to the pump and auxiliary apparatus. The quartz 

 apparatus was made by Mr. Reynolds of the Silica Syndicate. 

 The apparatus was supported inside by a double coil of the 

 Helmholtz type, specially designed to receive it, in such a 

 way that the copper cylinder lay at the centre of the coil 

 system. These coils were 8 - 25 cm. in radius and 8'25 cm. 

 apart, and so wound that fields up to 100 lines could be 

 maintained in the space between them. The dimensions 

 were such that one ampere gave a field of 20 gauss. The 

 calculated values of the field were carefully checked with a 

 fluxmeter and found correct within the limits of error of the 

 fluxmeter. The field w r as studied with a special small test- 

 coil on the fluxmeter and found remarkably uniform both 



* Cf. Buckley, Proc. Nat. Acad, of Sci. vol. ii. p. 683 (1916). 



