924 Prof. Horton and Miss Davies on Electron Velocities 



long and 1 mm. wide, and was situated horizontally and with 

 its middle over the hole H (1*5 mm. in diameter) in the 

 centre of a platinum disk which nearly filled the cross- 

 section of the discharge chamber and served to screen off 

 the light of the glowing filament from the lower part of the 

 tube. Round the edges of the platinum disk was fixed 

 a cylindrical piece of fine platinum gauze G, about 1*5 cm. 

 long. The hole H was also covered with a small piece of 

 similar fine gauze so as to improve the electric screening 

 of the space below H, The anode A was a circular platinum 

 plate, 1 cm. in diameter, fixed horizontally just above the 

 level of the open end of the gauze as indicated in the figure. 

 The total length of the ionization chamber was about 2'5 cm., 

 so that it could be easily arranged between the pole pieces N 

 and S of an electromagnet, which, by producing a strong 

 magnetic field along the axis of the tube, concentrated the 

 stream of electrons passing through H into a parallel beam. 

 When the velocity of the electron stream Avas raised to a 

 suitable value, the luminosity thus concentrated along the 

 axis of the tube was very much brighter than the diffused 

 glow which filled most of the space when no magnetic field 

 was used. The spectrum of the luminosity was viewed by 

 means of a Hilger wave-length spectroscope, the slit of 

 which was arranged so as to be illuminated by the light 

 produced along the axis of the tube between the grid and 

 anode. It was screened by the platinum disk from the 

 direct light of the glowing filament. All the platinum 

 electrodes were boiled for several days in strong nitric acid 

 before being fitted into the apparatus, and the usual pre- 

 cautions were taken for ridding the glass walls of occluded 

 gases. 



In the diagram the connexions to the pump &c. are not 

 shown. The arrangements for circulating pare neon through 

 the apparatus during the observations were similar to those 

 described in the paper already referred to. The gas passed 

 from the storage bulb down a fine capillary tube to the 

 discharge chamber, which it entered from a U-tube con- 

 taming a little carbon and immersed in liquid air. Another 

 tube connected the apparatus to the pumping system. 



In some of the experiments which were made with this 

 apparatus, in addition to observing the spectrum of the 

 luminosity produced in the gas under different conditions, 

 the currents between the electrodes were measured at the 

 same time in order to ascertain whether sudden changes in 

 the spectrum of the gas were connected with changes in the 

 gas ionization. The currents were measured by means of a 



