182 



Prof. R. W. Wood and Mr. S. Okano on 



below the filament and close to it was a very thin film of 

 mica, which stopped the electrons but transmitted the light. 

 The sodium was placed below the filament and vaporized by 

 a small bunsen flame. The cathode end of the tube was 



Fig. 3. 



CLASS 



wrapped with black cloth, and as the sloping walls of the 

 glass tube reflected no light in the direction from which 

 observations were made, the background was practically 

 black. 



In this experiment the steel tube was made the anode, and 

 with a potential difference of 0'5V. between it and the 

 cathode, the yellow glow of the sodium vapour appeared 

 above the mica plate but not below it. This indicated that 

 the light from the cathode played no part in the production 

 of the phenomenon. We also tried illuminating the tube 

 shown in fig. 1 with a concentrated beam from the arc, but 

 no difference in the brilliancy of the D line or the potential 

 at which it appeared could be detected. 



There remained apparently only the possibility that the 

 effects might be due in part to a contact difference of 

 potential. We made a number of experiments to test this 

 point, using various materials (copper, platinum, tungsten, 

 &c.) as anodes, and obtaining always the value 0*5 V. The 

 sodium vapour, however, usually condenses on the anode, 

 making the experiment inconclusive. 



In one case, to prevent the condensation, we mounted a 

 tungsten anode wire along the axis of the incandescent spiral 

 cathode wire. This gave a value of 1'4 Y. for the minimum 

 potential ; but we feel certain that the higher value was due 

 to the deflexion of the electrons away from the anode by the 

 magnetic field of the spiral. 



The most conclusive experiment was made with a cathode 

 of the form indicated by fig. 4. The portions C are of copper 

 wire, while the spiral and loop (W and W) are of 5 mil. 

 tungsten wire. 



In this case we are not using an auxiliary electrode, and 



