the Ionizing Potential of Sodium Vapour. 179 



The cathode was a spiral of fine tungsten wire (5 mils, 

 diameter) attached to two stout copper wires b and c, the 

 ends of which were split and then squeezed together. The 

 sodium was placed in d, and after exhaustion of the bulb, 

 distilled into A, after which it was sealed off. The anode 

 a was of platinum. The electrodes were sealed in with 

 sealing-wax, the lateral tubes through which the cathode 

 wires pass having a bore only slightly larger than the diameter 

 of the wire to prevent the diffusion and condensation of 

 sodium vapour on their walls. There is bound to be a loss 

 of the vapour through the tube leading to the pump, but this 

 cannot be helped ; for if we seal off the tube from the pump 

 the vacuum is rapidly impaired by the liberation of hydrogen 

 from the sodium. Experiments on the resonance radiation 

 of sodium vapour have shown that it is practically impossible 

 to remove all of the hydrogen from the metal by repeated 

 distillation in vacuo, for the metallic vapour carries down 

 hydrogen with it, when it condenses on the wall. The 

 tungsten wire was heated by the current from a small 

 storage-battery, and the potential applied as shown in fig. 1, 

 by a potentiometer, consisting of a wire of 10 ohms resistance 

 stretched on a metre stick, and from one to three or more 

 dry cells. 



In our first experiment we started out with an applied 

 potential of 6 volts between the cathode spiral and the 

 anode wire. The tungsten was raised to normal incan- 

 descence, and the bulb heated by brushing its surface as 

 uniformly and rapidly as possible with the flame of a Meker 

 burner. A bright yellowish glow appeared around the 

 anode, and the spectroscope showed, in addition to the 

 D lines, the red -and green lines of the subordinate series. 

 On diminishing the applied potential we found that the 

 subordinate series faded gradually and disappeared entirely 

 at 2*34 Y. The D lines, however, remained bright. On still 

 further reducing the potential we found that the yellow glow 

 at the anode wire disappeared at a potential of 0*5 V., though 

 we could still see the D lines in the spectroscope at still 

 lower potentials, or even with the connexion at A broken. 



This we subsequently found was duei to the fact that the 

 potential drop along the tungsten filament was sufficient to 

 cause a glow around the positive leading-in wire, and some 

 of this light was reflected into the spectroscope from the 

 wall of the tube. If the potential difference between the 

 terminals of the filament exceeds about three volts, arcing 

 takes place when the bulb is heated, without the application 

 of any potential between a and c, the yellow glow filling the 



