418 Dr Frank Horton, The ionisation produced by certain 
be started again by earthing the parallel plate anodes of the dis- 
charge tube by touching with the finger. 
In a previous paper* the author has drawn attention to the 
different appearances of the luminosity obtained with a hot 
lime-covered platinum cathode. Under certain conditions of 
temperature, gas-pressure, and potential difference, the luminosity 
surrounds the anode only, and this luminosity always appears 
gradually when the conditions are slowly changed (e.g. the tem- 
perature slowly raised), and there is no sudden increase in the 
current passing through the tube. If the temperature is still 
further raised, or the potential difference increased, a point is” 
reached when the luminosity suddenly leaves the anode and 
surrounds the cathode, and then, in some cases, luminous pencils 
of cathode rays are seen. These effects were also obtained with 
the lime-covered Nernst filament. With the temperature at 
1500° C., and the gas-pressure 0001 mm., a current of 28 milli- 
amperes was obtained. In this case 210 volts were put on from 
the high potential battery, but the magnitude of the discharge 
current was limited by a wire resistance of about 5500 ohms 
included in the circuit. The passage of the discharge greatly 
increased the temperature of the filament, which became much 
hotter near its lower end than at any other point. The gas- 
pressure in the apparatus was so low that no definite pencil of 
cathode rays could be seen, but there can be little doubt that 
the discharge was taking place mainly from the hottest point . 
of the filament. The whole bulb was filled with a bluish glow 
(Hg vapour and CO), which could be best seen by looking in such 
a direction that the light from the filament itself was screened 
by one of the platinum plates forming the anode. The applied 
P.D. was lowered to 40 volts and the filament became cooler and 
equally luminous all over. The glow could still be seen in the 
bulb, but as the current was now very much reduced, this 
luminosity was doubtless an “anode glow.” With 40 volts applied 
through a resistance of 5500 ohms to the terminals of the dis- 
charge tube, the following measurements of the thermionic current 
at different temperatures were taken : 
Temperature ( C.) 1510 1626 1698 1780 
3°3 4:3 D3 
bo 
(SS) 
Current (milli-amps. ) 
These currents do not increase so quickly with the temperature 
as is usually the case with the negative emission from glowing 
* Phil. Trans. Roy. Soc. A. ccvit. p. 149, 1907. 
