The spectrum of the discharge from a glowing cathode, etc. 501 
The spectrum of the discharge from a glowing lime cathode in 
mercury vapour. By F. Horton, M.A., St John’s College. 
[Read 10 February 1908.] 
While investigating the discharge of negative electricity from 
a Wehnelt cathode in various gases at different pressures, it was 
observed that the colour of the luminous discharge in certain gases 
altered as the gas pressure in the apparatus or the temperature of 
the cathode was varied. The experiments described in the present 
paper were commenced with the object of investigating the spectrum 
of the luminous discharge under different circumstances. 
The special form of discharge tube* used is shown diagrammati- 
cally in Fig. 1. The cathode A is a strip 
of platinum foil about 2 centimetres long 
and 2 millimetres wide, covered with lime, 
or, in later experiments, with a mixture of 
lime and baryta. The ends of the platinum 
strip were welded on to two stout platinum 
wires through which the heating current fff 
was conducted. These were sealed through £TC 
a ground glass stopper which was made air- 
tight by surrounding it with mercury. The 
anode B was of aluminium. The centre 
part of the tube was of small diameter 
(about 2 millimetres) and here the brilliancy 
of the positive column was enormously in- 
creased. 
With a glowing lime cathode a luminous 
discharge can be obtained — at certain gas 
pressures — with a difference of potential of 12 or 14 volts. In 
the present case much larger potential differences were necessary 
to maintain a bright discharge on account of the constriction in 
the discharge tube. The difference of potential used was usually 
between 120 and 400 volts. The difficulty was to get the discharge 
to start. When once started a smaller voltage would suffice to 
maintain the current. For this reason the discharge was generally 
started with an induction coil. 
Commencing with the cathode hot and the tube filled with air 
and gradually reducing the gas pressure in the apparatus, the 
* The Author wishes to record his indebtedness to the Government Grant 
Committee for Scientific Investigations for the means of purchasing several of the 
tubes used in these experiments. 
33—2 
