98 _ Mr Watson, Some Eaperiments on the 
potential at pressures above the critical pressure is due to the fact | 
that a positive column of gradually increasing length has to be 
maintained, while below this pressure, the negative glow is 
crushed. Except at very high pressures, far above the critical 
pressure, no positive column appeared in any of my experiments, — 
and it has been shown that moving the anode up and down in the — 
negative glow has no appreciable effect upon the voltage unless — 
the dark space is reached. Moreover, the present experiments 
tend to show that the cathode fall is not equal to the minimum 
spark potential at least for helium and neon, for the voltage across 
the tube invariably fell when the current started. 
Assuming the above mentioned results, it follows that the 
cathode fall is equal to the minimum voltage at which the current 
will pass when once started, and can be easily measured by 
increasing the resistance in the circuit until the discharge ceases, 
and measuring the voltage. This is very constant over a wide 
range of pressure, but varies greatly as already mentioned with 
the purity of the gas, and also with the material of the cathode. 
In what follows, whenever cathode fall is mentioned, it is to be 
understood that it is measured by the mmimum running voltage. — 
In the case of the tube already mentioned, with aluminium 
cathode, the value of this quantity was 164 volts for helium, and 
170 for neon, figures which are 20 and 30 volts respectively lower 
than the minimum spark potentials. It is perhaps worthy of note 
that they correspond approximately to the calculated fall in 
potential in the Aston dark space for these gases, namely, 20 and — 
40 volts (Roy. Soc. Proc. A. 1907, 80, p. 45; 1911, 84, p. 535; 
1912, 86, p. 172). A short consideration of the theory put 
forward in these papers to explain the existence of this dark space, 
will show that, if in a gas with no Aston dark space the cathode 
fall is equal to the minimum spark potential, then in the gases 
which do possess it, the cathode fall should be less than the 
minimum spark potential by the amount of the voltage drop in 
this region. 
Experiments with other Electrodes. 
A number of other experimental tubes were constructed, some 
of which were as follows. . 
1. A short tube about 4 cm. in diameter with a plane circular 
aluminium electrode touching the sides, and about 4 cm. from the 
rounded end which contained sodium potassium alloy introduced 
after complete evacuation of the tube. Some of the alloy was 
shaken on to the aluminium to which it adhered. A series of 
spark potentials for neon from a pressure of 11°3 mm. downwards 
was taken with this electrode as cathode, and a curve exactly 
