92 Mr Watson, Some Haperiments on the ; 
In making an experiment upon the spark potential, the — 
voltage was increased slowly until the discharge passed through 
the tube, and the potential difference noted. This was found to 
be independent of the current passing when once established, a 
fact which was determined by using different voltages on the — 
main battery B, and consequently different values of A; the fact 
that the voltmeter was in parallel with the tube was also found to 
be without influence, for the spark potential was unchanged if the — 
voltmeter was removed and the number of cells in the battery 
reduced until there were only sufficient to give the required — 
voltage. 
As noticed by other observers, there was a distinct lag in the — 
lighting up of the tube after the potential difference was applied. 
This was most marked in the case of neon, and was frequently a 
minute in duration. In the present experiments, which were 
carried out in an indifferently lighted room, no external stimulus 
was given to start the discharge, but the potential difference was 
applied for three minutes, and if no discharge occurred in this 
time, it was considered that the sparking potential was not 
attained. This method gave very consistent results, the difference 
between successive values at the same pressure being nearly 
always less than one volt, and hardly noticeable on the voltmeter. 
It could be detected, however, by the variation in the position of 
the sliding contact D. Three observations were made at each 
pressure for each side of the cathode. It was rather remarkable 
that after a change of pressure, the first value of the spark 
potential was always two or three volts below those obtained 
subsequently. The figures given below refer to the steady values, 
as the first discharge potential was uncertain. 
The gases were those used in a previous investigation on 
electrical discharge (Roy. Soc. Proc. A. 1912, 86, p. 168), and 
were very carefully purified before every experiment. They were 
first mixed with oxygen, and phosphorus ignited in the mixture 
to remove hydrogen, a very convenient method when dealing with 
small quantities of gas. The residue was then transferred to a 
tube containing charcoal immersed in liquid air, and allowed to 
stand for at least half an hour, the gas in the connecting tubes 
being pumped away. After this treatment the spectrum is 
perfectly clear from foreign lines, except those of mercury, at 
100 mm. pressure. The gas was admitted directly from the 
charcoal tube to the discharge tube, passing however through a U 
tube immersed in liquid air on the way in order to remove 
mercury vapour. There is little doubt that mercury vapour 
exerts a considerable influence on the spark potential, especially 
when aluminium electrodes are used, and for this reason the 
discharge tube was shut off from the rest of the apparatus, and 
