90 Mr Watson, Some Experiments on the 
solit vertical 
Some Experiments on the Electrical Discharge in Helium and 
Neon. By Herpert EpMreston Warson, B.Sc. (Lond.), 1851 
Exhibition Scholar, Trinity College, Cambridge. (Communicated 
by Professor Sir J. J. Thomson.) ‘ 
[Read 11 November 1912.] 
THE investigation which is about to be described had as its © 
original object the determination of the cathode fall and the spark 
potential (or minimum potential necessary to produce a spark — 
between two electrodes) in all the inactive gases. Unfortunately, — 
before it could be completed, the author was obliged to leave — 
England, but as several results of interest had already been 
obtained, it seemed advisable to give a brief account of them. 
The spark potential in helium has already been carefully 
investigated by the Hon. R. J. Strutt (Phil. Trans. A. 1900, 193, 
p. 379). His experiments differ, however, from those about to be 
described, in that, in the former, the electrodes were brass plates 
0°755 mm. apart, a Wimshurst machine was used for producing 
the required potential difference, mercury vapour was allowed 
access to the sparking tube, and no allowance seems to have been 
made for the lag which is a very pronounced feature of the — 
phenomenon under consideration. His results will be referred to 
later. 
In 1904, Ritter (Ann. d. Physik, tv. 1904, 14, p. 118) examined 
the discharge in helium between a steel plate and sphere for 
pressures between 100 mm. and one atmosphere. The gas admit- 
tedly contained argon, and probably, judging by the results, other 
impurities as well, although it is difficult to compare experiments 
carried out under widely differing circumstances. In any cease, 
the investigation has no bearing on the present work. 
Bouty (Ann. Chim. Phys. 1911, 23, p. 5) has made numerous 
experiments on the cohésion diélectrique or rate of change of the 
spark potential with pressure, for helium and neon, and has 
recognised the need for the extreme purity of the gases under 
examination. His results are of great interest and will be men- 
tioned later. 
Apparatus. 
The annexed diagram shows the general arrangement of the 
apparatus used. The current was obtained from a battery B of 
small cells giving a maximum £.M.F. of 1000 volts, 20 cells B’ 
connected with the others in series were also connected with the 
