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LII. The Positive Column in Oxygen. 
By the Rev. P. J. Kirkby, Fellow of New College, Oxford*. 
SOME researches in which I have recently been engaged 
led me to investigate the electric force in the " positive 
column " when a steady electric discharge passes through 
oxygen at low pressures between two plane parallel electrodes 
placed in a straight glass tube. 
The ''positive column"" in such a cylindrical discharge 
may, in view of common experience, be defined as a region 
of the discharge terminating at or close to the anode or 
positive electrode where the electric force — i. e. the potential 
difference per centimetre — is constant or nearly constant. 
Close to the cathode there is an abrupt fall of potential 
called the " cathode fall/' Here there is a bright glow 
lining the cathode which is unmistakable. Between this 
glow and the positive column there is nothing to catch the 
eye at pressures of a few millimetres of mercury. The 
positive column is easily recognized as a long faintly luminous 
column of a colour which varies with the gas through which 
the discharge passes, being in oxygen either pale violet or a 
curious green. 
At pressures of the order 1 mm. it begins at a distance of 
about 6 cms. from the cathode and extends right up to the 
anode. The nearest point of the positive column to the 
cathode may be called the foot of the positive column. The 
electric force is remarkably constant throughout this region : 
it depends chiefly on the pressure, but is said to vary f to 
some extent with the diameter of the discharge-tube. 
The results of my experiments show that in oxygen the 
variation of the electric force in the positive column with 
the pressure presents exceptional features which do not 
appear to have been noticed. In the first place, the force is 
very much smaller than in the other common gases ; secondly, 
instead of diminishing continually with the pressure it reaches 
a minimum at about 2*0 mms. pressure ; and thirdly, there 
is a sharp discontinuity in its value at a pressure of *8 mm. 
as measured by the McLeod gauge. 
The methods hitherto employed to investigate the electric 
force in the positive column have generally involved the use 
of exploring wires fused into the sides of the tabes : their 
difference of potential during a discharge divided by their 
distance apart was assumed to give the electric force in the 
undisturbed gas. This method is open to the criticism that 
* Communicated by the Author. 
f J. J. Thomson, ' Conduction of Electricity through Gases/ ch. xv. 
2 P2 
