560 Rev. P. J. Kirkby on the 
the introduction of the wires in the sides must disturb the 
uniformity of the surface of the discharge-tube, and pre- 
sumably affect the discharge itself. 
The method which I employed did not involve the use of 
an exploring wire. It is the same which I used to determine 
the force in the positive column in a discharge through 
electrolytic gas*. 
It depends on the assumption, suggested by experience 
and, I believe, fully justified by these experiments, that the 
positive column can be lengthened without making any 
other sensible change in the steady discharge. Throughout 
this paper ; 
D represents the distance in cms. between the electrodes, 
always exceeding the distance between the foot of 
the positive column and the cathode; 
C the steady current passing through the gas and always 
•0025 ampere; 
X the voltage-difference of the electrodes during the 
discharge ; 
Y the electric force in volts per cm. in the positive 
column; and 
'p the pressure in mms. of mercury. 
Then if D is increased by d, the potential-difference of 
the electrodes required to maintain the same current C as 
before under the same conditions will be simply X + Yd. 
In fact if everything is kept constant in the discharge but 
X and D, X will obviously be a continuous function of D 
whatever D's value. If D is taken as abscissa and X as 
ordinate a curve can be plotted, and it is easy to verify 
under favourable conditions that when D exceeds a certain 
value the curve becomes straight within the limits of ex- 
perimental error, and that the value in question is about the 
distance of the foot of the positive column from the cathode. 
The slope of this straight part of the D — X curve is obviously 
the electric force Y in the positive column. The advantage 
of this method is first that the exploring wire is dispensed 
with, and secondly that the electric force can be determined 
and verified by several observations with different values 
of D. The disadvantage is that the observations are not 
simultaneous, so that if a small change occurs in the region 
of the discharge near the cathode, where the electric force is 
very great, relatively large errors may occur in the observa- 
tions. This difficulty should only be felt when Y is small ; 
but in oxygen Y is remarkably small, ranging for pressures 
* Phil. Mag. March 1907. 
