Secriox III, 1902 [135 ] Trans. R. S. C. 
XVI. The Hall of Potential Method as applied to the Measurement of the 
Resistance of an LHlectrolyte in Motion. 
By H. T. Barnes, M.A.Sc., D.Sc. 
Assistant Professor of Physics, McGill University, Montreal, 
AND 
J. Guy W. Jounson, B.A. 
McGill University, Montreal. 
(Communicated by Professor John Cox, M.A., F.R.S.C., and read May 27, 1902.) 
In a note which one of the authors had the honour to send to 
the Royal Society of Canada, in 1900, it was pointed out that the 
change in density with concentration for many hydrated electrolytes 
is discontinuous. It was also suggested that this discontinuity 
represented a change in dissociation of the hydrate. Recently the 
authors have carried out some experiments on the change in resistance 
of a hydrated electrolyte with density, in order to study the effect, if 
any, of the discontinuity. If the effect is due to a change in dissocia- 
tion the conductivity of the solution should be indicative of it. 
The salt which we selected for the measurement was MgC1,— 
one which exhibits well the change in question. This was obtained 
pure from Merck, and various solutions made in strengths above and 
below the concentration at which the change occurs. 
Since the effect we were looking for is small and had been entirely 
overlooked in the work of other observers we were obliged to select 
a method for measuring the electrical resistance which would give us 
the greatest possible accuracy and delicacy. We selected a modification 
of the fall of potential method rather than the standard telephone 
method of Kohlrausch, because we found we could obtain much 
greater accuracy with the instruments at our disposal. We might 
state at the outset that it was the possession of such an instrument 
as the Thomson-Varley slide potentiometer with the corresponding 
high resistance (100,000 Ohms) galvanometer of the Thomson reflect- 
ing type, designed for use with it, which made our work possible by 
this method. There can be no question that the application of such 
instruments to the measurements rendered the results more accurate 
than could have been obtained by a telephone method. 
As the effect of temperature on the resistance of an electrolyte 
is large, it was necessary for us to take special precautions to guard 
against this error. We, therefore, arranged to have the electrolyte 
flow slowly through the resistance cell, first passing through a spiral 
