58 
PBOFESSOK HUGH L. CALLENDAIt OX 
with which I was intimately acquainted, had shown that the electrical units were 
probably in error, and appeared to indicate a smaller rate of variation of the specific 
heat than that given by Rowland. In reconsidering the problem, in 1893, I 
therefore determined to attempt the absolute measurement of the ohm and the 
Clark cell, in addition to the variation of the specific heat of water over as wide 
a range as possible. The method of steady-flow calorimetry appeared to be 
particularly adapted to the latter object, as it afforded much greater facility than 
that of Griffiths or Rowland in varying the conditions of experiment over a wide 
range. For the absolute measurement of the ohm, I immediately obtained estimates 
for a Lorenz apparatus of Professor Y. Jones’ pattern, which was eventually ordered 
in October, 1894, and is briefly described in Section 6 of this paper. For the 
absolute measurement of the Clark cell in terms of the ohm, after spending some 
time in designing various forms of electrodynamometer, I decided to employ the 
British Association pattern, with certain modifications, which are explained below, 
Sections 10 to 16. At the same time I commenced a series of investigations 
into the defects of the form of Clark cell described in the Board of Trade 
Memorandum, in which 1 was assisted by Mr. H. T. Barnes. This work included 
an accurate determination of the variation of the E.M.F. with temperature and 
with strength of solution, in addition to measurements of the solubility of zinc 
sulphate and of the density of its solutions. It extended further than I had at first 
anticipated, and was not completed till the summer of 1896. The results were 
published in the ‘Proceedings of the Royal Society,’ vol. 62, pp. 117-152. 
In the meantime I had been engaged, during the winter of 1895 and the summer 
of 1896, in testing various methods of temperature regulation, and in studying the 
theory of the flow of water in fine tubes under the conditions presented by the 
proposed method of calorimetry. This was a most important part of the work, 
as the determination of the variation of the specific heat over a large range of 
temperature exacted great accuracy of regulation, and close attention to details of 
design. The method of regulation and circulation finally adopted may appear very 
simple and obvious, but it was not reached wfithout considerable expenditure of time 
and thought. The experiments on the flow of water heated by an electric current 
(Section 33) threw some light on the causes of failure of the rough preliminary 
experiments, and supplied the data necessary for the design of the glass-work of the 
calorimeter and vacuum-jacket, which was ordered of Messrs. Muller, in Bonn, early 
in October, 1896. 
At this stage of the investigation, finding that I should not have sufficient leisure 
during the work of the session to carry out the research single-handed, as I had at 
first intended, I secured the assistance of Mr. Barnes, who had already proved his 
ability in the making and testing of Clark cells. Our first experiments were made 
on mercury, which, being itself a conductor of electricity, presented fewer difficulties 
than water, The wafer apparatus was fitted up and tested shortly before the 
