CONTINUOUS ELECTRIC CALORIMETRY. 
57 
Part I.—Introduction. 
( 1 .) General Account of the Origin ancl Progress of the Investigation. 
The method of Continuous Electrical Calorimetry, described in the following 
paper, was originally devised as part of a Fellowship Dissertation on applications of 
the platinum thermometer, at Trinity College, Cambridge, in the year 1886, but, on. 
account of unforeseen difficulties, the experiments did not at that time get beyond 
the preliminary stage. In the first rough apparatus, a steady flow of water, passing 
through a tube about 30 centims. long and 3 millims. in diameter, was heated by an 
electric current in a fine spiral of platinum wire of about 5 ohms resistance, nearly 
fitting the tube. The steady difference of temperature between the inflow and the 
outflow was measured by a pair of delicate mercury thermometers, which it was of 
course intended to replace in the final apparatus by a differential pair of platinum 
thermometers. The electrical energy supplied was measured by the potentiometer 
method in terms of a set of 5 Clark cells and a large German-silver resistance 
of 5 ohms in series with the platinum spiral. The potentiometer was specially made 
for the work, and consisted of a metre slide-wire, and ten resistances, each equal to 
the slide-wire, for extending the scale so as to secure sufficient accuracy of reading. 
This potentiometer was still in existence at the Cavendish Laboratory in 1893. 
The set of 5 Clark cells were tested by Glazebrook and Skinner (‘ Phil. 
Trans.,’ A, 1892), and were still in good condition at a later date. The external heat- 
loss in these experiments was found to be much larger than had been anticipated, 
and so variable that the results were of little or no value. In order to remedy this 
defect, I designed the vacuum-jacket, which was suggested by some experiments of 
Sir William Crookes ( £ Eoy. Soc. Proc.,’ vol. 31, 1881, p. 239), which appeared to 
indicate that the rate of cooling of a mercury thermometer in a very good vacuum 
was ten to twenty times less than in air. I therefore regarded the vacuum-jacket as 
a most essential part of the experiment, and expected a great improvement to result 
from its use. Unfortunately I failed to make the jacket for want of sufficient skill 
in glass-work, and abandoned the experiment for the time, until my appointment as 
Professor of Physics at* McGill College, Montreal, gave me greater facilities for 
carrying out the work. Eventually it proved that the effect of the vacuum-jacket in 
diminishing the external loss of heat was not nearly so great as I had been led to 
imagine, but it possessed several advantages as a heat insulator over such materials 
as cotton wool or flannel. The thermal capacity of a vacuum being negligible, the 
time required for attaining a steady state was much shortened. Moreover there was 
no risk of error from damp, which is the worst drawback of ordinary lagging. 
I had not originally intended to employ the electrical method for determining the 
variation of the specific heat of water, but only for comparing the electrical and 
thermal units at ordinary temperatures. In the meantime the work of Griffiths, 
VOL. CXCIX.-A. 
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