8 
PROF. H. L. CALLENDAR ON THE VARIATION OF THE SPECIFIC 
passes through a conductor of bare platinum wire, extending throughout the length 
of the fine-flow tube, and connected at either end to massive terminals of negligible 
resistance in the form of thick copper tubes enclosing the thermometers. These 
terminals serve to define accurately the resistance of the heater, and also to equalise 
the temperatiire along the length of the bulbs of the thermometers. The fineness of 
the flow tube renders any insulation of the conductor unnecessary. Sufficient stirring 
is obtained by causing the water to flow spirally round the thermometer bulbs and 
along the fine tube, or by subdividing the heating conductor. The heat generated by 
the friction of the water current in passing through the tube could be accurately 
measured by observing the difference of pressure between the ends of the tube, but 
its effect on the results was in all cases less than 1 part in 40,000. The electric 
current of 4 to 6 amperes was maintained steady by a battery of very large cells 
(300 ampere rate of discharge) because this permitted an order of accuracy of 1 or 2 
parts in 100,000 in the electrical readings and appeared preferable to discontinuous 
hand regulation. For this and other reasons the potential difference on the heating 
conductor was not directly balanced against an integral number of Clark cells, but 
was measured on a carefully calibrated potentiometer, together with the potential 
difference due to the same current passing through a specially designed standard 
resistance of platinum-silver maintained at a constant temperature in an oil-bath. 
As is usual in calorimetric experiments, the accuracy obtainable was limited chiefly 
by the determination of the heat-loss, which was deduced from experiments in which 
the electric and water currents were varied in such a way as to maintain the same 
rise of temperature. The heat-loss for the same rise of temjDerature was found to be 
not quite independent of the flow. It was also found to vary slightly owing to slight 
changes in the vacuum, when the apparatus was maintained for some time at a high 
temperature. It was inferred, however, that the uncertainty from all causes combined 
could not have exceeded 1 in 1,000 even at the limit of the range, and was probably 
much less at lower temperatures. 
The experiments have been criticised chiefly on the ground of the uncertainty of 
the absolute values of the electrical units ten years ago, but this would not affect the 
question of the variation of the specific lieat with temperature, as there was no 
question of the constancy of the standards employed. It is true that it was not at 
that time possible to secure a direct comjjarison of the Clark cells with the inter¬ 
national standards which have since been established. In default of this I made, 
with the assistance of Mr. King, an absolute determination of the E.M.F. of the cells 
at that time in use in the laboratory, l:)y means of an electro-dynamometer speciallj^ 
designed to read to 1 or 2 parts in 100,000. I was confident that the result of this 
determination, namely, 1’4334 volts for this type of cell at 15° C., would prove 
accurate to at least 1 in 10,000. This lias since been verified independently by 
Wolff and Waters (‘Bull. American Bureau of Standards,’ vol. 4, p. 64, 1907), who 
give the value 1*43330 volts at 15° C., in terms of modern standards, for Clark cells 
