CONTINUOUS ELECTRIC CALORIMETRY. 
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venient unit, either in joules per gramme per degree Centigrade, or in foot-pounds 
per pound per degree Fahrenheit, without any implied reference to the properties of 
water. On the other hand, it is not unusual in elementary text-books to define specific 
heat as the ratio of the thermal capacity of a given mass of the substance considered 
to that of an equal mass of water at a standard temperature. The two definitions 
lead to the same numerical results, provided that the unit of heat is the thermal 
capacity of unit mass of water at the standard temperature. But there is nothing in 
the derivation of the term “ specific heat ” to imply that it denotes a ratio with 
respect to water at a standard temperature, and I think that this definition unduly 
restricts the meaning, and has given rise indirectly to a good deal of confusion. 
(39.) Choice of a Standard Temperature for the Thermal Unit. 
In addition to the absolute unit of heat, which is naturally the same as the unit of 
mechanical energy, it is necessary for practical calorimetry to adopt as a standard of 
reference a thermal unit equal to the quantity of heat required to raise unit mass 
of water one degree at a standard temperature. For purely academic purposes, it 
would suffice to adopt either of the time-honoured standards at 0 C. or 4° C., which 
have been frequently proposed, and are still to be found in the majority of text¬ 
books. But it has been conclusively shown that the specific heat of water at these 
low temperatures is considerably higher than over the range which is commonly 
employed for calorimetric determinations. The units at 0 and 4° would be practically 
inconvenient on this account. A still more serious objection is that the specific heat 
at these temperatures cannot readily be determined with the same order of accuracy 
as at ordinary temperatures. The unit of 4 - 200 joules proposed by the British 
Association Committee, which was supposed at that time to represent the specific 
heat of water in joules per gramme degree at 10° C., is open to the additional objection 
that it is really an absolute unit in disguise, and that, as such, it is superfluous, and 
does not satisfy the requirement of a thermal unit defined in terms of the specific 
heat of water at a definite temperature. 
For practical purposes, it is evidently necessary to define the thermal unit in terms 
of the mean specific heat over a range of temperature rather than at a definite point. 
The range of 1°, which is generally taken in definitions, is evidently too small for 
the accurate measurement of the rise of temperature in terms of the fundamental 
interval. These very small ranges of temperature have frequently been employed in 
calorimetry, as in Joule’s earlier experiments, for the purpose of reducing 
the uncertain correction for external heat-loss ; but with modern appliances for 
accurate thermal regulation, and provided that the duration of the experiment is not 
unduly prolonged, it is quite possible to employ a rise of 10° or more without the 
uncertainty of the heat-loss exceeding the probable theormometric error. 
So far as the thermometric error alone is concerned, the obvious interval to select 
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