The unit of heat assumed in the table is the calorie at 20° C., which is taken as equal to 4.180 joules, as explained in the article Calorimetry.
Its Relation To The Calorie At Any Given Temperature, Such As 15° Or 20°, Cannot Be Determined With The Same Degree Of Accuracy As The Ratio Of The Specific Heat At 15° To That At 20°, If The Scale Of Temperature Is Given.
Measure is equivalent to 4.177 joules per calorie at 16.5° C., on the scale of Joule's mercury thermometer.
In order to be independent of the accuracy of the thermometer employed for observing the initial temperature of the water introduced, it has been usual to employ water at ioo° C., adopting as unit of heat the " mean calorie," which is one-hundredth part of the heat given up by one gramme of water in cooling from ioo° to o° C. The weight of mercury corresponding to the mean calorie has been determined with considerable care by a number of observers well skilled in the use of the instrument.
System of expression in ergs per gramme-degree-centigrade, or " calorie," is the most appropriate, as being independent of the value of gravity.
There Can Be No Doubt, However, That The Final Result Is The Most Accurate Direct Determination Of The Value Of The Mean Calorie Between O° And Ioo° C. In Mechanical Units.
The method requires very delicate weighing, as one calorie corresponds to less than two milligrammes of steam condensed; but the successful application of the method to the very difficult problem of measuring the specific heat of a gas at constant volume, shows that these and other difficulties have been very skilfully overcome.
The calorie employed by Regnault is to some extent uncertain, but the difference is hardly beyond the probable errors of experiment, since it appears from the results of recent experiments that Regnault made an error of the same order in his determination of the specific heat of water at ioo° C.
The Mean Calorie Cannot Be Accurately Realized In Practice In Any Simple Manner, And Is Therefore Unsuitable As A Standard Of Comparison.
In the practical use of the instrument it is not necessary to know both the latent heat of fusion of ice and the change of volume which occurs on melting; it is sufficient to determine the change of volume per calorie, or the quantity of mercury which is drawn into the bulb of the apparatus per unit of heat added.
Expressed In J Oules Per Calorie The Result Is 4.1832, Which Agrees Very Closely With The Value Foand By Rowland As The Mean Over The Range 15° To 20° C. The Value 4.183 Is Independently Confirmed In A Remarkable Manner By The Results Of The Electrical Method Described Below, Which Give 4.185 Joules For The Mean Calorie, If Rowland'S Value Is Assumed As The Starting Point, And Taken To Be 4.180 Joules At 20° C.
The dynamical equivalent of the calorie is 4.18 X Io 7 ergs or C.G.S.
0906 c.c., and was taken as being equivalent to 79 calories (I calorie =15.59 mgrm.
The Most Practical Unit Is The Calorie At 15° Or 20° Or Some Temperature In The Range Of Ordinary Practice.