ON THE MECHANICAL EQUIVALENT OF HEAT 435 



2d. For the weight of the tape by which the weights are hung. 



rm,- "0006 

 This i 



3d. For the expansion of torsion wheel, D' being the diameter at 

 20 C. This is -000018 (t" 20). Hence, 



' " 



where t i' is the rise of the temperature corrected for radiation. 



2. RADIATION 



The correction for radiation varies, of course, with the difference of 

 temperature between the calorimeter and jacket; but, owing to the 

 rapid generation of heat, the correction is generally small in proportion. 

 The temperature generated was generally about 0-6 per minute. The 

 loss of temperature per minute by radiation was approximately -00140 

 per minute, where is the difference of the temperature. This is one 

 per cent for 10 -7, and four per cent for 14 -2. Generally, the calori- 

 meter was cooler than the jacket to start with, and so a rise of about 

 20 could be accomplished without a rate of correction at any point 

 of more than four per cent, and an average correction of less than two 

 per cent. An error of ten per cent is thus required in the estimation 

 of the radiation to produce an average error of 1 in 500, or 1 in 250 

 at a single point. The coefficients never differ from the mean more 

 than about two per cent. The observations on the equivalent, being 

 at a great variety of temperatures, check each other as to any error in 

 the radiation. 



The losses of heat which I place under the head of radiation include 

 conduction and convection as well. I divide the losses of heat into the 

 following parts: 1st. Conduction down the shaft; 2d. Conduction by 

 means of the suspending wires or vulcanite points to the wheel above; 

 3d. True radiation; 4th. Convection by the air. To get some idea of 

 the relative amounts lost in this way, we can calculate the loss by 

 conduction from the known coefficients of conduction, and we can get 

 some idea of the relative loss from a polished surface from the experi- 

 ments of Mr. Nichol. In this way I suppose the total coefficient of 

 radiation to be made up approximately as follows: 



Conduction along shaft ............ -00011 



Conduction along suspending wires. . . . -00006 



True radiation .................... -00017 



Convection ........................ -00106 



Total . -00140 



