98 



Mr. J. S. Macdonald. Studies in the 



corrections for changes in the temperature of the calorimeter as may he 

 ascertained hy appropriate calibration experiments. I have therefore placed 

 five incandescent lamps within the calorimeter, reading the power absorbed 

 by them by means of a wattmeter, and changing this value by means of 

 suitable resistances. In such a way it is possible to compensate alterations 

 in the heat-production of the subject, and to dispense with the necessity for 

 any but the smallest adjustments in the balance of the calorimeter. It is 

 clear that such a process of compensation may be managed with greater 

 abruptness and precision than complicated modifications in the balance, using 

 the constantly observed temperature of the calorimeter as an index of success 

 or failure. 



In addition to the subject and these lamps two other sources of heat have 

 been present within the calorimeter during these experiments. Thus, in the 

 first place, there is the power-absorption of electromagnets forming part of 

 an electrical brake applied to the cycle-ergometer upon which the subject 

 performs definite amounts of mechanical work, and this is followed by a 

 voltmeter and ammeter arranged in this circuit. Two such electromagnets 

 placed in separate positions each environ a part of the path traversed by the 

 periphery of a copper disc by which the hind wheel of the cycle has been 

 replaced. Benedict has recently described the calibration of such an 

 instrument provided with a single electromagnet and with a thicker copper 

 disc. He is fortunate in possessing in this machine an instrument in which, 

 at a certain useful revolution rate, small changes of speed occasion directly 

 proportionate variations in the amount of mechanical work performed. This 

 useful property has been found by Benedict and Cady to be attributable to the 

 reaction of the eddy-currents in the copper disc upon the value of the 

 electromagnet exciting their presence, and is dependent upon the dimensions 

 of the poles of the magnet and upon the thickness of the copper disc and its 

 resistance. I am not so fortunate, and the mechanical work performed on my 

 cycle varies more rapidly, rising with the square of the revolution rate. In 

 this case it should be noted that the maintenance of a uniform speed is of 

 greater importance. 



To return to the sources of heat within the calorimeter, I have in the 

 second place to mention the power-absorption due to a fairly powerful fan 

 placed within the calorimeter and maintaining there the conditions of a 

 moderate breeze. Here again a voltmeter and ammeter are used to assess the 

 heat-production due to this source. This value added to that • due to the 

 brake and to that due to the incandescent lamps provide a " subtraction " 

 which has to be taken from the total heat measured at any time before the 

 heat-production of the subject is known. 



