m Terms of the Mechanical Equivalent of Heat. 23 



ing through a wire of resistance R, for a time t, generates an 



c'Rt . 



amount of heat represented by h= — =- where J is the mechani- 



cal equivalent of heat. The wire being immersed in a calori- 

 meter and put in a circuit with a galvanometer, h, c and t can 

 be measured. Then if R is measured in B. A. units the experi- 

 ment will give a relation between the value of that unit and 

 the mechanical equivalent. In this research R was measured 

 during the actual experiment by connecting its terminals with 

 those of a large resistance R' and measuring the current c', 

 which flowed through the latter. With this arrangement 



c' cc'Rl' 



cR=c / R / , or R = -R'. Hence J = — ; — ' in which R does not 

 c h 



appear, and the uncertainty attaching to its temperature has no 

 effect. 



The calorimeter was a cylindrical cup of sheet copper hold- 

 ing about 800 cc . On the bottom of the cup lay a sheet cop- 

 per frame which supported three vertical glass rods. Around 

 these the wire, R, was coiled, forming a helix. The ends of the 

 wire were soldered to stout copper wires which, insulated by 

 short vulcanite tubes, passed through the wall of the calori- 

 meter and turned down so that they could be placed in mer- 

 cury cups. The cover of the calorimeter rested in contact with 

 the water to secure uniformity of temperature. The cover had 

 an expansion tube' and a smaller central tube which formed one 

 bearing for the stirring apparatus, another bearing being given 

 by a brass socket on the bottom of the calorimeter. The 

 stirrer consisted of a spiral blade of sheet copper supported on 

 a brass frame, the upper part of which was tubular, and passed 

 through the central tube of the cover. The stirrer was kept in 

 motion during the experiment by a silk thread, which passed over 

 a vulcanite wheel at the top of the stirrer and ran to a driving 

 clock. The stirrer formed the escapement of the clock, which 

 ran very uniformly with this arrangement. I estimated the 

 heat generated by the stirrer as two-thirds of the whole work of 

 the weights. This is about one thousandth part of the heat 

 generated by the current and only a rough determination of the 

 correction is needed. The thermometer passed through the 

 tubular upper part of the stirrer, and was clamped to a shelf 

 above in such a manner that its bulb was in the centre of the 

 calorimeter and surrounded by the stirring blade which, in 

 turn, was surrounded by the wire which carried the current. 

 The wire was composed of an alloy of platinum and iridium, 

 and was varnished to prevent conduction to the water. Its 

 resistance was about 1'8 ohm. The calorimeter was supported 

 on legs of vulcanite within a copper vessel with double walls, 

 the space between which was filled with water. This water- 



