2 Dr. Fletcher on the Determination of the 13. A. Unit 



value. H. F. Weber J, in 1878, used a similar method, em- 

 ploying the Siemens unit, the value of which he also measured 

 in C.G.S. units. Weber's value of the mechanical equivalent 

 is about one part in two hundred greater than Joule's water- 

 friction value, and one part in four hundred greater than 

 Rowland's water-friction value. 



In both Joule's and Weber's experiments a possible source 

 of error seems to have been ignored. The wire was assumed 

 to be at the temperature of the water in which it was immersed, 

 and its resistance was calculated on this assumption. It is 

 evident, however, that the wire was hotter than the water, 

 inasmuch as it was giving heat to the water. The error due 

 to this cause is of uncertain amount. If corrected for this 

 error, the values of the equivalent w^ould be increased and 

 their excess over the water-friction values would become 

 greater than before. To avoid this source of error, the research 

 described below was planned. The suggestion and general 

 plan of the research I owe to Professor Rowland. 



The theory of the method is as follows: — A current c, flow- 

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



c 2 Rt 

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

 chanical equivalent of heat. The wire being immersed in a 

 calorimeter 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 

 experiment 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 cB, = c'Bf, or Rzzi-R'. Hence J= ^^ in 



c h * 



which R does not appear, and the uncertainty attaching to its 

 temperature has no effect. 



The calorimeter was a cylindrical cup of sheet copper hold- 

 ing about 800 cubic centim. On the bottom of the cup lay a 

 sheet-copper 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 calorimeter and turned down so that they could be placed 

 in mercury-cups. The cover of the calorimeter rested in con- 

 tact 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 calori- 



