490 PROCEEDINGS OF THE AMERICAN ACADEMY 



Investigations on Light and Heat, made and pcnusnED wholly or in paet with 

 Appropbiation from the Rumfobd Fund. 



XXVI. 



CONTRIBUTIONS FROM THE PHYSICAL LABORATORY OF 

 HARVARD UNIVERSITY. 



ON A NEW METHOD FOR DETERMINING THE ME- 

 CHANICAL EQUIVALENT OF HEAT. 



By A. G. Webster. 



Communicated by Professor Trowbridge, May 26, 1885. 



In 1867 Joule published the results of his experiments for deter- 

 mining the mechanical equivalent of heat, by means of observations 

 on the thermal effect of an electric current. In his experiments a 

 calorimeter was used holding over a gallon of water, the temperature 

 of which was taken by a thermometer. The method about to be 

 described differs from Joule's in that the temperature is measured by 

 the change of resistance of a wire, which is heated by a current, and 

 no water is employed. The idea of the method was suggested by 

 Professor John Trowbridge. Accuracy is not claimed for the results 

 which follow, as the experiments were undertaken only with the view 

 of ascertaining the practicability of the method. 



The method of conducting the experiments was as follows. A thin 

 ribbon of steel about 45 cm. in length and 1 mm. in breadth, and 

 weighing .23 gr., was included in one side of a Wheatstone's bridge, 

 by which its resistance was measured. It was then thrown into 

 another circuit, and a transient current from twelve large Bunsen cells 

 was passed through it. The quantity of electricity transmitted was 

 measured by a ballistic galvanometer, and the difference of potential 

 of the ends of the steel strip was compared with the electromotive 

 force of a Daniell's cell by means of a quadrant electrometer. The 

 rise in temperature of the steel was found by immediately measuring 

 its resistance again. It had been previously found, by a series of 

 experiments made between the temperatures of 90° and 10° C, that 

 the resistance of the steel used was represented by the equation 



