Intelligence and Miscellaneous Articles. 217 



ON A NEW METHOD FOR DETERMINING THE MECHANICAL EQUI- 

 VALENT OF HEAT. BY A. G. WEBSTER. 



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

 mining the mechanical equivalent of hea^, 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 tempera- 

 ture 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 DanielFs 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 



E = a (1 + -00503 0), 



6 being the temperature. 



If then E be the initial resistance of the strip, and R x the resist- 

 ance after the passage of the current, 



K o = a(l+/3 o ), 



and the rise in temperature is 



R — ~R n 



0i — 0o=' 



a[3 



If w be the weight of the strip, and s its specific heat, the quantity 

 of heat imparted to it by the current is 



h = ws(di _ 6o)= ^(\^) (1) 



But if Q is the quantity of electricity transmitted, and E the 

 Phil. Mag. S. 5. Vol. 20. No. 123. August 1885. Q 



