MEASUREMENT OF DISCHARGES. 529 



be higher than that of the calorimeter. Fletcher* turns the diffi- 

 culty by placing a shunt of great resistance R' communicating with 

 the galvanometer at the ends of the wire R, which plunges in the 

 calorimeter. If I' be the intensity of the external current, we have 

 IR = I'R', and therefore 



JQ = I 2 R/=H'RV. 



We thus replace the measurement of R' by that of R; we 

 measure the current I', and the total current I + I'. 



In order to eliminate the determination of the equivalent of heat, 

 Lippmannf proposed to place in the same calorimeter the motor 

 which heats the liquid by friction and the wire which heats it when 

 the current passes. If, then, the motor is first made to act by the 

 pull of the weight, and then by the current, the experiment may be 

 made so that the same final heating, equilibrated in respect of 

 radiation, is attained in both cases. A simple thermometer is then 

 sufficient to show that the temperatures are equal. The mechanical 

 energy W expended in unit time, is equal to the electrical energy 

 I 2 R corresponding to the same heating, which gives the value of R. 



1114. MEASUREMENT OF DISCHARGES. In order to obtain an 

 induced discharge by the displacement of a frame, recourse is 

 usually had to the terrestrial field. The frame may be turned 

 through 1 80, either about the vertical or about a horizontal line 

 situated in the meridian, starting from a position in which it is 

 almost perpendicular to the component of the field normal to the 

 axis. Thus either the horizontal component H, or the vertical Z 

 of the field is used. 



In the former case, which is that most generally employed, the 

 axis must be carefully adjusted, or at any rate situate in a vertical 

 plane, perpendicular to the meridian. If, being situate in the 

 meridian, it made an angle e with the vertical, the vertical com- 

 ponent Z would affect the phenomenon; by a rotation of 180, if 

 I is the inclination, the variation of the flow of force will be 



2HS cos e + 2ZS sin e = 2HS cos e [i + tan I tan e]. 



The defect of adjustment being very small, cos e does not 

 appreciably differ from unity; but it cannot be assumed that the 



* LAWRENCE FLETCHER. Phil. Mag. [5], Vol. xx., p. i. 1885. 

 t LIPPMANN. Comptes rendus, Vol. xcv., p. 634. 1882. 

 VOL. II. MM 



