528 MEASUREMENT OF RESISTANCES IN ABSOLUTE VALUE. 



We shall review the principal experiments. 



1113. CALORIMETRIC METHOD. The uncertainty which pre- 

 vails as to the true value of the mechanical equivalent of heat 

 does not enable us to determine with sufficient accuracy the abso- 

 lute value of a resistance by a calorimetric measurement. The 

 law of Joule should be considered as furnishing a relation between 

 J and R which would enable us to calculate one of these quan- 

 tities when the other is exactly known. If, in the fundamental 

 equation (i), we put JQ for the calorific energy W, corresponding 

 to the disengagement of a quantity of heat Q, and make Q, I, and / 

 equal to unity, R = J. The numerical expression of the mechanical 

 equivalent of heat is then the absolute value of the resistance in 

 which a current equal to unity liberates a thermal unit per second. 



We need not return to the details of the calorimetric experiment 

 (917). Joule measured the intensity of the currents by a tangent 

 galvanometer with a circular frame; and, in order get at each 

 instant the value of H, he observed at the same time an electro- 

 dynamometer placed in the same circuit (866). We have 



The constant k was determined once for all by a series of ob- 

 servations made with two instruments, while the value of H was 

 determined by Gauss' method. 



Expressing the resistance in B.A. units, Joule found J = 4-212. io 7 . 

 Experiments on the friction of water had given 4-1624. io 7 . Con- 

 sidering this latter number as exact, he concludes that 



io 9 = 0-9881. io 9 . 



4212 



According to this experiment the British Association unit is too 

 small by 0-0119.* 



The principal source of error inherent to the method is the 

 uncertainty as to the temperature of the wire, which should always 



* JOULE. Phil. Mag. [3], Vol. xxxi., p. 173. 1847. Scientific Papers, 

 p. 277. British Association Report. Dundee, 1867. Reprint, p. 175. 

 Scientific Papers, p. 542. Phil. Trans., Roy. Soc., February, 1878, Pt. ii. 

 Scientific Papers, p. 632. 



