CALORIMETRICAL MEASUREMENTS. 517 



CHAPTER VII. 



MEASUREMENT OF RESISTANCES IN 

 ABSOLUTE VALUE. 



1103. VARIOUS METHODS. As the resistance of a conductor 

 in electromagnetic units has the same dimensions as a velocity 

 (526), the determination of a resistance in absolute value neces- 

 sarily implies the measurement of a length and of a time, and the 

 other quantities which come into play in the experiments always 

 reduce to numerical ratios. x 



The principal methods used in measuring electrical resistance 

 may be classed in two groups. In the first case, the resistance 

 of a conductor is deduced from the thermal energy disengaged 

 in this conductor in a certain time by a current of known intensity. 

 In the second case, the resistance of a circuit is determined by the 

 current which is produced by a known electromotive force, constant 

 or variable. This electromotive force should itself be related to the 

 fundamental units ; but the only electromotive forces of which the 

 value can be directly determined in electromagnetic units are those 

 arising from the effects of induction. 



We might still compare a resistance with a coefficient of self- 

 induction (1100), and determine this latter either by direct calcu- 

 lation or by a comparison with a coefficient of mutual induction, 

 which would be deduced from the dimensions of the conductor; 

 but it can be conceived that the experiment would possess no great 

 accuracy. 



1104. CALORIMETRICAL MEASUREMENTS. The thermal energy 

 W developed by a current I in unit time / in a resistance R satisfies 

 equation 



according to Joule's law (917), a measurement of the quantities 

 W, I, and / will give the value of R. 



