456 Prof. Maxwell and Mr. F. Jenkin on the Elementary 



ductor of great resistance, the work may be spent in heating the 

 conductor, or it may electrolyze a solution, or be thermoelectri- 

 cally or mechanically used; but in all cases the change effected, 

 measured as equivalent to work done, will be the same, and equal 

 to EQ. Hence the electromotive force between two points is 

 unity, if a unit of mechanical work is spent (or gained) in the 

 transfer of a unit of electricity from one point to the other. This 

 general definition is due to Professor W. Thomson. 



The direct measurement of electromotive force would be given 

 by the measure, in any given case, of the work done by the trans- 

 fer of a given quantity of electricity. The ratio between the 

 numbers measuring the work done, and the quantity trans- 

 ferred, would measure the electromotive force. This measure- 

 ment has been made by Dr. joule and Professor Thomson, by 

 determining the heat developed in a wire by a given current 

 measured as in (§ 18)*. 



28. Indirect Measurements of Electromotive Force. — The 

 direct method of measurement is in most cases inconvenient, 

 and in many impossible ; but the indirect methods are nume- 

 rous and easily applied. The relation between the current, C, 

 the resistance, R, and the electromotive force, E, expressed by 

 Ohm's law (equation 6), will determine the electromotive force 

 of a battery whenever R and C are known. A second indirect 

 method depends on the measurement of the statical force with 

 which two bodies attract one another when the given electro- 

 motive force is maintained between them. This method is fully 

 treated in Part IV. (43). The phenomenon on which it is based 

 admits of an easy comparison between various electromotive 

 forces by electrometers. This method is applicable even to those 

 cases in which the electromotive force to be measured is inca- 

 pable of maintaining a current. The laws of chemical electro- 

 lysis and electromagnetic induction afford two other indirect 

 methods of estimating electromotive force in special cases (54 

 and 31). 



29. Measurement of Electric Resistance. — We have already 

 stated that the resistance of a conductor is that property in vir- 

 tue of which it limits the amount of work performed by a given 

 electromotive force in a given time, and we have shown that it 



E 



may be measured by the ratio p of the electromotive force 



between two ends of a conductor to the current maintained by 

 it. The unit resistance is therefore that in which the unit 

 electromotive force produces the unit current, and therefore per- 

 forms the unit of work, in the unit of time. If in any circuit 



* Phil. Mag. S. 4. vol. ii. (1851), p. 551. 



