Relations between Electrical Measurements. 513 



or by comparison with a succession of discharges of known quan- 

 tities from an absolute condenser. 



43. Practical Measurement of Electromotive Force. — The rela- 

 tions expressed by equations (17) and (23) show that in any given 

 circumstances the force exerted between two bodies due to the 

 effects of statical electricity will be proportional to the electro- 

 motive force or difference of potential (47) between them. This 

 fact allows us to construct gauges of electromotive force, or in- 

 struments so arranged that a given electromotive force between 

 two parts of the apparatus brings an index into a sighted posi- 

 tion. In order that the gauge may serve to measure the electro- 

 motive force absolutely, it is necessary that two things should 

 be known, — first, the distribution of the electricity over the two 

 attracting or repelling masses (or, in other words, the capacity 

 of each part); secondly, the absolute force exerted between them. 

 For simple forms, the distribution, or capacity of each part, can 

 be calculated from the fundamental principles (33) ; the force 

 actually exerted can be weighed by a balance. By these means 

 Professor W. Thomson* determined the electromotive force of a 

 Daniell's cell to be O0021 in British electrostatic units, or 

 0*0002951 in metrical electrostatic unitsf. This proposition is 

 equivalent to saying that two balls of a metre radius, at a dis- 

 tance d apart in a large open space, and in connexion with the 

 opposite poles of a Daniell's cell, would attract one another with 



, (0-0002951) 2 , - L 0-000,000,00888 

 a force equal to -r-^ — — absolute units, or -^ 



gramme weight J. 



An apparatus by which such a measurement as the foregoing 

 can be carried out is called an absolute electrometer. It will be 

 observed that, although the definition of electromotive force is 

 founded on the idea of work, its practical measurement is effected 

 by observing a force, inasmuch as when this force exerted be- 

 tween two conductors of simple shape is known, the work which 

 the passage of a unit of electricity between them would perform 

 may be calculated by known laws. 



44. Comparison of Electromotive Forces by their Statical Effects. 

 — This comparison is simpler than the absolute measurement, 

 inasmuch as it is not necessary, in comparing two forces, to 

 know the absolute values of either. Instruments by which the 

 comparison can be made are called electrometers. Their arrange- 



* Paper read before the Royal Society, February 1860. Vide Proceedings 

 of the Royal Society, vol. x. p. 319, and Phil. Mag. S.4.vol.xx.( 1860) p.233. 



f [Note added May 5, 1865. — In electromagnetic measure this would 

 make the electromotive force of a Daniell's cell equal to about 91,700. 

 Other observers have found a value of about 100,000 (metrical system, 

 based on metre, gramme, and second).] 



X This value was erroneously given in the original paper. 



