Relations between Electrical Measurements. 455 



that would be produced by tbe unit electromotive force is said 

 to measure the electric capacity of a conductor. Thus, generally, 



the capacity of a conductor S = r— } where Q is the whole quan- 

 tity in the charge produced by the electromotive force E. When 



the electromotive force producing the charge is capable of main- 

 taining a current, the capacity of the conductor may be obtained 

 without a knowledge of the value either of Q or E, provided we 

 have the means of measuring the resistance of a circuit in elec- 

 tromagnetic measure. For let R be the resistance of a circuit, 

 in which the given electromotive force, E, will produce the unit 

 deflection on a tangent galvanometer, then, from equations (6) 

 and (12), we have 



S=2 ^1T ! ' (14) 



where t and i retain the same signification as in equation (13) 

 (§ 25). 



27. Direct Measurement of Electromotive Force. — The meaning 

 of the words " electromotive force " has already been explained 

 (§ 16). This force tends to do work by means of a current or 

 transfer of electricity, and may therefore be said to produce and 

 maintain the current. In any given combination in which elec- 

 tric currents flow, the immediate source of the power by which 

 the work is done is said to produce the electromotive force. The 

 sources of power producing electromotive force are various. Of 

 these, chemical action in the voltaic battery, unequal distribution 

 of temperature in circuits of different conductors, the friction of 

 different substances, magnetoelectric induction, and simple elec- 

 tric induction are the most familiar. An electromotive force 

 may exist between two points of a conductor, or between two 

 points of an insulator, or between an insulator and a conductor, 

 — in fine, between any points whatever. This electromotive 

 force may be capable of maintaining a current for a long time, 

 as in a voltaic battery, or may instantly cease after producing a 

 current of no sensible duration, as when two points of the atmo- 

 sphere at different potentials (§ 47) are joined by a conductor; 

 but in every case in which a constant electromotive force, E, is 

 maintained between any two points, however situated, the work 

 spent or gained in transferring a quantity, Q, of electricity from 

 one of those points to the other will be constant ; nor will this 

 work be affected by the manner or method of the transfer. If 

 the electricity be slowly conveyed as a static charge on an insu- 

 lated ball, the work will be spent or gained in accelerating or 

 retarding the ball; if the electricity be conveyed rapidly through 

 a conductor of small resistance, or more slowly through a con- 



