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



the condition of the wire in the two cases. The wire in the 

 latter case is therefore said to convey a permanent current of 

 electricity, the magnitude or strength of which is defined as pro- 

 portional to the quantity conveyed per second. This definition 

 is expressed by the equation 



C =T ••••••• W 



where C is the current, Q the quantity, and t the time. A per- 

 manent current flowing through a wire may be measured by the 

 force which it exerts on a magnet ; the actual quantity it conveys 

 may be obtained by comparing this force with the force exerted 

 under otherwise similar conditions, when a known quantity is 

 sent through the same wire by discharges. The strength of a 

 permanent current is found at any one time to be equal in all 

 parts of the conductor. Conductors conveying currents exert a 

 peculiar force one upon another ; and during their increase or 

 decrease they produce currents in neighbouring conductors. 

 Similar effects are produced as they approach or recede from 

 neighbouring conductors. The laws according to which currents 

 act upon magnets and upon one another will be found in the 

 writings of Ampere and Weber. 



16. Meaning of the words "Electromotive Forced — Hitherto 

 we have spoken simply of statical effects; but it is found that a 

 current of electricity, as above defined, cannot exist without 

 effecting work or its equivalent. Thus it either heats the con- 

 ductor, or raises a weight, or magnetizes soft iron, or effects 

 chemical decomposition ; in fine, in some shape it effects work, 

 and this work bears a definite relation to the current. Work 

 done presupposes a force in action. The immediate force pro- 

 ducing a current, or, in other words, causing the transfer of a 

 certain quantity of electricity, is called an electromotive force. 

 This force is necessarily assumed as ultimately due to that part 

 of a circuit where a "degradation" or consumption of energy 

 takes place : thus we speak of the electromotive force of the vol- 

 taic or thermoelectric couple; but the term is used also inde- 

 pendently of the source of power, to express the fact that, how- 

 ever caused, a certain force tending to do work by setting elec- 

 tricity in motion does, under certain circumstances, exist between 

 two points of a conductor or between two separate bodies. But 

 equal quantities of electricity transferred in a given time do not 

 necessarily or usually produce equal amounts of work ; and the 

 electromotive force between two points, the proximate cause of 

 the work, is defined as proportional to the amount of work done 

 between those points when a given quantity of electricity is 

 transferred from one point to another. Thus if, with equal/u^ 

 rents in two distinct conductors, the work done in the one is 



