46 Mr maxwell, ON FARADAY'S LINES OF FORCE. 



conducted with different facility in different directions. The body when suspended in a 

 uniform magnetic field will turn or tend to turn into such a position that the lines of force shall 

 pass through it with least resistance. It is not difficult by means of the principles in (28) 

 to express the laws of this kind of action, and even to reduce them in certain cases to numerical 

 formulae. The principles of induced polarity and of imaginary magnetic matter are here 

 of little use; but the theory of lines of force is capable of the most perfect adaptation to this 

 class of phenomena. 



Theory of the Conduction of Current Electricity, 



It is in the calculation of the laws of constant electric currents that the theory of fluid 

 motion which we have laid down admits of the most direct application. In addition to the 

 researches of Ohm on this subject, we have those of M. Kirchhoff, Ann. de Chim. xli. 496, and 

 of M. Quincke, xlvii. 203, on the Conduction of Electric Currents in Plates. According to 

 the received opinions we have here a current of fluid moving uniformly in conducting circuits, 

 which oppose a resistance to the current which has to be overcome by the application of 

 an electro-motive force at some part of the circuit. On account of this resistance to the motion 

 of the fluid the pressure must be different at different points in the circuit. This pressure, 

 which is commonly called electrical tension, is found to be physically identical with the potential 

 in statical electricity, and thus we have the means of connecting the two sets of phenomena. 

 If we knew what amount of electricity, measured statically, passes along that current which 

 we assume as our unit of current, then the connexion of electricity of tension with current 

 electricity would be completed*. This has as yet been done only approximately, but we 

 know enough to be certain that the conducting powers of different substances differ only in 

 degree, and that the difference between glass and metal is, that the resistance is a great 

 but finite quantity in glass, and a small but finite quantity in metal. Thus the analogy 

 between statical electricity and fluid motion turns out more perfect than we might have 

 supposed, for there the induction goes on by conduction just as in current electricity, but 

 the quantity conducted is insensible owing to the great resistance of the dielectrics -j-. 



On Electro-motive Forces. 



When a uniform current exists in a closed circuit it is evident that some other forces 

 must act on the fluid besides the pressures. For if the current were due to difference of 

 pressures, then it would flow from the point of greatest pressure in both directions to the 

 point of least pressure, whereas in reality it circulates in one direction constantly. We 



• See £jrp. Res. (371). f Exp. Res. VoL HI. p. 513. 



