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



on one another. — Hitherto we have spoken of the measurement 

 of currents as dependent on their action upon magnets ; but this 

 measurement in the same units can as simply be founded on their 

 mutual action upon one another. Ampere has investigated the 

 laws of mechanical action between conductors carrying currents. 

 He has shown that the action of a small closed circuit at a dis- 

 tance is the same as that of a small magnet, provided the axis 

 of the magnet be placed normal to the plane of the circuit, and 

 the moment of the magnet be equal to the product of the current 

 into the area of the circuit which it traverses. 



Thus, let two small circuits having areas A and Aj be placed 

 at a great distance D from each other in such a way that their 

 planes are at right angles to each other, and that the line D is 

 in the intersection of the planes. Now let currents C and C 1 

 circulate in these conductors ; a force will act between them 

 tending to make their planes parallel, and the direction of the 

 currents opposite. The moment of this couple will be 



G=^-. (10) 



Hence the unit electric current conducted round two circuits 

 of unit area in vertical planes at right angles to each other, one 

 circuit being at a great distance, D, vertically above the other, 



will cause a couple to act between the circuits of a magnitude ^3. 



The definition of the unit current (identical with the unit founded 

 on the relations given in § 18) might be founded on this action 

 quite independently of the idea of magnetism. 



20. Weber's Electro-Dynamometer. — The measurement de- 

 scribed in the last paragraph is only accurate when D is very 

 great, and therefore the moment to be measured very small. 

 Hence it is better to make the experimental measurements in 

 another form. For this purpose, let a length (/) of wire be 

 made into a circular coil of radius k; let a length (/J of wire 

 be made into a coil of very much smaller radius, k v Let the 

 second coil be hung in the centre of the first, the planes being- 

 vertical and at the angle 6. Then, if a current C traverses both 

 coils, the moment of the force tending to bring them parallel 



will be // 1 



G= >C^ si „0 (ii) 



This force may be measured in mechanical units by the angle 

 through which it turns the suspended coil, the forces called 

 into play by the mechanical arrangements of suspension being 

 known from the construction of the instrument. Weber used 

 a bifilar suspension, by which the weight of the smaller coil 



