466 THE ELECTROMAGNETIC FIELD. [PT. III. CH. XI. 



The repulsion between the wires per unit length is 

 W_ 



~ * 



dd ~ dd d 



By means of the principles here stated, Maxwell has calculated 

 the inductances of coils of wire, by supposing the diameter of the 

 coil to be so great in comparison with the distance apart of the 

 different turns that the coil may be treated like a group of 

 straight conductors. The treatment of the magnetic field due 

 to currents even in straight conductors whose inductivity is 

 different from that of the surrounding medium, except in the case 

 of concentric cylinders, is a problem of considerable complexity, 

 and the results given by Maxwell, for the case of two wires, 

 Art. 685, are only approximately correct. 



