28 ALTERNATING CURRENTS 



conductor ; whereas a surface filament is only linked with the ex- 

 ternal lines. As a consequence, the e.m.f. induced along a surface 

 filament when an alternating current is sent through the conductor 

 is less than that induced along a central filament, and since the 

 various filaments may be regarded as different branches of a parallel 

 circuit, it is obvious that, for a given cross-section of filament, the 

 impedance will be greater, and hence the current less, in the case of 

 a filament near the surface than in one near the centre of the con- 

 ductor. Thus the current density will be greater in the surface 

 layers, the current being unevenly distributed over the cross-section 

 of the conductor. This uneven distribution of current is equivalent 

 to a reduction of cross-section, or to an increase of resistance, of the 

 conductor, and is frequently spoken of as the skin effect. 



The skin effect with cylindrical copper conductors is, at ordinary 

 frequencies, inappreciable until a diameter of about inch is reached ; 

 it then increases very rapidly with the diameter. But with con- 

 ductors constructed of magnetic materials, such as iron or steel, a 

 very marked effect occurs even with conductors of small cross-section. 

 This effect is of considerable practical interest in cases where an 

 alternating-current system is used for working electric railways or 

 tramways, and where the ordinary track rails are used as one of the 

 conductors of the system. In the case of a steel conductor of large 

 cross-section, the effective area of cross-section over which the current 

 is distributed is confined to a comparatively thin surface layer not 

 exceeding -| inch at all ordinary frequencies. Hence the loss occurring 

 with a given current in such a conductor is very much greater for an 

 alternating than for a continuous current. 



