56 THE MAGNETIC CIRCUIT [ABT. 23 



flux travels along the air-gap with the projecting poles, and cuts 

 the an nature conductors. 



(g) Whenever the current varies in a conductor, e.m.fs. are 

 induced not only in surrounding conductors but also in the con- 

 ductor itself. This e.m.f. is called the e.m.f. of self-induction. 

 Such e.m.fs. are present in alternating-current transmission lines, 

 in the armature windings of alternating-current machinery, etc. 

 While the e.m.f. of self-induction does not differ fundamentally 

 from the transformer action mentioned above, its practical aspect 

 is such as to make a somewhat different treatment desirable. 

 Inductance and its effects are therefore considered separately in 

 chapters X to XII below. 



All of the foregoing cases can be reduced to the following two 

 fundamental modes of action of a magnetic flux upon an electrical 

 conductor: 



(1) The exciting magnetomotive force and the winding in which 

 an e.m.f. is to be induced are both stationary, relatively to one 

 another; in this case the voltage is induced by a varying magnetic 

 flux. Changes in the flux are produced by varying either the 

 magnitude of the m.m.f ., or the reluctance of the magnetic circuit. 

 This method of inducing an e.m.f. is usually called the transformer 

 action. 



(2) The exciting magnetomotive force and the winding in which 

 the e.m.f. is to be induced are made to move relatively to each 

 other, so that the armature conductors cut across the lines of the 

 flux. This method of inducing an e.m.f. is conventionally referred 

 to as the generator action. 



By analyzing the transformer action more closely it can be 

 reduced to the generator action, that is to say to the " cutting " of 

 the secondary conductor by lines of magnetic flux or force. This is 

 so, because in reality the magnetic disturbance spreads out in all 

 directions from the exciting winding, and when the current in the 

 exciting winding varies the magnetic disturbance travels to or 

 from the winding in directions perpendicular to the lines of force 

 (Fig. 11). This traveling flux cuts the secondary conductor and 

 induces in it an e.m.f. However, the question as to whether an 

 e.m.f. is induced by a change in the total flux within a loop, or by 

 the cutting of a conductor by a magnetic flux is still in a somewhat 

 controversial state; 1 although Bering's experiment is a strong 

 Hering, "An Imperfection in the Usual Statement of the Funda- 



