360 



ELEMENTARY LESSONS ON [CHAP. x. 



coil lies within the secondary produces the same effect 

 as plunging it suddenly into the coil. 



So long as a steady current traverses the primary 

 circuit there are no induced currents in the secondary 

 circuit, unless there is relative motion between the two 

 circuits : but moving the secondary circuit towards the 

 primary has just the same effect as moving the primary 

 circuit towards the secondary, and vice versa. 



We may tabulate these results as follows : 



394. Fundamental Laws of Induction. When 

 we reflect that every circuit traversed by a current has a 

 field of magnetic force of its own in which there are lines- 

 of-force running through the circuit (Art. 192), and that a 

 coil of many turns has a field in which the lines-of-force 

 are distributed almost identically as those of a magnet 

 are, we shall see that the. facts tabulated in the preceding 

 paragraph may be summed up in the following funda- 

 mental laws : 



(i.) A decrease in the number of lines-of-force which 

 pass through a circuit produces a current rotmd 

 the circuit in the positive direction (i.e., produces 

 a "direct" current); while an increase in the 

 number of lines-of-force which pass through the 



