156 ELECTRICITY 



of lines of force cut by the . circuit. In other words, so 

 long as there is any change in the intensity of that part 

 of the field which is within the circuit, an electro-motive 

 force ( 156) will be set up in the circuit ; and this 

 E. M. F. will vary according to the rate of change in 

 the number of lines of force which fall within the area 

 of the circuit. Of course it is- at once clear that such 

 induced currents will last but a moment, unless we can 

 arrange to have the number of lines that fall within the 

 area of the circuit continually changing. This can be 

 done either by varying the intensity of the magnetic 

 field, or moving the magnet, or by moving the wire cir- 

 cuit in the field. In practice the wire circuit is com- 

 monly made to rotate in the field, thus moving through 

 a changing number of lines of force. 



180. The Dynamo. - - The dynamo is a device for 

 producing induced currents. From what we have just 

 learned it is evident that such a device must consist of 

 a magnet and a wire circuit, together with some arrange- 

 ment for varying the number of lines within the area of 

 the circuit. The method in common use is to arrange 

 the wire circuit on an axle so that it can be rotated 

 within the magnetic field. 



In the section of a dynamo, Fig. 114, m is the mag- 

 net; the space between its poles, p and p\ is crossed 

 by lines of magnetic force. The coil of wire a (called 

 the armature] is made to turn about an axle e, and in 

 turning it cuts the lines of force. This sets up a cur- 

 rent in the coil, which passes out through one brush 

 b to the outside circuit c, supplying instruments on 



