MAN'S CO-LABORER: THE DYNAMO 



revolve by a belt adjusted to its axis and driven by a 

 steam engine. The wire coils of the armature thus made 

 to revolve cut across the so-called lines of magnetic 

 force which connect the two poles of the magnet, and 

 in so doing generate a current of induced electricity, 

 which flows away to reach in due course the third rail 

 or the trolley- wire, and ultimately to propel the motor. 

 It is hardly necessary to state that in actual practice 

 this generating dynamo is a complex structure. The 

 armature is a complex series of coils of wire; the elec- 

 tromagnets surrounding the armature are several or 

 many; and there is an elaborate system of so-called 

 commutators through which the currents of electricity 

 which would otherwise oscillate as the revolving coil 

 cuts the lines of magnetic force in opposite directions 

 are made to flow in one direction. But details aside, 

 the foundation facts upon which everything depends 

 are (i) that a coil of wire when forced to move so that 

 it cuts across the lines of force in any magnetic field 

 develops a so-called induced current of electricity; and 

 (2) that such an induced current possesses power of 

 magnetic attraction and repulsion. These facts were 

 discovered more than sixty years ago, and carefully 

 studied by Michael Faraday, Joseph Henry, and others. 

 Faraday found that such an induced current could be 

 produced not merely with the aid of an iron magnet, 

 but even by causing a wire to cut the lines of force that 

 everywhere connect the north and south poles of the 

 earth, the earth being indeed, as William Gilbert long 

 ago demonstrated, veritably a gigantic magnet. More- 

 over, these relations are reciprocal; so that if a wire 



