124 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



67. The dynamo. The dynamo is a machine for the produc- 

 tion and maintenance of an electric current when the machine is 

 supplied with mechanical power, or, conversely, for the develop- 

 ment of mechanical power when the machine is supplied with 

 electric current. When used for the former purpose the dynamo 

 is called an electric generator, and when used for the latter pur- 

 pose, the dynamo is called an electric motor. 



The action of the dynamo as a generator is essentially as 

 follows: A wire is forced by an external source of mechanical 

 power to move side wise across a magnetic field. This motion 

 induces an electromotive force in the wire and this electromotive 

 force produces a current in the circuit which is connected to the 

 ends of the wire. The induced current causes the magnetic field 

 to push on the moving wire in a direction opposite to its motion, 

 and the work done in overcoming this opposing force is the work 

 that goes to maintain the induced current. 



The action of the dynamo as a motor is essentially as follows: 

 An electric current from an external source is forced through a 

 wire which is allowed to move sidewise in a magnetic field in the 

 direction of the side push upon it, thus developing mechanical 

 power. The motion of the wire induces in it an electromotive 

 force which opposes the flow of current through the wire, and'the 

 work done by the external source of electric current in forcing 

 the current through the wire in opposition to this induced electro- 

 motive force is the work which appears as mechanical energy in 

 the motor. 



The above-described action of the dynamo as a generator and 

 as a motor constitutes a complete statement of what is called 

 Lenz's Law, namely, that an induced current leads to the pro- 

 duction of a force which opposes the action which produces it 

 and the work done in overcoming this opposing force is the work 

 that goes to produce the induced current. 



Types of dynamos. There are two distinct types of dynamo 

 electric machines, namely, (a) alternating-current machines and 

 (3) direct-current machines. The alternating-current generator 



