78 CHEMICAL PHYSICS. 



vice versa, electric currents, termed magneto-electric currents, can be 

 generated by the action of magnets on metallic wires. Indeed, when- 

 ever a magnet is brought near to, or taken away from, a wire, or is 

 moved about in its neighborhood ; or if, vice versa, a mass of wire is 

 moved around a magnet i. e., whenever metallic wires are made to 

 pass through a magnetic field temporary electric currents are always 

 set up in the wire. These induced currents are the result of the 

 conversion of mechanical energy i. e.. the energy required to move 

 a wire or a magnet into electrical energy. 



Use is made of these facts for the generation of electric currents 

 by suitably constructed machines, known as magneto-electric machines. 

 In the smaller ones a permanent horseshoe magnet is used. Between 

 or in front of its poles revolve two coils of insulated wire with soft 

 iron cores, known as armatures. During rapid revoluting the soft 

 iron cores are magnetized while opposite one of the poles of the per- 

 manent magnet, demagnetized while equidistant from the two poles, 

 and reversed while passing to the opposite pole. The magnetization 

 and demagnetization of the iron cores have the same effect on the 

 coils of wire as if a magnet were suddenly introduced into the coil 

 and as quickly withdrawn. Thus currents will be induced in the 

 wire, and they will run in opposite directions as the polarity of the 

 cores changes with each half revolution. Such currents are known 

 as alternating currents. 



The same principle is made use of in the dynamo-electrical machines 

 for generating the currents employed for motive power, electric 

 lighting, etc. Instead of permanent magnets, powerful electro- 

 magnets are here used, and in place of an armature with two coils 

 of wire, armatures with many coils are employed. Moreover, an 

 arrangement known as the commutator is often used to change the 

 alternating to a direct or continuous current i. e., the generated 

 electricity is collected in such a manner that it moves in one direction 

 continuously. 



Electric motors are essentially dynamos in which the action is re- 

 versed i. e., electric currents convert iron into electromagnets, which 

 by successive attraction and repulsion of the iron in the armature 

 cause its rapid rotation, which motion may be communicated to other 

 machinery. 



Voltaic induction. It has been mentioned that a body charged 

 with static electricity causes, by induction, an electric disturbance in 

 all bodies near by. Similarly an electric current passing through 



