ELECTRIC MOTOR 



1995 



ELECTRIC MOTOR 



d- 



Underlying Principle. The electric motor 

 depends for its operation upon the power of 

 an electric current to produce motion; wher- 

 ever there is an electric current there is mag- 

 netic action all around the conductor along 

 which the current 

 flows. This may 

 be illustrated by 

 a simple experi- 

 ment (Fig. 1). 

 Procure a wine 

 glass or small 

 tumbler, a small 

 bar magnet, a 

 small quantity of 

 mercury (quick- 

 silver), an elec- 

 tric cell and sev- 

 eral pieces of 

 wire. Small cop- 

 per wire for the 

 b a 1 1 ery connec- 



tions is the best. 



FIGURE 

 Explanation of 



sim- 



plest form of electric motor 

 The other wire is given in the text. 



should be large and strong. Assemble the ap- 

 paratus as shown in the cut. Fasten the mag- 

 net (a) into the glass (b) with glue so it will 

 remain in an upright position. Pour the mer- 

 cury (c) into the glass. Attach the wire (e) to 

 the standard (d) so that it will swing freely, and 

 make it long enough to touch the surface of 

 the mercury. When the wires (/, /') are at- 

 tached so as to form a circuit with the battery, 

 .the wire (e) will rotate around the magnet. If 

 the direction of 

 the current is re- 

 versed, the wire 

 will rotate in the 

 opposite d i r e c - 

 tion, the motion 

 being caused by 

 the magnetic 

 force which is 

 mutual between 

 the magnet and 

 the wire. This 

 simple experi- 

 ment, first per- 

 formed by Fara- 

 day, pointed the 

 way to the in- 

 vention of the 

 electric moto'r 

 (see FARADAY, 

 MICHAEL). If in- 

 stead of the bar magnet and the straight wire 



we use a horseshoe magnet and place a coil 

 of wire between the poles, as shown in Fig. 2, 

 we have a simple electric motor. Since the 

 causes of the motion described above are fully 

 explained in the articles ELECTROMAGNET and 

 ELECTROMAGNETISM, the reader is referred to 

 those articles for this explanation. 



Operation. The necessary parts of an elec- 

 tric motor are the same as those of a dynamo 

 (which see) ; these are the field magnets, the 

 armature and the commutator. The current 

 conveyed to the motor induces magnetism in 

 the armature and also excites the magnetic 

 field. The armature is so magnetized that the 

 line of its poles is at right angles to that in 



FIGURE 3. a 



The coil cannot turn farther until direction U 



changed. 



FIGURE 3. 6 



The current has changed> direction In the coll. 

 and the coil continues to turn. 



the magnetic field produced by the field mag- 

 nets. The attraction of opposite poles causes 

 the armature to rotate to such a position as 

 will place the line connecting its poles parallel 

 to that of the magnetic field, but before it 

 reaches this position the connections of two 

 windings of the armature are automatically 

 charged by the commutators and the current is 

 reversed, shifting the polarity of the armature 

 back to its original position, and the armature, 

 still impelled to adjust itself to the new con- 

 ditions, continues to rotate. This reversal of 

 the current of the commutator is shown in 

 Fig. 3 a and Fig. 3 6. 



For the sake of simplicity the motor de- 

 scribed is of the ring-armature pattern, in 

 which the armature consists of a soft iron 





