326 Introduction to the Study of Science 



Another step will make clearer the application of the principle 

 in the actual generator. The armature is usually made of a 

 core of sheet iron disks insulated from each other by thin paper 

 or other substance and placed together to form a drum, and this 

 drum wound with several turns of continuous insulated wire, 

 the ends of which are connected with the collectors, as in Fig. 

 104. The field magnet is made of a certain kind of steel 

 when it is to be a permanent magnet, or of soft iron wound 

 with copper wire when it is to be an electromagnet. When 

 the armature revolves rapidly in the magnetic field, electric 

 pressure or electromotive force is generated. Although this 

 may appear to be more complicated, it is identical in principle 

 with the simple form first presented. 



The magneto. On automobiles or in certain telephone sys- 

 tems there is a small generator called the magneto. This com- 

 prises an armature with a soft iron core wound with many turns 

 of fine wire, and a permanent U magnet, between the ends of 

 which the armature is made to revolve. 



Summary. It is necessary to keep clearly in mind that 

 when a conducting wire is moved sidewise within a magnetic 

 field (the field, that is, of a magnet), there is generated in 

 the wire an electric current. This is the principle of the elec- 

 tric generator. When a conducting wire carrying an electric 

 current comes within a magnetic field, it is made to move 

 in a direction which is the reverse of that which would gen- 

 erate an electric current in it. This reverse motion may be 

 continued and be made to do work as a motor (pages 321-2). 

 A motor is made with a large number of turns of wire and 

 with a powerful magnet so that a great twist or torque is given 

 to the armature. By making the armature with a sufficient 

 number of different coils of wire, the twist or torque is made 

 continuous. 



The motor and generator connected. The electric motor may 

 be operated by energy derived from a battery of dry or other 

 cells. It is, however, usually operated by electric energy derived 



