232 ELEMENTS OF GENEKAL SCIENCE 



reader; when the current in the wire is reversed, the heavy con- 

 ductor moves toward the reader. The wire moves in each case 

 because a push is developed as a result of the action between 

 the invisible magnetic field of the current in the wire and 

 the lines of force of the U -magnet. This motor is very imper- 

 fect, but it illustrates how a machine may be made to change 

 the energy of an electric current into mechanical energy. 



FIG. 118. Demonstration materials for studying the motor 



At the right are the parts of the simple motor described in the text. The 



voltmeter consists of a U-magnet, a coil of wire attached to a pointer, and a 



graduated scale 



Loops of wire can be placed in the magnetic field of the 

 U-magnet, with the coil arranged to rotate in either direction, 

 depending upon the path of the current through the wire. 



The voltmeter (or ammeter) also illustrates the motor. 

 The instrument (fig. 118) is made to indicate three different 

 ranges of voltages ; namely, 150 volts, 30 volts, and 30 milli- 

 volts (a millivolt is y^ volt). If a dry cell is connected to 

 the 150-volt terminals, the pointer indicates a slight move- 

 ment of the coil. When the cell is connected to the 30-volt 

 terminals, the motion of the coil is about five times as great, 



