240 



ELEMENTS OF GENERAL SCIENCE 



In the alternating-current system a generator consisting 

 of a huge armature, driven by steam, gas, or water power, 

 rotates in the magnetic field produced by electromagnets. 

 The voltage produced by this dynamo is raised to a higher 

 voltage by means of " step-up " transformers. The energy 

 is transmitted several miles with small losses, and then by 



FIG. 122. Use of incandescent lamps 



The number of incandescent lamps of different kinds sold yearly from 1880 to 



1916 is an interesting comment upon the different stages in the development of 



the lighting industry 



means of " step-down " transformers it is made available for 

 use. Smaller distributing transformers, such as are in most 

 common use, receive the current at 1100 or 2200 volts and 

 reduce it to 110 or 220 volts, which is a safe voltage for 

 community use. 



Wires from the low-voltage coils of the distributing trans- 

 formers are brought to the basement of the residence, where 

 suitable fuses and switches are installed. The watt-hour 



