ELECTRICAL INVENTIONS 245 



until it is " stepped down" to a lower voltage by a transformer. 

 One type of transformer easily comprehended may be briefly 

 described. A fine insulated wire is wound many times around a 

 small cylinder of soft iron outside of which is a larger cylindrical 

 frame wrapped with a few turns of coarse insulated wire. When 

 an alternating current of high voltage is sent through the inner 

 coil, it induces a low-voltage current of greater amperage in the 

 coarse wire. Thus a current of 5,000 volts and i ampere might 

 be stepped down to one of 100 volts and 50 amperes. Conversely, 

 if an alternating current of low voltage is sent through an inner 

 coil of coarse wire it will induce a high-voltage current of pro- 

 portionately less amperage in the fine wire of an outer coil. In 

 the latter case the transformer is built to step up the current. 



When the transformer is used on a continuous current it is 

 provided with an interrupter, one type of which is similar to the 

 device used to rapidly make and break the current of an electric 

 bell. For it is only when the moving lines of magnetic force pro- 

 duced by one coil cut the wires of the other coil that a current is 

 produced in this second coil. This occurs incessantly with an alter- 

 nating current since the direction of the flow is constantly chan- 

 ging; it is assured in a constant current by the use of the interrupter. 



It was not until the invention of the dynamo made it possible 

 to produce electricity cheaply and abundantly that motors, 

 electric lights, electric heaters, and similar contrivances became 

 commonplace. 



When an electric current is forced under high voltage through 

 a fine wire, the electric energy is partly transformed into heat 

 energy. If you send a current from a dry battery through a 

 fine copper wire, you will feel the wire become hot, or if you wrap 

 the wire several times about the bulb of a thermometer, it will 

 very soon register a rise in temperature. The incandescent 

 electric light is made by sending a current through such a fine 

 resistant wire that a strong glow of light is the result. In the 

 earlier types of electric lights a fine filament of carbon was used 

 in place of a wire. Since carbon heated to the glowing point in 



