OTHER USES FOR ELECTRICITY 401 



be placed in a house, or too much current running over the wire will 

 cause the wire to set fire to the house. 



Fuses. Sometimes through short circuits (What are short cir- 

 cuits?) a great deal more current goes over the wire than should. 

 If this excess of current continues to flow, the result will be disastrous. 

 For this reason, fuses are used. These are made of metal which 

 melts at a low temperature. When the current becomes too "strong" 

 the heat melts them, " blows out " and breaks the current of elec- 

 tricity, and prevents damage. 



We can find out the resistance offered by lights, flatirons, electric 

 stoves, etc., by the formula 



Volts (pressure) 



Amperes (current) = 



Ohms (resistance) 



If the plate on the flatiron reads 5 amperes you must find out 

 what the voltage of your current is. If it is 110 volts you can easily 

 find the amount of resistance of the flatiron. 



110 volts 



5 amperes = , 



ohm 



or 22 ohms of resistance. If the amount of current is given in 

 watts, we must divide the number of watts by the voltage to get 

 the amperes, since watts equals volts X amperes. 



Electric Heaters. Electric stoves, heaters in cars, electric bed 

 warmers, electric flatirons, etc., all work on the principle of getting 

 small wires very hot. 



Electric Incandescent Lamp. The carbon or Tungsten lamp 

 contains small wires which offer great resistance to electric currents. 

 They become white hot, giving us light for our homes. 



Amount of Resistance Required. We have already mentioned 

 that the amount of current which can get over a wire depends upon 

 the amount of resistance, ohms of resistance, the wire offers to 

 the current. It also depends upon the pressure volts with which 

 the current is being driven over the wire. The harder the cur- 

 rent is "pushed" or driven over the wire the hotter the wire 

 becomes. 



