386 GENERAL SCIENCE 



Now, the measuring of electricity may be compared to this 

 example. The amount of electricity passing through the wires, cor- 

 responding to the gallons of water per second, is called amperes, 

 and the pressure of the electricity, corresponding to the water pressure 

 in pounds per square inch, is called volts. For example, a 110-volt 

 current refers to the pressure by which the current is being forced 

 over the wire. By multiplying the amperes by the volts we obtain 

 a unit which expresses the rate of doing work or power, called watts. 



FIG. 265. An instrument for meas- 

 uring electrical pressure, volts. FIG. 266. Watthour meter. 



An instrument which requires a watt of any energy would consume 

 in one hour a watthour of energy. 



The commercial unit for the measuring of electrical energy is 

 called a kilowatt hour, which means 1000 watthours. A 25 watt 

 lamp will use 25 watthours in an hour, or the electrical energy 

 supplied to the lamp in forty hours would equal 1 kilowatt hour 

 (1000 watt hours). 



The Watthour Meter. A watthour meter is essentially a small 

 electrical motor driving a registering dial. All electricity which is 

 used in the house must flow through this motor. If only one lamp 

 is in use a small amount of current passes through. If two lamps 

 are used, twice as much current passes through the meter. The 

 increase will be in amperes, since the volts remain constant. For 



