HOW ELECTRICITY IS MEASURED 



261 



What important law governs electric currents? 

 George S. Ohm, a German scientist, discovered that 

 electric currents-, like other phenomena in nature, 

 are controlled by certain laws which may be expressed 

 as mathematical equations. 



Ohm discovered that in any electric circuit the num- 

 ber of amperes flowing is always equal to the volts 

 of pressure divided by the ohms of resistance. 



Rate of flow (amperes) = 



pressure (volts) 



resistance (ohms) 



An ordinary electric iron offers about 22 ohms of 

 resistance to the current flow. Now if one wishes to 

 know how many amperes flow when the iron is con- 

 nected to a 110-volt line he uses Ohm's Law as fol- 

 lows: 



volts 110 

 'amperes = , or amperes = = 5 amperes 



ohms 



22 



An electric heater uses six amperes when connected 

 to a 110-volt line, and if one needs to know the resis- 

 tance of the heating element in ohms he uses the 

 law thus: 



volts volts 



amperes = ; therefore, ohms = 



ohms 



amperes 



110 



or ohms = = 18^3 ohms 



6 



The rate at which a lamp, motor, or heater uses 

 energy is measured in a unit called the watt, named 

 in honor of James Watt of steam-engine fame. We 

 have come to know electric-light bulbs as 40-watt, 

 75-watt, or 100-watt bulbs, and if you will look on 

 the name plate of any other electrical device such 

 as an iron, toaster, or motor, you will see marked on 

 it a certain number of watts. These numbers merely 



550 WATTS 



100 WATTS 



U WAT T S 



6 5 WATT 5 



FIG. 422. DIFFERENT RATES OF USE OF ELECTRICITY 



indicate the rate at which the device uses electrical 

 energy. 



If the rate of using energy is not marked on the 

 device in watts, it is easy to calculate when the current 

 in amperes and the pressure in volts are known. 



Watts = volts X amperes. 



For example, an electric percolator which permits 

 five amperes of current to flow when the pressure is 

 1 10 volts would use energy at the rate of 550 watts. 



Watts = volts X amperes, or watts = 5 X 110 = 

 550 watts 



Electricity is usually used at such a rapid rate in 

 the modern home and in industry that the watt as a 

 unit is too small for practical purposes ; hence, a 

 larger unit, the kilmvatt, is used. 



1 kilowatt = 1000 watts. 



Thus, if an electric iron is rated at 550 watts, that 

 means 0.550 kilowatts. 



How is the cost of electricity determined? The 

 meter which you learned to read in your experiment 

 did not read in kilowatts but in kilowatt hours. This 

 will be clearly understood by a simple example. If 

 you can walk at the rate of three miles an hour, in 

 five hours you can walk fifteen miles. You obtained 

 this by multiplying the rate at which you walk by 

 the total time you were walking. In a similar way, 

 if electricity is being used at the rate of one kilo- 

 watt and it is used for an hour, the total amount of 

 energy used will be one kilowatt hour. If an electric 

 flatiron which uses energy at the rate of 0.550 kilo- 

 watts is used for two hours in doing an ironing it 

 will consume 2 X 0.550 or 1.10 kilowatt hours of 

 electrical energy. If this energy costs 10 cents per 

 kilowatt hour the ironing will cost $.10 X 1.10 or 11 

 cents. 



The reading of electric meters is usually recorded 

 once a month. When the next reading is taken, the 

 one for the previous month is subtracted, the dif- 

 ference between the two being the number of kilo- 

 watt hours used in the intervening time. This is 

 then multiplied by the price per kilowatt hour to find 

 the amount. 



Exercise. Electricity in a certain city is sold at the 

 rate of eight cents per kilowatt hour. Calculate the bill 

 for a month of thirty days in a home where the follow- 

 ing electrical appliances were used for the specified times. 



