Artificial Lighting 173 



68. Measuring electrical energy. An electrical meter is 

 placed in the house circuit to register the amount of electrical 

 energy consumed. The amount is indicated by the positions 

 of the hands over the dials. The dials indicate the energy 

 consumed in kilowatt hours. A kilowatt hour is equal to 

 1000 watt hours. Suppose that there are 40 of the 25 watt 

 tungsten lamps in the circuit. These in operation would con- 

 sume electrical energy at the rate of one kilowatt (40 lamps of 

 25 watts each equal 1000 watts). If the lamps are kept in 

 operation for one hour, the dials on the meter will indicate a 

 reading of one kilowatt hour. How long must twenty of such 

 lamps be operated to show the same amount of energy con- 

 sumed ? Ten lamps ? One lamp ? 



Reading the meter. It is a simple matter to read the meter. 

 The first dial shows up to ten kilowatt hours, each division 

 being equivalent to one kilowatt hour. The second dial shows 

 ten kilowatt hours for each division or one hundred for the 

 complete circle. The third registers 100 kilowatt hours for 

 each division of the dial or 1000 for the circle. A fourth 

 indicates 1000 kilowatt hours for each division. 



Exercise. Suppose, for example, the previous reading of a meter 

 was 1362; and the present reading is 1379. The difference, or 17, 

 indicates the amount of electric energy consumed, or 17 kilowatt 

 hours. If the rate charged for electric energy is ten cents per kilo- 

 watt hour, the total amount of the bill for the kilowatt hours consumed 

 would be 0.10 X 17, or $1.70. 



The readings for the previous state of the meter are shown in Fig. 

 49 by dotted lines ; and the present state by the heavy lines. How 

 many kilowatt hours have been consumed since the last reading? 

 What would be the cost at the rate of 12 cents per kilowatt hour ? 



The electric current should do useful work. The electric 

 current which enters the house by one wire should be made to 

 do the largest amount of useful work in any appliance before 

 it is allowed to leave by the other wire. It is always cheaper 

 to use the best electrical apparatus not only for lighting, but 



