NATURAL AND ARTIFICIAL LIGHTING 69 



.005 or .006 of a cent per candlepower-hour. This is ap- 

 proximately y*> the cost of the open flame gas light per candle- 

 power-hour. 



Electricity is sold by the WATT-HOUR or KILOWATT-HOUR. 

 The kilowatt-hour is equal to 1000 watt-hours. The ordinary 

 house meter reads off directly the number of kilowatt-hours of 

 current used. This WATT HOUR-METER, or "WATT-METER," 

 as it is usually but erroneously called, is really little more than 

 a very small and easy running motor (Fig. 62). A very small 



FIG. 62. A watt-hour meter. 



portion of the current passing through this meter runs the 

 motor which, in turn, operates a chain of gear wheels which 

 turn the hands before the dials (Fig. 63). The usual cost of 

 electric current for lighting purposes is from 8 to 15 cents per 

 kilowatt-hour. 



The common CARBON FILAMENT lamps are made in several 

 sizes; the sizes most commonly used are the 50-watt lamps 

 giving about 16 candlepower of light and the 100-watt lamp 

 giving about 32 candlepower of light. Therefore, at 10 cents 

 per kilowatt-hour for current, these carbon lamps cost about 



