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lamp to give a light equal to an ordinary gas burner is known to consume a certain amount of electric energy, and we are thus enabled to determine what it costs to produce lights or to burn each lamp for a given number of hours.
In addition to these instruments we have a device which is placed in each house to show how much current passes to the lamps therein; This is the "Electric Meter."
There are a number of methods of arranging the incandescent lamps with relation to the wires leading from the generator. What is termed the two wire system is shown in Fig. 8, and is the one generally employed in what is known as isolated lighting; that is^ in cases, where the machinery for generating the current, is located at the place where the light is to be used; as in factories, large hotels, and like places not located in a district lighted from a central station.
When large districts of a city are lighted from a central station, what is termed the three-wire system, is generally em-
