This method dates back to the introduction of the incandescent light, and, although it has been frequently demonstrated that a small current of high potential could be employed to work incandescent lamps, the series system has never been installed on a commercial scale, and is confined to arc lighting. In the United States the usual pressure for arc lighting is 2,000 volts, and it is not an uncommon occurrence to have forty arc lamps in series upon a line over 10 miles in length, carrying a current of 10 ampères. To economically use this high pressure for glow lamps in series, they must be of such design as to enable the whole of the current to be passed through them without injury. The filament of an ordinary high-resistance glow lamp would be immediately destroyed, so that low-resistance lamps, having a much larger sectional area, must be employed. The Bernstein or the Cruto lamp, which can be made to have a “hot” resistance of about 0·7 ohm, and requires a current of 9·75 ampères, could be used, and the current might be economically brought from a great distance. Mr. Bernstein calculates that it would be possible to operate 6,000 of these 7-volt lamps from twenty dynamos, each giving a current of 10 ampères at a potential of 2,000 volts, and still have a margin for loss of current in the leads. An economical feature of this scheme is the easy way in which power could be saved when only comparatively few lights were required; for instance, in the daytime all the circuits could be looped together and fed by one dynamo, and, as the number of lights increased, so other machines could be switched in by having an auxiliary bank of lamps as a resistance. From the central-station twenty pairs of carefully insulated copper wires, say of No. 6 B. W. G., would lead to the houses; and, as a good-sized ordinary house takes on an average twenty lights, the conductor would pass through fifteen houses before it returned to the station. It is in the house that the practical difficulty commences, as in this series system the circuit must never be opened, so that the switches and safety appliances must be such that, whatever happens, there must remain some path for the current, otherwise all the lights on that particular circuit would be extinguished. Mr. Bernstein gives the designation of “short closed” if the current goes through the switch-lever, and “long closed” if the current is led through the lamps or other electrical devices. [Illustration: FIG. 20.]