The arc lamp could not practically be made in a unit smaller than the so-called “1200 candlepower” (6.6 ampere) or “half” size, which really gave about 350 spherical candlepower. A demand therefore arose for a small street lighting unit, and Edison designed his “Municipal” street lighting system to fill this requirement. His experience in the making of dynamos enabled him to make a direct current bipolar constant potential machine that would deliver 1000 volts which later was increased to 1200 volts. They were first made in two sizes having an output of 12 and 30 amperes respectively. Incandescent lamps were made for 3 amperes in several sizes from 16 to 50 candlepower. These lamps were burned in series on the 1200-volt direct current system. Thus the 12-ampere machine had a capacity for four series circuits, each taking 3 amperes, the series circuits being connected in multiple across the 1200 volts. The number of lamps on each series circuit depended upon their size, as the voltage of each lamp was different for each size, being about 1½ volts per cp. A popular size was the 32-candlepower unit, which therefore required about 45 volts and hence at 3 amperes consumed about 135 watts. Allowing 5 per cent loss in the wires of each circuit, there was therefore 1140 of the 1200 volts left for the lamps. Hence about 25 32-candlepower or 50 16-candlepower lamps could be put on each series circuit. Different sizes of lamps could also be put on the same circuit, the number depending upon the aggregate voltage of the lamps. [Illustration: EDISON MUNICIPAL LAMP, 1885. Inside the base was an arrangement by which the lamp was automatically short circuited when it burned out.]