Lamps for 220-volt circuits were developed as this voltage was desirable for power purposes, electric motors being used, and a few lamps were needed on such circuits. They are less efficient and more expensive than 110-volt lamps, their use being justified however only when it is uneconomical to have a separate 110-volt circuit for lighting. The lamps were made in sizes from 16 to 50 candlepower. [Illustration: Edison. Thomson-Houston. Westinghouse. Brush-Swan. Edi-Swan Edi-Swan United States. Hawkeye. (single contact). (double contact). Ft. Wayne Jenny. Mather or Perkins. Loomis. Schaeffer or National. Indianapolis Jenny. Siemens & Halske. VARIOUS STANDARD BASES IN USE, 1892.] [Illustration: THOMSON-HOUSTON SOCKET.] [Illustration: WESTINGHOUSE SOCKET.] Electric street railway systems used a voltage in the neighborhood of 550, and lamps were designed to burn five in series on this voltage. These lamps were different from the standard 110-volt lamps although they were made for about this voltage. As they were burned in series, the lamps were selected to operate at a definite current instead of at a definite voltage, so that the lamps when burned in series would operate at the proper temperature to give proper life results. Such lamps would therefore vary considerably in individual volts, and hence would not give good service if burned on 110-volt circuits. The candelabra screw base and socket and the miniature screw base and socket were later developed. Ornamental candelabra base lamps were made for use direct on 110 volts, smaller sizes being operated in series on this voltage. The former gave about 10 cp, the latter in various sizes from 4 to 8 cp. The miniature screw base lamps were for low volt lighting. [Illustration: Thomson-Houston. Westinghouse. ADAPTERS FOR EDISON SCREW SOCKETS, 1892. Next to the Edison base, the Thomson-Houston and Westinghouse bases were the most popular. By use of these adapters, Edison base lamps could be used in T-H and Westinghouse sockets.]