The simplicity of the constant current transformer soon drove the constant direct-current dynamo from the market. An enclosed arc lamp was therefore developed for use on alternating constant current. Two sizes of lamps were made; one for 6.6 amperes consuming 450 watts and having an efficiency of about 4½ lumens per watt, and the other 7.5 amperes, 480 watts and 5 l-p-w (clear glassware). These lamps soon superseded the direct current series arcs. They have now been superseded by the more efficient magnetite arc and tungsten filament incandescent lamps. SERIES INCANDESCENT LAMPS ON CONSTANT CURRENT TRANSFORMERS Series incandescent lamps were made for use on constant current transformers superseding the “Municipal” and “Shunt Box” systems. The large Edison, now called the Mogul Screw base, was adopted and the short circuiting film cut-out was removed from the base and placed between prongs attached to the socket. [Illustration: Holder. Socket. Holder and socket. SERIES INCANDESCENT LAMP SOCKET WITH FILM CUTOUT, 1900. The “Large Edison,” now called Mogul Screw, base was standardized and the short circuiting device put on the socket terminals.] The transformers made for the two sizes of arc lamps, produced 6.6 and 7.5 amperes and incandescent lamps, in various sizes from 16 to 50 cp, were made for these currents so that the incandescent lamps could be operated on the same circuit with the arc lamps. The carbon series incandescent lamp, however, was more efficient if made for lower currents, so 3½-, 4- and 5½-ampere constant current transformers were made for incandescent lamps designed for these amperes. Later, however, with the advent of the tungsten filament, the 6.6-ampere series tungsten lamp was made the standard, as it was slightly more efficient than the lower current lamps, and was made in sizes from 32 to 400 cp. When the more efficient gas-filled tungsten lamps were developed, the sizes were further increased; the standard 6.6-ampere lamps now made are from 60 to 2500 cp. THE NERNST LAMP Dr. Walther Nernst, of Germany, investigating the rare earths used in the Welsbach mantle, developed an electric lamp having a burner, or “glower” as it was called, consisting of a mixture of these oxides. The main ingredient was zirconia, and the glower operated in the open air. It is a non-conductor when cold, so had to be heated before current would flow through it. This was accomplished by an electric heating coil, made of platinum wire, located just above the glower. As the glower became heated and current flowed through it, the heater was automatically disconnected by an electro-magnet cut-out. [Illustration: NERNST LAMP, 1900. The burners consisted mainly of zirconium oxide which had to be heated before current could go through them.]