When at G. This is closed when the lamp is turned on. the proper current is sent through the carbon filament, it becomes incandescent, but does not burn, as there is no air in the bulb. After a lamp has been used for some time, part of the carbon becomes deposited on the inside of the bulb, and absorbs a great deal of the light sent out by the filament. When this has happened, the best economy is to replace the lamp with a new one. 66 471. Metallized Filaments. If the carbon filament is metallized" by subjecting it to the intense heat of an electric furnace, it is rendered much more refractory, that is, capable of being brought to a higher temperature without melting. The higher temperature causes the metallized filament to give out more light than the ordinary carbon filament for the same expenditure of electrical energy. A carbon lamp gives 16 candle power for about 50 watts, requiring 3.1 watts per candle power. The metallized filament lamp gives about 20 candle power for 50 watts, or 2.5 watts per candle power. 472. True Metal Filament Lamps of high efficiency have also been developed. These lamps depend upon the possibility of drawing certain of the less common metals, like tantalum and tungsten, into a flexible wire of very small diameter-0.1 mm. or less and also upon the ability of these wires to carry a current that brings them to incandescence without bringing them to the melting point. The tungsten lamp (Fig. 435) is of much higher efficiency than the carbon or metallized filament lamp. A 40-watt tungsten lamp will give 32 candle power or more, requiring less than 1.25 watts per candle power. This is most im FIG. 435 portant to the consumer, because what he pays for is watt hours and what he wishes to use is light, hence a tungsten gives him twice the returns that a metallized filament lamp gives. + D B C E 473. The Incandescent Circuit. Incandescent lamps are coupled in parallel across the mains, or wires leading from the dynamo. Figure 436 illustrates a simple incandescent circuit. The dynamo D is first run until its voltage is 110 volts, and then any lamp or group of lamps in the circuit can be turned on. The hot resistance of a 110-volt, 16-candle-power lamp is about 220 ohms; consequently each lamp requires a current of half an ampere. The parallel resistance of 2, 3, or n lamps being only, و FIG. 436 th part of the resistance of a single lamp, the same dynamo that will light one lamp FIG. 437. Branch Circuits Each arch in the line where one conductor crosses another indicates that the two do not touch, or that they are insulated from each other. by coupling submains to them, that is, by attaching to each wire a branch wire large enough to carry the current for its group (Fig. 437). It is customary to put a fuse (Fig. 339) between each branch circuit and the main. 474. The Three-wire System. In order to reduce the expense of distribution, Edison devised the three-wire system, in which two similar dynamos D, D' are coupled in series (Fig. 438). The main feeding wires are attached one FIG. 438.-Three-wire System to the positive of the first dynamo, and the other to the negative of the second. A third wire is attached to the negative of the first and to the positive of the second dynamo. If there are equal numbers of lamps burning on both sides of the middle wire, it will carry no current, but if there are 50 lamps on one side and 55 on the other, for instance, it will carry the current for 5 lamps. The middle or neutral wire is often grounded. re 475. Alternating Current Transformer. Since alternating dynamos are built to give high voltage, some method is necessary for changing this voltage to that which can be used in a lamp. A step-down transformer is used for this purpose; it is virtually a reversed Ruhmkorff coil, versed because the work to be done is to change a high potential current into one of a low potential. The principle of the step-down transformer is shown in Fig. 439. The current from the dynamo flows through a long coil of fine wire which has an iron core to increase the magnetic field. Surrounding the same core is a second coil, shorter and of larger wire. To the terminals of this coil are attached the lamps to be lighted. The proper size and ALTERNATOR O FIG. 439 LAMPS length of the wire in each coil are determined by the respective voltages of the dynamo and of the lamps to be FIG. 440 used. A step-down transformer reduces the voltage in the same ratio as that of the numbers of turns in the two coils. For instance, to reduce a 1000-volt current to one of 100 volts, the number of turns in the primary must be 10 times the number in the secondary. Very little energy is lost in the transformation; if the 1000-volt current sent into the transformer is of 1 ampere, the 100 volt current taken from it will be of very nearly 10 amperes. Figure 440 shows in cross section the details of a step-down transformer with the relation of its parts, P being the windings of the primary coil and S of the secondary; and Fig. 441 shows the instrument as set up for use. This is the kind used on a line pole to reduce the 1000-volt current from the station to a lower voltage current for house use. When in use, it is filled with oil for insulation. A step-up transformer changes a current of low voltage to one of high voltage; it has a greater number of turns in the secondary than in the primary. FIG. 441 476. Electric Railways. The trolley lines and third-rail railways of the United States have for their essential parts |