parallel with the call bell. As this parallel circuit has a resistance of less than 50 ohms, the current is sufficient to light the lamp at central, giving the signal for the line. The slight current passing from B through the storage cells at D while the receiver is on the hook is sufficient to keep D fully charged, ready to send its current through the local circuit when the receiver is off the hook. The transmitter T is shown in series with the primary P of the induction coil. Central calls the subscriber by closing the jack and cutting out the lamp, when the current will be large enough to ring the bell C. 465. Electric Lighting is done chiefly by the use of the arc lamp and the incandescent lamp. There are two different methods of producing the current for either lamp: by the use of the direct-current dynamo, and by the use of the alternator. 466. Arc Lighting. The simplest arc light is produced by sending a current of electricity from one carbon rod to another across a short air gap. The current is started by bringing the carbons together and then separating them, when the arc will be formed. In order to maintain the ordinary arc a difference of potential of from 45 to 50 volts is required, and this sends through the arc a current of from 9 to 10 amperes. The upper or + carbon (Fig. 429) is very much hotter than the carbon, and as the current passes, the carbon becomes incandescent and some of it crosses the space between, forming a conductor for the current. This gives to the + carbon a cuplike shape in the middle, and this cup is the seat of the most intense artificial light and heat that have been produced. FIG. 429 A type of lamp much used in projection lanterns is the 90° angle lamp, one form of which is shown in Fig. 430. When used with the direct current the horizontal carbon is made the positive pole and its crater is kept in the axis of the lantern. The positive carbon wears away twice as fast as the negative, but both carbons can be kept in place as they wear away by being pushed forward by separate feeding screws. This form of lamp can also be used with the alternating current, in which case they are fed at an equal rate. SERIES SHUNT COIL RESISTANCE FIG. 430 FIG. 431 467. The Inclosed Arc Lamp has a small globe nearly air-tight surrounding the arc and a few inches of the carbon. In this form of lamp the carbon is burned much more slowly than in the open arc, since the supply of air is cut off by the globe. Figure 431 is a diagram of the connections of a direct current, series multiple arc lamp. The fall of potential across the arc is between 80 and 90 volts. A resistance is coupled in series with the arc so that the lamp can be used on a 110-volt circuit. There are two electromagnets that control the length of the arc. One is a series coil that lengthens the arc, and the other is a shunt coil that shortens it. 468. The Luminous Arc. — The ordinary carbon arc itself is not luminous, the light coming from the hot crater of the positive carbon. If, however, carbons are used which have a core made of a mixture of carbon and some metallic salts, the hot vapors given off by these salts become luminous when heated by the current. If calcium salts are used, the color is a golden yellow, and this arc is sometimes called the flaming arc, from its resemblance to a flame. The arc formed is much longer than the ordinary carbon arc and the light is exceedingly brilliant. Another exceedingly brilliant source of light is the magnetite arc. This has a heavy copper rod for the positive upper terminal and a tube of sheet iron, packed with the mineral magnetite for the lower. The terminals wear away very slowly and the lamp is of high efficiency. 469. The Mercury Vapor Lamp. — When a tube, having a terminal at each end and inclosing a small quantity of mercury, is exhausted of air, the mercury vapor in the tube will carry the current when it is once started and it will become a source of light. In the Cooper-Hewitt lamp, Fig. 432, the mercury is held in the large bulb at one end of the tube and serves as the cathode. The anode is a small iron cup at the other end of the tube. Platinum wires sealed in the glass carry the current to the electrodes. In the simplest form of the lamp the tube is tilted by pulling on the suspended chain until the mercury runs through the tube in a thin stream. This mercury connects the electrodes and starts the cur The quality of the light is peculiar in that it contains no red rays. In uses in which the red rays are required, as in the judgment of colors, the lamp is supplied with a transforming reflector which converts a portion of the violet rays into red and improves the quality of the light for such purposes. G 470. Incandescent Lighting. When a heavy current is sent through a small copper wire, the copper melts. If the same experiment is made with a platinum wire, the wire will not melt, but will become intensely hot, and glow with a very bright light. Similar incandescence can be produced also in some other substances. The incandescent lamp (Fig. 433) consists of a glass bulb, into the base of which there are sealed two platinum wires which carry a loop of carbon filament. The air is exhausted from the bulb before it is sealed. After the lamp is screwed into the base there is but one gap, in the circuit FIG. 433 |