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one point to another a given number of watts, say, 10,000, it is possible to have either an E.M.F. of 100 volts and a current of 100 amperes or an E.M.F. of 1000 volts and a current of 10 amperes. In the two cases, however, the loss of energy in the wire which carries the current from the place where it is generated to the place where it is used will be widely different. For,
watts = amperes x volts;
but, from Ohm's law,
amperes × ohms.
amperes2 × ohms = I2R. If, then, R represents the resistance of this transmitting wire, the socalled "line resistance," and I the current flowing through it, the heat developed in it will be proportional to I2R. Hence the energy wasted in heating the line will be but 100 as much in the case of the 1000 volt, 10-ampere current as in the case of the 100 volt, 100-ampere current. Hence long-distance transmission, where line losses are considerable, it is important to use the highest possible voltages.
On account of the difficulty of insulating the commutator segments from one another, voltages higher than 1200 or 1500 cannot be obtained with direct-current dynamos of the kind that have been described. With alternators, however, the difficulties of insulation are very much less on account of the absence of a commutator. The large 10,000horse-power alternating-current dynamos on the Canadian side of Niagara Falls generate directly 12,000 volts. This is the highest voltage thus far produced by generators. In all cases where these high pressures are employed they are transformed down at the receiving end of the line to a safe and convenient voltage (from 50 to 500 volts) by means of step-down transformers.
It will be seen from the above facts that alternating currents are best suited for long-distance transmission. The Big Creek plant in California transmits power 241 miles at a pressure of 150,000 volts. (See opposite p. 241.) The Southern Sierras Power Company of
FIG. 332. Transformer on electriclight pole
California sends current 830 miles across the desert. Transmission at 220,000 volts is now under consideration for a line to extend the length of California, over 1100 miles. In all such cases step-up transformers, situated at the power house, transfer the electrical energy developed by
FIG. 333. High-voltage long-distance transmission line
the generator to the line, and step-down transformers, situated at the receiving end, transfer it to the motors or lamps which are to be supplied (Fig. 333). The generators used on the American side of Niagara Falls produce a pressure of 2300 volts. For transmission to Buffalo, 20 miles away, this is transformed up to 22,000 volts. At Buffalo it is transformed down to the voltages suitable for operating the street cars, lights, and factories of the city. On the Canadian side the generators produce currents at 12,000 volts, as stated, and these are transformed up, for long-distance transmission, to 22,000, 40,000, and 60,000 volts (see Fig. 166, p. 150).
374. The tungar rectifier. Negative electrons are found to escape from a filament that is heated to incandescence, and if this filament is then made more than, say, 25 volts negative with respect to a near-by anode any gas that surrounds the filament is found to be ionized (split into positively and negatively charged parts) by the violence of the blows which the electrons strike against its molecules. It is thus rendered conducting. These facts are utilized in the tungar rectifier of the alternating current. The bulb (Fig. 334) is filled with argon to a pressure of 3 to 8 cm. The anode is a small cone of graphite or tungsten,
FIG. 334. Tun
and the cathode is a coiled tungsten filament. When the rectifier is in operation, the cone and the filament are alternately + and −, one being while the other is When the cone is + and the filament the negative electrons from the filament are forced across the space from the filament to the cone, and the argon, which is thereby ionized, carries the current from the cone to the filament. When the cone is and the filament +, the negative electrons cannot escape from the filament; hence the gas does not become conducting. The principle of operation. can be understood from Fig. 335.
The rectifier is connected to the alternating-current line at C and D. The alternating current in the primary coil P of the transformer T causes an induced current in S, which keeps the filament F incandescent. Under the action of the current, A and F are alternately + and When F is -, the electrons escape and ionize the gas, permitting the current to pass. When F is + the negative electrons are driven back into the filament and cannot escape to ionize the gas. Hence no current passes. In this way a unidirectional pulsating current passes through the storage batteries or other load. This rectifier is used largely for charging storage batteries for small-power purposes.
375. Principle of the carbon microphone. Let a dry cell, an ammeter, and two pieces of electric-arc carbon be arranged in series (Fig. 336). Press the carbons very gently and observe the reading of the ammeter. Press gradually harder, then gradually less, watching the instrument. The current increases with increase in pressure, and decreases with decrease in pressure.
FIG. 336. The principle of the carbon transmitter
FIG. 335. Principle of operation of the tungar rectifier
This peculiar behavior of carbon in offering a variable resistance with variation in pressure is taken advantage of in constructing the carbon transmitter of the telephone. In the modern transmitter, however, the current is made to traverse many particles of granular carbon, which, lying loosely together, furnish a very great number of loose contacts (see Fig. 339).
376. Principle of the telephone. The telephone was invented in 1875 by Alexander Graham Bell of Washington (see on opposite page) and Elisha Gray of Chicago. The simple local-battery system is shown in Fig. 337.
The current from the battery B (Fig. 337) is led first to the back of the diaphragm E, whence it passes through a little chamber C, filled with granular carbon, to the conducting back of the transmitter, and thence through the primary p of the induction coil, and back to the battery.
When a sound is made in front of the microphone, the vibrations produced by the sounding body are transmitted by
FIG. 337. The telephone circuit (local-battery system)
the air to the diaphragm, thus causing the latter to vibrate back and forth. These vibrations of the diaphragm vary the pressure upon the many contact points of the granular carbon through which the primary current flows. This produces considerable variation in the resistance of the primary circuit, so that as the diaphragm moves forward, that is, toward the carbon, a comparatively large current flows through p, and as it moves back a much smaller current. These changes in the current strength in the primary p produce changes in the magnetism of the soft-iron core of the induction coil. Currents are therefore induced in the secondary s of the induction coil, and these currents pass over the line and affect the receiver at the other end. A step-up induction coil is used to get sufficient potential to work through the high resistance of a long line.