the generating stations and generators, feed wires, trolley wires or third rails, the cars, and the road bed. The generators are usually direct-current dynamos producing current Feed Wire Trolley Wire

at a pressure of about 500 volts. The positive poles of these dynamos are connected to the feed wires, trolley wires, or third rails, while the negative poles are connected to the rails, which serve as return

conductors.

Feed wires are used when the line is a long one; they are connected to different sections of the trolley wire. The current goes from the trolley wire to the wheel in contact with it, down the conductor carried by the pole, to the motor, then through the car wheels to the track and then to the dynamo. The cars are in parallel, like incandescent lamps on a lighting circuit, and each takes its own current, independent of the rest. The amount of current taken by each car depends largely upon the load in the car and the grade of the road. Since the motors are

subject to the severest kind of usage, they are protected with water-tight cases.

In the system illustrated in Fig. 444 an alternating current is used at a pressure of 11,000 volts. The trolley wire is of steel, with a copper supply wire about an inch above it. These wires are supported by a steel wire, and this in turn is supported by steel bridges.

The use of the motor on electric railways has solved the problem of how to transport people to and from their places of business in large cities. Another important use is the application of individual motors to machines such as printing presses, lathes, band saws, etc. Each machine can then be run separately without large waste of power through the friction of shafts and belting.

477. The Transmission of Electrical Energy. The function of a transmission line is to transmit electrical energy to a distance. There is always a certain loss in transmission, due chiefly to the heating of the line. As the heating of a conductor is proportional to the square of the current, it is plain that the current sent should be as small as possible. To transmit 10,000 watts of electrical power by using 1 ampere at a pressure of 10,000 volts is much more economical than to send 10 amperes at a pressure of 1000 volts. For this reason transmission lines are high tension lines, and require only a small wire to carry the current. For instance, the alternators of the electric road from Philadelphia to Atlantic City give 6600 volts, but this current is changed to one of 33,000 volts by the use of step-up transformers. It is sent over the line at this voltage to the substations where the voltage is reduced to 430 volts by step-down transformers and then fed into the alternating side of a rotary converter, from the direct current side of which a direct current of 650 volts is taken. This is fed into the third rail which feeds the motors on the cars.

Questions

1. Make a drawing showing how the lines of force go through the diaphragm of a telephone receiver.

2. How do you explain the heavy current that passes through the carbons of an arc lamp when the lamp is first turned on?

3. Why is platinum used instead of copper for the sealing in wire of incandescent lamps?

4. Why is it a good plan- whenever a dynamo supplies current to a number of groups of lamps — to place a fuse between each group and the mains?

5. Why is it necessary to use a step-down transformer between the street mains of an alternating system and the house lighting? 6. Why is it best to use a step-up transformer between the alternating current generator and the transmission line?

Problems

1. A small arc lamp requires a current of 4 amperes and a difference of potential at its terminals of 45 volts. What is the resistance of the lamp? What resistance must be put in series with it on a 110-volt circuit?

2. What current will pass through the lamp when the carbons are first put in contact?

3. The positive carbon of a projection lantern burns off an inch in length in 40 minutes. How much will each carbon be shortened if the lantern is run for a lecture lasting an hour and a half?

4. An open arc requires a voltage of 45 volts and a current of 9 amperes. What is the resistance of the arc? How much resistance must be put in series with the arc to burn it across a 110-volt circuit? How much must be put in if two arcs are put in series across this circuit? How much current will they take?

5. A certain inclosed arc lamp requires 4.5 amperes, but the voltage at its terminals must be 80 volts to run it properly. What is the resistance of the inclosed arc lamp? How much resistance must be put in series with it to burn it across a 110-volt circuit?

6. The current required by the searchlight projector shown in Fig. 445, is 100 amperes. What resistance must be placed in

A

series with it on a 110-volt circuit, when the potential difference at the terminals of the lamp is 60 volts? What is the resistance of the lamp?

7. The searchlight projector at Fort Monroe has a diameter of 60 inches. With 150 amperes at 60 volts the arc gives 194,000,000 candle power. What is the resistance of the arc? How many candle power does it give per watt?

8. A Cooper-Hewitt mercury vapor lamp requires 3.5 amperes and gives 700 candle power on a 110-volt circuit. What are the total watts of the lamp? How many watts per candle power?

FIG. 445

9. The fall of potential in the same lamp, when running, is 74 volts in the tube alone. What is its resistance? What resistance must be put in series with it on this circuit?

10. A carbon filament, 110-volt, 16-candle-power lamp requires 0.5 ampere. What is its hot resistance? How many such lamps can be run per electrical horse power? How many per kilowatt? How many watts are required per candle power?

11. A metallized filament, 110-volt, 20-candle-power lamp requires 0.454 ampere. What is its hot resistance? How many watts are required per candle power? How many such lamps can be run per horse power? How does its efficiency compare with that of the carbon filament lamp?

12. Suppose 12, 16-candle power, carbon filament lamps are burning on one side of the neutral wire in a three-wire system, and 7 on the other. How much current does the neutral wire carry? How much does each of the other wires carry?

13. Suppose that 25, 16-candle power, carbon filament lamps are burning on one side of the neutral wire and 25 metallized filament lamps on the other. How much current will the neutral wire carry ? How many candle power will each side give?

14. What is the heat loss per hour, in each of the lamps mentioned in problem 13?