=310. Uses of Transformers.=--In electric lighting systems, dynamos often produce alternating currents at 1000 to 12,000 volts pressure. It is very dangerous to admit currents at this pressure into dwellings and business houses, so that transformers are installed just outside of buildings to "step-down" the high voltage currents to 110 or 220 volts.

The lighting current that enters a house does not come directly from a dynamo. It is an induced current produced by a transformer placed near the house. (See Fig. 307.) In a perfect transformer the efficiency would be 100 per cent. This signifies that the energy that is sent into the primary coil of the transformer exactly equals the energy in the secondary coil. The best transformers actually show efficiencies better than 97 per cent. The lost energy appears as heat in the transformer.

"The transfer of great power in a large transformer from one circuit to another circuit entirely separate and distinct, without any motion or noise and almost without loss, is one of the most wonderful phenomena under the control of man."

=311. The mercury arc rectifier= is a device for changing an alternating current into a direct current. It is frequently used for charging storage batteries where only alternating current is supplied by the electric power company. It consists of an exhausted bulb containing two carbon or graphite electrodes marked _G_ in Fig. 308 and a mercury electrode marked _M_. It is found that current will pa.s.s through such a bulb only from the graphite to the mercury but not in the reverse direction. In operating the device, the secondary terminals of an alternating current transformer _T_ are connected to the graphite terminals of the rectifier. A wire connected to the center of the secondary of the transformer at _C_ is attached to the _negative_ terminal of the storage battery _SB_. The _positive_ terminal of the battery is connected to the mercury electrode of the rectifier tube through a reactance or choke coil _R_. This coil serves to sustain the arc between the alternations. _Sw_ is a starting switch, used only in striking the arc. It is opened immediately after the tube begins to glow.

[Ill.u.s.tration: FIG. 308.--Diagram of a mercury arc rectifier.]

Important Topics

Transformer, induction coil, mercury arc rectifier, construction, action; uses of each.

Exercises

1. Does the spark of an induction coil occur at "make" or at "break?"

Why?

2. What must be the relative number of turns upon the primary and secondary coils of a transformer if it receives current at 220 volts and delivers current at 110? Also show by diagram.

3. Would the transformer work upon a direct current? Why?

4. Explain why the interrupter is a necessary part of the induction coil and not of the transformer.

5. If a building used eighty 110-volt incandescent lamps, what would be necessary to light them if they were joined in series? Why would this not be practical?

6. If a 16-candle-power lamp requires 0.5 ampere upon a 110-volt circuit what current and voltage will be needed to operate 12 such lamps in parallel?

7. What will it cost to run these lamps 4 hours a night for 30 days at 10 cents per kilowatt hour?

8. If a mercury arc rectifier uses 5 amperes of current at 110 volts alternating current to produce 5 amperes of direct current at 70 volts, what is the efficiency of the rectifier?

9. Compute the heat produced in a 40 watt tungsten lamp in 1 minute.

10. Compute the heat produced in a 60 watt carbon incandescent lamp in 1 hour.

(4) THE TELEPHONE

=312. The Electric Telephone.=--This is an instrument for reproducing the human voice at a distance by an electric current. The modern electric telephone consists of at least four distinct parts (see Fig.

312); viz., a _transmitter_, an induction coil, an electric battery, and a _receiver_. The first three of these are concerned in sending, or _transmitting_ over the connecting wires a fluctuating electric current, which has been modified by the waves of a human voice. The receiver, is affected by the fluctuating current and reproduces the voice. It will be considered first, in our study.

=313. The telephone receiver= was invented in 1876 by Alexander Graham Bell. It consists of a permanent steel magnet, U shaped, with a coil of fine insulated copper wire about each pole. (See Fig. 310.) A disc of thin sheet iron is supported so that its center does not quite touch the poles of the magnet. A hard rubber cap or ear piece with an opening at its center is screwed on so as to hold the iron disc firmly in place.

[Ill.u.s.tration: FIG. 309.--The simplest telephone system. It consists of two telephone receivers connected in series on a circuit. It will work, but not satisfactorily.]

_The action of the receiver_ may be understood from the following explanation: The electric current sent to the receiver, comes from the secondary coil of the induction coil; it is an alternating current, fluctuating back and forth just in time with the waves of the voice affecting it at the transmitter. This alternating current flows around the coils on the poles of the permanent magnet. When this current flows in one direction, its magnetic field a.s.sists the field of the permanent magnet, strengthening it. This stronger magnetic field draws the thin iron disc in front of the poles of the magnet a little closer to them.

When the current in the coils flows the other way, its magnetic field weakens the field of the steel magnet, and the disc is drawn back by the force of its own elasticity. Thus the disc of the receiver vibrates with the alternations of the current, and reproduces the same sounds that were spoken into the transmitter.

[Ill.u.s.tration: FIG. 310.--A telephone receiver. This receiver has a permanent horseshoe magnet with a coil about each pole.]

=314. The Telephone Transmitter.=--The telephone receiver just described has great sensitiveness in reproducing sound, but it is not satisfactory as a transmitter or sending apparatus. The _transmitter_ commonly used is represented in cross-section in Fig. 311. In this figure, back of the mouthpiece, is a thin carbon disc, _D_. Back of this disc is a circular compartment containing granular carbon, _g_. The wires of the circuit are connected to the carbon disc and to the back of the case containing granular carbon. The circuit through the transmitter also includes a voltaic or storage cell and the primary coil of an induction coil. (See Fig. 312.)

[Ill.u.s.tration: FIG. 311.--A telephone transmitter.]

[Ill.u.s.tration: FIG. 312.--Telephone instruments at one end of a talking circuit.]

=315. The action of the transmitter= is explained as follows: When the sound waves of the voice strike upon the carbon disc, the latter vibrates, alternately increasing and decreasing the pressure upon the granular carbon. When the pressure _increases_, the electrical resistance of the granular carbon is _lessened_, and when the pressure upon it is _decreased_, its resistance _increases_. This changing resistance causes fluctuations in the electric current _that_ correspond exactly with the sound waves of the voice affecting it.

=316. A complete telephone system= operating with a local battery is shown in Fig. 312. A person speaking into the transmitter causes a fluctuation in the electric current in the transmitter as described in Art. 315. This fluctuating current pa.s.ses through the primary coil of the induction coil _Ic_. This fluctuating current produces a fluctuating magnetic field in its core. This fluctuating field induces an _alternating_ current in the secondary coil which alternates just as the primary current fluctuates, but with a much higher E.M.F. than the latter. The alternating current pa.s.ses to the receiver which reproduces the speech as described in Art. 313. The line circuit includes the secondary of the induction coil, the receiving instrument and the receiver of the sending instrument so that the voice is reproduced in both receivers. An electric bell is placed at each station to call the attention of parties wanted. The movement of the receiver hook when the receiver is lifted, disconnects the bell and closes the talking circuit.

The latter is opened and the bell connected when the receiver is hung up again.

[Ill.u.s.tration: FIG. 313.--Diagram of a telephone system as used in a large exchange.]

In cities and towns, the telephone system in use differs from the one described in usually having one large battery placed in the central exchange, instead of dry cells at each instrument. (See Fig. 313.) Also the operator at _central_ is called by simply taking the receiver from the hook instead of being "rung up" by the subscriber. The operations of the transmitter, induction coil and receiver, however, are the same in all telephones.

Important Topics

1. Receiver: parts, action.

2. Transmitter: parts, action.

3. Induction coil, bell, line wires, etc.

4. Action of the whole device.

Exercises

1. State three important electrical laws or principles that are employed in the operation of the telephone. What is the application of each?

2. Connect the binding posts of a telephone receiver with a sensitive galvanometer and press on the diaphragm of the receiver; a deflection of the galvanometer will be noticed. Release the diaphragm and a reflection in the opposite direction is seen. Explain.

3. Is the current pa.s.sing through the transmitter the one going to the receiver of the instrument? Explain.

4. Does the receiver at the telephone used by a person repeat the speech of the person? Explain.

5. How many 0.5 ampere lamps can be used with a 6 ampere fuse?

6. Why is it necessary to have a rheostat connected in series with a stereopticon or moving picture machine while a rheostat is not used with arc lights out doors?

7. How many candle power should a 60 watt carbon incandescent lamp give, if its efficiency is 3.4 watts per candle power?

8. Three incandescent lamps having resistances of 100, 150, and 240 ohms, respectively, are connected in parallel. What is their combined resistance?

Review Outline: Induced Currents