SUMMARY. Currents are usually measured with galvanometers. These may have either (1) a movable coil with a stationary magnet or (2) a movable magnet with a stationary coil.

QUESTIONS AND PROBLEMS

1. What is the principle involved in the chemical method of measuring the strength of an electric current? in the magnetic method?

2. How could you test whether or not the strength of an electric current is the same in all parts of a circuit? Try it.

3. Explain from the diagram of the commercial ammeter (Fig. 269) the principle of the suspended-coil, or d'Arsonval, type of galvanometer.

4. In a galvanometer consisting of a stationary coil and a magnetic compass, what force opposes the motion due to the magnetic field of the current?

5. What force acts to oppose the motion of the coil (1) in the suspended-coil galvanometer; (2) in the commercial ammeter represented in Fig. 269?

6. In calibrating an ammeter the current which produces a certain deflection is found to depositg. of silver in 50 min. What is the strength of the current?

ELECTRIC BELL AND TELEGRAPH

313. The electric bell. The electric bell (Fig. 270) is one of the simplest applications of the electromagnet. When the button P is pressed (Figs. 270 and 271), the electric circuit of the battery is closed, and a current flows in at A, through the coils of the magnet, over the closed contact c, and out again at B. But as soon as this current is established, the electromagnet E pulls over the armature a, and in so doing breaks the contact at c. This stops the current and demagnetizes the magnet E. The armature is then thrown back against c by the elasticity of the spring s which supports it. No sooner is the contact made at c than the current again begins to flow and the former operation is repeated. Thus the circuit is automatically made and broken at c, and the hammer H is in consequence set into rapid vibration against the rim of the bell.

314. The telegraph. The electric telegraph is another simple application of the electromagnet. The principle is illustrated in Fig. 272. As soon as the key K, at Chicago for example, is closed, the current flows over the line to, we will say, New York. There it passes through the electromagnet m, and thence back to Chicago through the earth. The armature b is held down by

the electromagnet m as long as the key K is kept closed. As soon as the circuit is broken at K the armature is pulled up by the spring d. By means of a clockwork device the tape c is drawn along at a uniform rate beneath the pencil or pen carried by the armature b. A very short time of closing of K produces a dot upon the tape; a longer time, a dash. As the Morse, or telegraphic, Chicago

New York

Cz

m

Earth

Earth

FIG. 272. Principle of the telegraph

alphabet consists of certain combinations of dots and dashes, any desired message may be sent from Chicago and recorded in New

York. In modern practice the message is not ordinarily recorded on a tape, for operators have learned to read messages by ear, a very short interval between two clicks being interpreted as a dot, a longer interval as a dash.

The first commercial telegraph line was built

by S. F. B. Morse (see opposite page 282) between Baltimore and Washington. It was opened on May 24, 1844, with the now famous message, "What hath God wrought!"

315. The relay and sounder. Since the current that comes over a long telegraph line is of small amperage, the armature of the electromagnet of the receiving instrument must be made very light to respond to the action of the current. The electromagnet of this instrument is made of many thousand turns of fine wire, to secure the requisite number of ampere turns (§ 310) to work the armature. The clicks of such an armature are not sufficiently loud

to be read easily by an operator. Hence at each station there is introduced a local circuit, which contains a local battery and a second and heavier electromagnet, which is called a sounder. The electromagnet on the main line is called

the relay (Figs. 273 and 275). The sounder has a very heavy armature (Fig. 274, A), which is so arranged that it clicks both when

it is drawn down by its electromagnet against the stop S and when it is pushed up again by its spring, on breaking the current, against the stop t. The interval which elapses between these two clicks indicates to the operator whether a dot or a dash is sent. The small current in the main line simply serves to close and open the circuit in the local battery which operates the sounder (Fig. 275). The electromagnets of the relay and the sounder differ in that the latter consists of a few hundred turns of coarse wire and carries a comparatively large current.

316. Plan of a telegraphic system. The actual arrangement of the various parts of a telegraphic system is shown in Fig. 275. When an operator at Chicago wishes to send a message to New York, he first opens the switch which is connected to his key, and which is always kept closed except when he is sending a message. He then begins to operate his key, thus controlling the clicks of both his own sounder and that at New York. When the Chicago switch is closed and the one at New York open, the New York operator is able to send a message back over the same line. In practice a message is not usually sent as far as from Chicago to New York over a single line, except in the case of transoceanic cables. Instead it is automatically transferred, say at Cleveland, to a second line, which carries it on to Buffalo, where it is again transferred to a third line, which carries it on to New York. The transfer is made in precisely the same way as the transfer from the main circuit to the sounder circuit. If, for example, the sounder circuit at Cleveland is lengthened so as to extend to Buffalo, and if the sounder itself is replaced by a relay (called in this case a repeater), and the local battery by a line battery, then the sounder circuit has been transformed into a repeater circuit, and all the conditions are met for an automatic transfer of the message at Cleveland.

SUMMARY. The multiple-stroke electric bell depends for its continued action upon an automatic circuit breaker.

The Morse telegraph system is of the closed-circuit type, the current flowing constantly when the line is not in use.

A telegraph relay receives a very weak current through its helix, which consists of thousands of turns of wire.

A telegraph sounder has only a few turns in its helix, which must carry a relatively strong current to get the necessary ampere turns for operation.

SAMUEL F. B. MORSE (1791-1872)

The inventor of the electromagnetic recording telegraph and of the dot-and-dash alphabet known by his name was born at Charlestown, Massachusetts, graduated at Yale College in 1810, invented the commercial telegraph in 1832, and struggled for twelve years in great poverty to perfect it and secure its proper presentation to the public. The first public exhibition of the completed instrument was made in 1837 at New York University, signals being sent through 1700 feet of copper wire. It was with the aid of a $30,000 grant from Congress that the first commercial line was constructed in 1844 between Washington and Baltimore