"

with silk. The two ends of the coil communicate with the binding screws, m and n, by means of which they may be made to communicate with the poles of a magnetic couple. A metallic frame supports a hook, from which is suspended a delicate silken cord, s. This cord supports two fine magnetic needles, the one, ab, above the graduated circle, and

the other, B, within the coil, only a part of which is visible in the figure. The two needles are so united that one can not turn without the other, and their poles being placed in opposite directions, the action of the earth upon them is completely neutralized. Hence they are free to obey the least force.

Uses of the Galvanic Multiplier.

432. The Multiplier is used to indicate the feeblest currents of electricity. By means of it, BECQUEREL established

(432) What is the use of the Multiplier?

the fact that a current is developed in every chemical action, in the imbibition of liquids, and in many other phenomena. By using a galvanometer with many thousands of turns of wire, the existence of electrical currents in animals and vegetables may be demonstrated.

Fig. 298.

If the current cease, the iron bar at once loses its magnetism.

We may

in like manner form a permanent magnet

by using a bar of steel instead of a bar of iron.

Illustrate. (433.) How is an iron bar converted into a magnet by galvanism? In what way may a bar of steel be converted into a magnet?

The bar of steel may also be magnetized by passing through the wire a spark from a Leyden jar. To do this, one end of the wire is made to touch the external covering of the jar, and the other end is brought into contact with the button of the jar. The steel bar is magnetized instantaneously, thus showing the identity between the electricity of the galvanic current and that of the Leyden jar.

II. ELECTRO-MAGNETIC

434.

TELEGRAPHS. -THE ELECTRO-MOTOR.

The Electro-Magnet.

An ELECTRO-MAGNET is a magnet obtained by the use of electricity.

Electro-magnets are generally made of soft iron, bent in the form of a horse-shoe, as shown in Fig. 299. Upon each branch is wound a great number of coils of wire, insulated by being covered with silk, The wire is coiled in different directions upon the two branches, and its extremities are then connected with the poles of a battery.

Fig. 299.

In this way magnets may be constructed of immense power, so powerful, in fact, as to support the weight of ten or twelve persons. Fig. 300 represents the method of arranging the details of a magnet which is intended to exhibit a great sustaining power.

The plate in contact with the two poles is called an arma

ture.

When the instrument is of soft iron, it is magnetized instantaneously by the passage of a current of electricity through the wire, and

In what other way may it be done? What inference is drawn from this fact? (434.) What is an Electro-Magnet? How are they constructed? What is an armature?

460

as instantaneously loses its magnetism when the current is stopped, or broken. This property has been utilized in the electro-magnetic telegraph.

The Electrical Telegraph.

currents.

In 1820, AMPERE proposed to transmit signals by passing currents over magnetic needles, making use of as many wires and needles as

What is the principal use of the electro-magnet? (435.) What is an Electrical Telegraph? Give an outline of the history of magnetic telegraphs.

there are letters. In 1837, STEINHEIL, of Munich, actually constructed such a telegraph.

In 1831, Prof. HENRY, now of the Smithsonian Institute, published the results of his researches on the subject of electro-magnetism, and in subsequent years, exhibited experiments illustrative of their application to the transmission of power to a distance, for the purpose of producing telegraphic effects.

In 1837, Prof. MORSE invented a machine for recording signals upon paper, and in 1844, the first working line of telegraph for practical purposes was built from Washington to Baltimore.

Many modifications of the telegraphic apparatus have been made since its first invention. Three principal varieties are now in use, all of which are based upon a common principle, which is very simple.

At the station from which a telegram is dispatched, is an electrical battery, and at the one where it is to be received, is an electro-magnet. The two are connected by a wire running between the stations. When the current is transmitted through the wire, the iron becomes magnetized and attracts an armature of soft iron, which in turn imparts motion to other pieces, by means of which the signals are imparted. When the current ceases, the iron loses its magnetism, and a spring forces the armature back to its primitive position. By successively breaking and restoring the current, the telegram is transmitted.

In one form of the telegraph, the electro-magnet causes a needle to move over a sort of dial, around which are printed the letters of the alphabet. The letter before which the needle stops, is the one to be transmitted. This machine requires as many signals as there are letters in the message. This is the dial telegraph.

In another form of the telegraph, there are two electromagnets, which set in motion two movable arms placed at the extremities of a horizontal black line on a white dialplate. The relative positions of the hands with reference to the fixed line, serve as conventional signals, nearly in the same way as was customary in the old-fashioned telegraph. This is the signal telegraph.

How many kinds are in common use? Explain their general principle. What is the dial telegraph? The signal telegraph?