1. An electro-magnet may be alternately endowed with and deprived of the property of attracting iron by connecting and disconnecting it with a galvanic battery.

2. The battery may be miles away from the magnet. If wires connect the two, the electric current will still be carried to the helix and produce the same effects.

3. A person stationed near the battery may complete and break the circuit at pleasure. As he does so, one end of a lever placed near the poles of the distant magnet will be attracted or released. When it is attracted, the other end of the lever, which is furnished with a point, is made to indent a strip of paper passed in front of it by machinery, with dots or dashes, according to the time that the operator by the battery keeps the circuit complete. If, now, different combinations of dots and dashes are agreed upon to represent certain letters, it is evident that a message can be communicated from the one point to the other. Fig. 326 represents Morse's recording apparatus.

A B is the electro-magnet, connected with the distant battery by the wires L, M, which are raised on poles and insulated by glass supports. C is an armature of soft iron attached to one end of the lever D D, so as to rest about one-eighth of an inch above the poles of the magnet. The other end of the lever carries a point or style, I, which is raised as C is depressed. A strip of paper, F, F, rolled on the spool E, is made to pass in front of the style, between the two cylinders G, H, by means of wheel-work set in motion by the weight J when the current passes. K is a spring, to pull down the end of the lever bearing the style when the other end is released by the magnet. A striking apparatus was formerly connected with the machinery in such a way as to give warning to the attendant with the first motion of the lever; but it is now generally dispensed with, as the clicking sound produced by the lever is found to be sufficient for the purpose.

Instead of carrying both wires over poles from the electro-magnet to the battery, the earth is now generally made to form one-half the circuit. This is effected by carrying down the wire from the magnet, and connecting it with a metallic plate buried in the ground; a similar plate must be buried where the battery is stationed, and a wire from the latter connected with it. If this is done, but one wire need pass over the poles to complete the circuit.

917. The apparatus used by the operator where the battery is stationed, to complete and break the circuit, is called the Signal Key. It is represented in Fig. 327.

Fig. 327.

THE SIGNAL KEY.

By pressing on the knob, the screws in which the wires are fastened are connected, and the circuit is completed. On removing the hand, the knob springs up, the circuit is broken, and the current ceases. If the knob is kept pressed down, the paper at the other end is indented with a continuous line; but by tapping on it so as to form different combinations of dots and dashes, which stand for letters, and

are understood at both ends of the line, a message is transmitted. According to Morse's system, the following combinations are used to represent the different letters and figures:

operation. What was formerly connected with the machinery? Why is it now dispensed with? Instead of carrying both wires over poles from the electro-magnet to the battery, what is now the more usual arrangement? How is the earth made to form half of the circuit? 917. What is the Signal Key? Describe it, and its mode

1

To prevent confusion, a small space is left after each letter, a longer one between words, and a still longer one at the end of a sentence. The operators in telegraph offices become so familiar with this alphabet that they understand a message from the mere clicks of the lever, without looking at the paper on which it is recorded.

918. An electric current is transmitted by a wire to a great distance, but not with undiminished power. When, therefore, the stations are very far apart, the electromagnet is charged too feebly to make the style indent the paper. In this case, the wire from the original battery is made to act on a very delicate armature, so as to complete the circuit of a second battery placed near the machine. This Relay Battery, as it is called, acts on the recording apparatus as described above, or transmits a fresh and vigorous current to another relay battery. In this way lines of any length may be formed.

As relay batteries do not interrupt the circuit, any number of them may be placed at intervals along a line. Each may work a recording apparatus of its own, and a given communication may thus be registered simultaneously at a multitude of different stations.

Relay batteries may be dispensed with by increasing the number of plates employed and distributing them in groups along the line. It has been computed that if a telegraph wire could be carried round the earth, 1200 of Grove's pint cups, distributed in equi-distant groups of fifties, would supply the galvanic power for the whole distance.

of operation. How are the different letters represented? 918. What difficulty is there when the current is transmitted to a great distance? How is this remedied? How does the Relay Battery act? How may a given message be registered simultaneously at different stations? What may be substituted for relay batteries? How many cups would supply the galvanic power for a telegraph round the earth?

919. House's and Bain's Telegraph.-Morse's apparatus, having been first introduced and being very simple and not likely to get out of order, is more used than any other, both in this country and in Europe. There are other ingenious systems, however, which are employed to some extent. Among these are House's Printing Telegraph and Bain's Electro-chemical Telegraph.

House's apparatus is one of the most wonderful achievements of inventive art. Making use of the electro-magnet in connection with ingenious and somewhat intricate machinery, it enables the operator, by playing on twentyeight keys like those of a piano (representing the twenty-six letters and two punctuation points), to print ordinary letters on a strip of paper at the other end of the line at the rate of about two hundred a minute. The great advantages of House's system are that there is little or no liability to mistake in transmitting a message, and that the latter, being produced in Roman capitals, need not be transcribed, but may be sent just as it comes from the machine to the person for whom it is intended.

In Bain's Electro-chemical Telegraph no magnet is used. The point of the wire, which is stationary, constitutes the pen, and rests lightly on a metallic plate, which is made to revolve by machinery. On this plate is placed paper which has been previously moistened with some chemical preparation decomposable by voltaic electricity. When the connection is made by the distant operator, the current passes from the wire to the plate through the paper, and in passing decomposes the chemical compound with which the paper is impregnated. The result is a deep blue spot on the paper, which renders the dot or dash visible, just as the indentation does according to Morse's system. As even a feeble voltaic current has the power of decomposition, there is not the same necessity for relay batteries on Bain's line as on either of the others.

920. Submarine Telegraphs.-Submarine Telegraphs are telegraphs connecting points separated by water, in which the wire is submerged. The first successful telegraph of this kind was laid in 1851 across the English Channel, and connected Dover with the French coast. This was followed by several others; and in 1858, after several unsuccessful attempts, a telegraph cable nearly 2,000 miles in length was laid across the Atlantic Ocean, between Valen

919. What other telegraph systems besides Morse's are in use? What is said of House's apparatus? What are its great advantages? What is the principle involved in Bain's Electro-chemical Telegraph? What advantage is there connected with this system? 920. What are Submarine Telegraphs? Where and when was the first submarine telegraph laid? In 1858 what great enterprise was carried through? De

tia Bay, Ireland, and Trinity Bay on the coast of Newfoundland. It consisted of a group of seven copper wires insulated and protected by a casing of gutta-percha, the whole surrounded by strands of iron wire, and sunk to the bottom of the ocean, at a depth nowhere exceeding 24 miles.

Public interest was strongly excited in this great enterprise; but it has thus far been doomed to disappointment. After transmitting several messages, the Atlantic Telegraph, for some unexplained reason, ceased to work, though signals have from time to time been received. There is little doubt, however, that the work is feasible, and that we shall soon have regular telegraphic communication between the opposite sides of the Atlantic.

921. History of the Telegraph.-The fact that frictional electricity could be conveyed by wires to a great distance was known more than a hundred years ago. Franklin, in 1748, set fire to alcohol by means of a wire from an electrical machine carried across the Schuylkill River. The first attempt to transmit a communication by electricity, however, was made in 1774 by Le Sage [luh sahzh], a Frenchman, at Geneva.

Le Sage used twenty-four wires insulated in glass tubes buried in the earth, each of which represented a letter of the French alphabet. The wires were connected with an electrical machine in the order necessary to spell out the words, and electroscopes attached to them at the other end indicated this order by their successive divergence to an attendant stationed there.

922. Volta's discovery in 1800 furnished a far more efficient agent for telegraphic communication than frictional electricity, and was followed in a few years by a plan for an electro-chemical telegraph, requiring thirty-five wires, to represent the different letters and figures, and to act by the decomposition of water.

The great discovery of electro-magnetism in 1819 called forth many new suggestions,-among others, the use of the deflections of the needle as signals; but none of the plans proposed were practicable on a large scale. A more per

scribe the Atlantic cable. What is said of the working of the Atlantic telegraph? 921. What fact relating to frictional electricity was known more than a hundred years ago? What experiment was performed by Franklin in 1748? Who made the first attempt to transmit a message by electricity? Describe the plan of Le Sage. 922. By what was the discovery of voltaic electricity followed? What suggestions were called