Such is the dry statement of a problem the more detailed working of which we are about to explain to the reader.
When a schoolboy places a sixpence and a piece of zinc in juxta-position with each other in his mouth, he immediately perceives a singular taste, which as instantly disappears upon their separation; it is an experiment which most of us have performed, wondered at for a moment, and then forgotten. How little did we ever dream that in so doing we were calling into life one of the most subtle, active, and universal agents in nature—a spirit like Ariel to carry our thoughts with the speed of thought to the uttermost ends of the earth—a workman more delicate of hand than the Florentine Cellini, and more resistless in force than the Titans of old!
If now we place a piece of zinc, Z, and of copper, C, in a glass of acidulated water, instead of in the saliva of the mouth, and if we then attach to the piece of zinc the wire D K, and to the piece of copper the wire B A, and approximate the two ends, A K, until they touch, we shall have the philosophic expression of the contrivance of the boy—a decomposition of the water will immediately take place, and either as its cause or consequence—for scientific men have not yet decided which—an electric current will flow in a continued stream from the zinc plate or positive pole to the copper plate or negative pole of the battery, and this action, provided the plates are kept clean and the acidulated water is supplied, will go on as long as the materials last. If this little instrument, which generates a very small amount of electric force, is combined with others, as in figure 2,—the zinc plate of one cell being connected with the copper plate of the next by a piece of wire—we shall have the celebrated battery invented by Volta in 1800, in which the accumulated current, after flowing from one cell into another, by means of the little hoops of wire, is transmitted along the large hoop, D K A B, from the one pole of the battery to the other. Within the narrow chambers of some such battery (which may be made of any number of cells, according to the force required), the motive power is generated by which the electric telegraph is worked, and the large hoop by which its two poles are connected represents the telegraphic wire we see running beside the railroad, whose office is to form a conducting pipe for the conveyance of the electricity. Different substances possess this property in various degrees; some, such as dry paper, not permitting the passage of the electric fluid to any sensible extent; and others transmitting it with great freedom. Of all known bodies, the metals are the most perfect conductors, and copper is in this respect superior to iron; but the latter, being cheaper and more durable, is commonly employed in the construction of the telegraph. Thus we have two of the indispensable requisites—a constant force and a channel which conveys it from place to place.
There was yet a third thing necessary—some contrivance by which the force could be made instrumental in forming signs or characters at its destined goal; and this final condition was supplied by Oersted’s discovery in 1819, that a magnetic needle is deflected by the passage of a circuit of electricity through a wire parallel and in close neighbourhood to it. The following cut will explain our meaning:—When the fluid passes from the U pole of the battery in the direction of B A K L M Z, and enters V, its opposite pole, “a current,” as it is called, is completed, running from left to right, the effect of which upon the needle, N, is to deflect it in the direction of the dotted line (seen in perspective) 2, 3, or to an angle of 90 degrees, with the wire, if the current is sufficiently strong. If, however, the current be reversed, and the electric fluid made to traverse the wire from right to left, in the direction of the letters V Z M L K A B to the U end of the battery, the needle will immediately reverse its position and place itself at 90 degrees in the opposite direction. This then is the whole principle and mystery of the needle telegraph, the one still most extensively used in this country. The break that occurs between the letters B U and Z V is intended to show the method in which the needle is made to work. “Whilst the wires are thus apart the circuit is broken,” or the fluid no longer passes along the wire, but immediately they are approximated the circulation again commences, and the needle “answers the helm.” By the opening and closing, then, of this small space, which is effected by a lever, the needle is made to oscillate at will.
The mere fact, however, of an electric current passing along a wire in proximity to a magnetic needle was not sufficient to enable any person to construct a telegraph. Would the needle be deflected by a wire, the battery of which was placed at any considerable distance? it would not; therefore, for all telegraphic purposes Oersted’s discovery was worthless. Schweigger, however, soon after ascertained that by passing a great number of times round the needle a wire, thoroughly insulated by a “serving” of silk thread, as shown in figure 4, the deflecting powers of the currant were multiplied, and the sensibility of the instrument marvellously increased.
