Great Facts / A Popular History and Description of the Most Remarkable Inventions During the Present Century

by Frederick C. Bakewell

The fact that an electric current might be sent through a long circuit had been established by Dr. Watson, in conjunction with other Fellows of the Royal Society, in 1747, when they sent the charge of a Leyden jar through two miles of wire, supported upon short sticks driven into the ground; the wire at each terminus being connected with the earth for the return current. This method of insulation and conduction fully answered the purpose, and served to determine the great velocity with which electricity is transmitted, for no perceptible interval occurred between the discharge of the Leyden jar at one end of the circuit, and its effect at the other extremity.

Mr. Ronalds made the next experiment on an extensive scale, by insulating eight miles of wire in glass tubes, the wire being carried backwards and forwards for that distance on his lawn at Hammersmith. That mode of insulation was found very efficient. It was, indeed, too perfect, for the difficulty arose of discharging the electricity from the wire after the charge had passed through it.

The length of telegraphic communication established at Munich, in 1837, by Dr. Steinheil, was an important practical advance in the system of extending and insulating the wires, and deserves consideration, not only from the extent to which it was carried into practical operation, but from the circumstance that the earth was employed to form the return circuit. The wires appear to have been carried through the city by extending them from the church towers and other elevated buildings. That plan, indeed, presents so many facilities for passing telegraph wires through towns, that it is not improbable it may be ultimately adopted in this country.

Though the conducting power of the earth was thus early made use of for one-half of the circuit, the fact seems to have been unknown in England at the time of laying down the telegraph wires to Slough in 1845, for a separate wire was then used for the return current. Some years afterwards, indeed, Mr. Bain laid claim to the discovery; but the fact that the conducting power of the earth had been previously applied to the purpose by Dr. Steinheil has been incontestably proved.

In the early stages of the practical application of electric telegraphs in this country, Mr. Cook took an active part in overcoming the numerous difficulties attending the proper protection and insulation of the wires. In the first instance, the plan of burying the wires in trenches was tried, but with very indifferent success, as the asphaltum and other resinous substances with which it was attempted to insulate them were inadequate for the purpose, and allowed the electricity to escape from wire to wire. The method of supporting the wires on tall posts was then adopted by Mr. Cooke, the wires being insulated from the posts at the points of suspension, by passing them through quills. Various improvements have since been made in the insulators, and the plan most in favour at present is to pass the wires through globular earthenware or glass insulators, attached to the posts, as shown in the annexed diagram. The wires themselves are about one-sixth of an inch in diameter; they are made of iron coated with zinc, or galvanized, as it is termed, to protect them from rust.

Notwithstanding the great care taken to insulate the wires at the posts, a large quantity of the electricity escapes in wet weather, and returns to the battery without having reached the most distant stations, and thus not unfrequently the communications are interrupted. The author is of opinion that the loss of electricity in wet weather is occasioned rather by communication from one wire to another through the moist atmosphere, than by defective insulation at the posts. In confirmation of this opinion it may be stated, that he has experimentally determined that a working electric current might be transmitted from London to Liverpool, if all the points of attachment were connected by water with the surface of the ground, provided that the rest of the wire were insulated.[7]

The use of gutta percha as an insulating covering for wire has given rise to a new era in telegraphic communication. Gutta percha is an excellent insulator, and wire covered with two coatings of that material, about one-sixteenth of an inch each, is so far protected, that 100 miles of it immersed in water transmits an electric current from a powerful voltaic battery with very trifling loss. This perfection in insulation has greatly facilitated the establishment of telegraphic communication between England and the Continent. The first attempt to establish a submarine circuit between Dover and Calais took place on the 28th of August, 1850. A single copper wire, about the thickness of a common bell wire, coated thickly with gutta percha, was laid across the English Channel experimentally, without any protection. It proved sufficient for the transmission of an electric current, and several messages were sent through it between Dover and Calais; but it was far too feeble to resist the action of the waves, and the following day it was cut through by friction against the rocks, and the communication was stopped.

The plan afterwards adopted for a permanent submarine line was to enclose five similar wires in a hollow iron wire cable. The wires were first slightly twisted, to prevent them from being broken when stretched. They were then covered with hempen yarn, to protect the gutta percha from attrition, and they were thus introduced into the hollow cable, of which they formed the core. The accompanying woodcut represents this structure of the cable; the five twisted wires are shown at C; B represents the same covered with hemp yarn; and A a portion of the completed cable, constructed of thick iron wire galvanized. This cable has now been laid down for seven years, and with perfect success. Its strength has often been severely tested, as it has been sometime drawn up by ships' anchors, and considerably strained; but it has not been broken, and the insulation is almost perfect. The success of this submarine cable has induced the extension of that means of communicating with the Continent, and similar submarine telegraph cables have been laid down from Dover to Ostend, from Harwich to the Hague, from Scotland to Ireland, and across the Mediterranean Sea as far as Malta. The weight and the cost of those cables present a serious obstacle to their adoption in forming a telegraphic communication with America; and when it was determined to attempt to establish electrical connection with the New World, a different form of cable was adopted. The conductor of the electric current in the Atlantic cable is composed of seven strands of fine copper wire twisted together, the aggregate thickness of which is not greater than the single copper wire of other submarine cables. This fine copper cord is covered carefully with gutta percha; it is then coated with tarred hemp, and is protected externally by an iron wire rope, composed of numerous strands of fine wire. The form and exact size of the cable are shown in the accompanying drawing and section. The central dots in the section are the conducting wires round which are the gutta percha and hemp, and the outer rim represents the iron wire casing.