The feasibility of uniting the two vast systems of telegraphy had engaged the consideration of some of those most prominently associated with electric telegraphy on both sides of the Atlantic. It had been already shown that cables could be successfully laid and maintained in comparatively moderate depths in the Mediterranean, Black Sea, etc., but the nearest points between the British Isles and Newfoundland are nearly 2,000 miles apart. The greatest length of submarine line which had hitherto been effectively submerged—110 miles—formed but an insignificant portion of such an enormous distance; and that, too, involving a depth of nearly three miles for a large proportion of the way, instead of about 300 fathoms.
Apart from the engineering difficulties entailed by this vast distance and depth, the question was then undetermined as to the possibility of conveying electric currents through such a length in an unbroken circuit, and at a speed that would enable messages to be passed rapidly enough in succession to prove remunerative. Various researches had been made—by Faraday among others—with a view to determining the law in relation to the velocity of electricity through a conducting-wire.
The retarding effect of the insulating covering had already been discovered; but the exact formula for the working speed of cables of definite proportions and lengths was not correctly arrived at till some years later. The similarity, in principle, of a cable to a Leyden jar was first pointed out by Mr. Edward Brailsford Bright in the course of a paper read before the British Association{31} in 1854. He showed that on charging a gutta-percha-covered wire, the insulating material tended to absorb and retain a part of the charge and to hold back, as a static charge, some of the electricity flowing as current through the conductor—just as the charge (of opposite potential) induced on the outside plate of a Leyden jar statically holds the primary charge on the inner plate, until either are neutralized. The brothers, Edward and Charles Bright, made a series of extensive experiments on long lengths of underground wires; and these investigations were supplemented later by Mr. Edward Orange Wildman Whitehouse (formerly a medical practitioner), who became electrician to the first Atlantic cable. Mr. Whitehouse was a man of very high intellectual and scientific attainments, and a most ingenious and painstaking experimenter.
The retardation of the electric current through an insulated wire due to induction—a phenomenon practically unknown with bare, aerial wires suspended on posts, and of no consequence with quite short cables—was overcome by using a succession of opposite currents. By this means the latter, or retarded, portion of each current was “wiped out” by the opposite current immediately following it; and thus a series of electric waves could be made to traverse the cable, one after the other, several being in the act of passing onward at different points along the conductor at the same time. The Messrs. Bright devised a special key (embodied with a patent for signaling through long cables) for transmitting these alternating currents from the battery; and this was followed by others to effect the same object—one{32} by Professor Thomson (now Lord Kelvin), who became electrical adviser to the enterprise.
