<> The following text was excerpted from http://60centuries.copper.org/electrical10_a.htm
This is a medium length article about Thomson's involvement with the Atlantic Telegraph expedition and his technological contributions to the project.

There ensued the much more ambitious project of laying a cable across the floor of the Atlantic Ocean which involved sinking the cable to the sea-bed four miles or more below the surface, together with the difficulties of landing at both ends, on Valentia Island and Nova Scotia respectively. In 1856 a company was formed with a capital of £350,000 and the great enterprise, which called for no less than 17,500 miles of 22 s.w.g. copper wire, began.

Apart from the mechanical difficulties of splicing and laying such a long cable, those concerned had to take one vital decision at the outset: it was a decision based on the very sparse information available at the time and unfortunately it was a wrong one.

Electrical messages suffer a slight amount of 'friction' in transmission so that what would be an instantaneous signal over a short distance becomes a long-drawn-out wave at the receiving end of a very long length of wire; how much greater, then, must this retardation of the electrical impulse be at a point two thousand miles away! On the other hand, the slower the message the less possible would it be either to identify the signal or to work the cable at a profit.

Kelvin, who was then Professor William Thomson, had already discerned this weakness of cables and had published some important notes on the subject, wherein he showed that the retardation, which is proportional to the length, depended also upon the capacity of the wire and of its gutta-percha covering. He therefore recommended a thick wire for this cable and a heavy covering, coupled with the use of only the most minute currents. This in turn involved the construction of a very accurate detector, for which purpose he subsequently invented the delicately balanced mirror galvanometer mentioned below. When at last the Atlantic cable project took shape he was appointed one of the eighteen directors, but without any particular duties or powers.

Whitehouse, in charge of the scheme, was a man of considerable ability but fixed ideas whose views were entirely opposite to Thomson's. Sir Charles Bright was the Chief Engineer. Whitehouse advocated a small cable and the use of a high current to overcome its resistance, and this procedure was adopted.

By July 1857 the cable was ready. It comprised 1200 pieces of copper wire each two miles long. Before being shipped they were joined into lengths of 300 miles; one half was carried by the U.S. frigate Niagara and the remainder by H.M.S. Agamemnon. Thomson sailed in an unofficial capacity on the Agamemnon, Whitehouse remaining ashore at the Irish end. The expedition commenced work from Valentia Island on 5 August, but after 400 miles had been successfully laid the cable broke and the ships returned home for the winter.

The experience gained during this voyage convinced Thomson that Whitehouse was wrong, but he had not sufficient power to enforce his views upon the Board. In the succeeding winter he invented his mirror galvanometer which was designed to detect even the faintest currents. It comprised a small mirror affixed to a tiny magnet; the mirror reflected the light from a lamp on to a scale that was graduated in degrees. The slightest trace of current made the magnet move together with the attached mirror, thereby causing a spot of light to travel along the scale. Despite its delicacy, this exquisite instrument could be used effectively even during a storm.

Thomson was also perplexed by the unexpected difficulty that had been found in getting currents to pass through copper. He had a number of samples analyzed and it was then found that they were not equally pure, although of very high grade (between 98.76 per cent and 99.2 per cent). He raised this question with the other Directors who did not think it of much consequence, and with the manufacturers, who said that it was commercially impossible to make the quality of copper that he required. Nevertheless the Professor stuck to his ground, with the result that the makers did achieve what had been considered impossible. Thus was born 'high conductivity copper', which every electrical engineer now specifies as a matter of course.

On 10 June 1858 the two ships sailed again, Thomson now being in charge of the electrical testing room on the Agamemnon. His 'stethoscope', the mirror galvanometer, went with him. After many misadventures-including a terrible storm in which the cable broke loose in the hold and was damaged, and the testing room was flooded with salt-water - the ships succeeded in landing both ends on the same day (5 August). The galvanometer rendered invaluable service throughout, its tiny spot of light being observed 'as anxiously as a physician watches a patient during a crisis'. The first message ever sent from Europe to America by wire flashed under the sea, a response was received, and as the cable seemed in perfect order it was handed over to Whitehouse. Great was the jubilation both in London and in New York. The directors of the enterprise became public heroes, Thomson in particular receiving a civic welcome on his return to Glasgow.

The rejoicings, however, proved to be premature. Messages steadily became more and more difficult to transmit, until finally, on 28 August, the cable failed completely. An investigation showed that Whitehouse, despite Thomson's warnings, had been using his powerful current on a cable that was already damaged by the storm at sea. A violent quarrel broke out between the electrical engineer and the directors, the shareholders lost their capital, and for some time the name of the Transatlantic Cable was not a safe one to use in City circles. However, Thomson, Bright and a few other leading men never lost faith in the ultimate practicability of the enterprise. After a discreet interval the project was revived, a new company found the necessary money, and a fresh cable was made and thoroughly tested. On this occasion Thomson's advice was followed, the conductor being made three times as thick as its predecessor with the current reduced to a minimum. The Great Eastern, the famous old paddle steamer and largest ship of its day, was specially adapted to carry it, and in July 1865 she started westwards. After 1200 miles had been laid the cable broke, its end falling to the sea floor two and a half miles below. The ship was laid across the line of her outward course and the cable was found again, but unfortunately her apparatus for grappling with it was not sufficiently strong. Once more the expedition had to return home but this time all on board were confident that success must eventually be theirs. A new cable was manufactured. Just a year later it was laid quietly and uneventfully. Furthermore, the broken end of the other cable was recovered, quite undamaged; the joint was spliced and this second line ran on to Newfoundland.
The heroes of this great achievement, including Thomson, were honored by knighthoods. (28