chapter 1._

ELECTRICITY: A. D. 1825-1874. The Perfected Telegraph.

"The European philosophers kept on groping. At the end of five years [after Oersted's discovery], one of them reached an obstacle which he made up his mind was so entirely insurmountable, that it rendered the electric telegraph an impossibility for all future time. This was [1825] Mr. Peter Barlow, fellow of the Royal Society, who had encountered the question whether the lengthening of the conducting wire would produce any effect in diminishing the energy of the current transmitted, and had undertaken to resolve the problem. ... 'I found [he said] such a considerable diminution with only 200 feet of wire as at once to convince me of the impracticability of the scheme.' ... The year following the announcement of Barlow's conclusions, a young graduate of the Albany (N. Y.) Academy--by name Joseph Henry--was appointed to the professorship of mathematics in that institution. Henry there began the series of scientific investigations which is now historic. ... Up to that time, electro-magnets had been made with a single coil of naked wire wound spirally around the core, with large intervals between the strands. The core was insulated as a whole: the wire was not insulated at all. Professor Schweigger, who had previously invented the multiplying galvanometer, had covered his wires with silk. Henry followed this idea, and, instead of a single coil of wire, used several. ... Barlow had said that the gentle current of the galvanic battery became so weakened, after traversing 200 feet of wire, that it was idle to consider the possibility of making it pass over even a mile of conductor and then affect a magnet. {773} Henry's reply was to point out that the trouble lay in the way Barlow's magnet was made. ... Make the magnet so that the diminished current will exercise its full effect. Instead of using one short coil, through which the current can easily slip, and do nothing, make a coil of many turns; that increases the magnetic field: make it of fine wire, and of higher resistance. And then, to prove the truth of his discovery, Henry put up the first electro-magnetic telegraph ever constructed. In the academy at Albany, in 1831, he suspended 1,060 feet of bell-wire, with a battery at one end and one of his magnets at the other; and he made the magnet attract and release its armature. The armature struck a bell, and so made the signals. Annihilating distance in this way was only one part of Henry's discovery. He had also found, that, to obtain the greatest dynamic effect close at hand, the battery should be composed of a very few cells of large surface, combined with a coil or coils of short coarse wire around the magnet,--conditions just the reverse of those necessary when the magnet was to be worked at a distance. Now, he argued, suppose the magnet with the coarse short coil, and the large-surface battery, be put at the receiving station; and the current coming over the line be used simply to make and break the circuit of that local battery. ... This is the principle of the telegraphic 'relay.' In 1835 Henry worked a telegraph-line in that way at Princeton. And thus the electro-magnetic telegraph was completely invented and demonstrated. There was nothing left to do, but to put up the posts, string the lines, and attach the instruments."

_P. Benjamin, The Age of Electricity,