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Page:Popular Science Monthly Volume 72.djvu/270

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POPULAR SCIENCE MONTHLY

regarded it merely as an extremely sensitive galvanometer without knowing how it came into existence. It was devised by Thomson to enable him to utilize his mathematical solution of signaling through a long submarine cable, and was regarded of such national importance that a private act of Parliament was sanctioned by the Privy Council to extend the normal life of fourteen years for the patent of this cable "speaking instrument," as it was originally called. The invention of this instrument marks the theoretical solution of a most important problem, which solution Thomson found great difficulty in getting the electrician of that day to accept.

As early as 1855—before any long submarine cable had been constructed—Thomson published in the Proceedings of the Royal Society the theory of the propagation of signals through a cable based on a correspondence which he had had with the late Sir Gabriel Stokes, and he showed that the book "Fourier de la Chaleur"—that "mathematical poem," as he used to call it—contained, in Fourier's mathematical equations of the flow of heat, the entire mathematical solution of the propagation of electric waves through a cable. From "Fourier's series" he deduced that, whereas on a short overhead telegraph line the signal reaches its full strength at the distant end practically as soon as the signaler at the near end of the line begins to send it, with a submarine cable it is retarded, spreads out, and blurs the next signal. There is a past history effect as in politics and in many natural phenomena. The passing of an act of Parliament can not suddenly change a people; indeed, it is well known that the actual effect of an act of Parliament promoted by well-wishers is often gradually found to be most harmful, and has to be repealed or curbed in its action.

Herbert Spencer in his "Sociology" strongly advocated legislators to study the science of politics. Thomson would perhaps have said, "Study Fourier's mathematical poem."

If it were attempted to send a series of electric signals through an Atlantic cable with the same apparatus and at the same speed as messages are sent between London and Brighton, the signaler at the far end would not have the slightest knowledge that the signaler at this end was trying to send a message, whatever were the strength of the current sent into the cable. To work a long submarine cable, either time must be allowed for each signal to grow at the distant end, or, as this would make the sending of messages very slow, the receiving instrument and the signaler receiving the message must, like a clever doctor diagnosing a disease, be able to interpret mere indications. Sending the letter "e," for example, produces at the other end of a long cable a totally different result, depending on what has preceded it. In no case, at a speed of, say, thirty words a minute with a 3,000-mile cable, will it be more than a suggestion, even at the beginning of a