This is an excerpt from George B. Prescott's "History, Theory and Practice of the Electric Telegraph". This is an excellent introduction to the history and necessity of the Morse telegraph repeater.
THE TELEGRAPHIC REPEATER.
THE electro-magnetic telegraph of Professor Morse, which was first put in operation between the cities of Baltimore and Washington, in 1844, was the same in principle as that employed at the present day, consisting of a key and relay magnet, in connection with a registering instrument, and a local battery at each station. Professor Morse had, in previous experiments, endeavored to work the registering instrument directly, by means of the circuit of the main line; but the strength of the electric current was found to be so greatly reduced by the resistance encountered in passing through conductors of great length, that sufficient magnetic power could not be produced to work the registering apparatus. He then substituted the arrangement of the local circuit above referred to, which is one of the most valuable parts of his invention in use at the present day.
Two or three years after the value and utility of this great invention had been demonstrated by the brilliant success of the experimental line between Baltimore and Washington, the wires were rapidly extended in every direction throughout the United States. It was soon discovered, however, that it was impracticable to operate the lines in circuits of very great length. A distance of from three to five hundred miles, in good weather, was as great a length of line as could be successfully worked, under the conditions of insulation that prevailed at that day, and but one or two lines were so well constructed as to admit of this. If it was desired to transmit communications to a greater distance, it became necessary to divide the line into two or more distinct circuits at intermediate points, and, of course, at every repeating station that was introduced, it was necessary to employ two additional operators for each wire. This not only increased the working expenses, but greatly increased the liability of errors occurring in the transmission of messages between the terminal stations of the line. Numerous experiments were then made in order to remedy this difficulty, resulting in the invention from time to time of various combinations, each accomplishing the desired result more or less perfectly, until at the present day it is not an uncommon feat for operators to converse by telegraph with the utmost facility between points situated three thousand miles apart. The instruments now used for the above purpose are constructed upon many different principles, but are all classed under the general term in this country of repeaters, and in Europe of translators.
The repeater is an apparatus for the purpose of duplicating from one electric circuit to another the breaks and completions received from the transmitting station, for the purpose of renewing the power lost by the escape of the electric fluid into the earth through imperfect insulation.
The earliest device for this purpose was perhaps suggested by the arrangement of circuits described in Professor Morse’s first patent, but abandoned by him upon the invention of the relay magnet and local circuit. In this plan, Morse proposed to divide the line into a number of circuits, each about twenty miles in length, and provided with an independent battery for each circuit. The electro-magnet at the extremity of the first circuit was so arranged as to complete the second circuit, by causing a forked wire to descend into two mercury cups. The second circuit of twenty miles was then charged, which in like manner actuated the third, and so on to an indefinite extent. This plan would work only in one direction; consequently a duplicate series of wires, batteries, and electro-magnets were necessary in order to telegraph in the opposite direction. By substituting metallic points for the mercury cups, and placing an electro-magnet at each end of the two adjoining circuits, and arranging the circuits of each wire so that it is opened and closed by the magnet of the other, a combination was formed capable of working in either direction. But it will at once be seen that in this plan the second circuit could not respond without a transfer of the switch or button at the central station, by a person employed for the purpose. When the second operator responded, the operator at the principal station, by means of his switch, changed the repeating magnet from the first to the second circuit, and vice versa. This apparatus constitutes the button repeater. It is frequently modified by using the levers of the sounders instead of the main magnets to open and close the other circuit. We are unable to state where or by whom the button repeater was first used. Mr. Charles S. Bulkley had one in operation at Columbus, Georgia, as early as 1847. A similar device was constructed by Ezra Cornell, of Ithaca, New York, and was known as the Cornell Switch, but, we believe, subsequent to the date of Bulkley’s invention above mentioned. It is quite probable that the same invention may have been made by several different persons without the knowledge of each other, as something of this kind could hardly have failed to suggest itself to almost any ingenious person who was endeavoring to find means of overcoming the difficulty of working lines in long circuits, or of obviating the difficulties experienced by the loss of insulation during heavy rain-storms.
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