The wonderful developments of wireless telegraphy must not make us forget that some very interesting and startling improvements have been made in connection with the ordinary wire-circuit method: notably in the matter of speed.
At certain seasons of the year or under special circumstances which can scarcely be foreseen, a great rush takes place to transmit messages over the wires connecting important towns. Now, the best telegraphists can with difficulty keep up a transmitting speed of even fifty words a minute for so long as half-an-hour. The Morse alphabet contains on the average three signals for each letter, and the average length of a word is six letters. Fifty words would therefore contain between them 900 signals, or fifteen a second. The strain of sending or noting so many for even a brief period is very wearisome to the operator.
Means have been found of replacing the telegraph clerk, so far as the actual signalling is concerned, by mechanical devices.
In 1842 Alexander Bain, a watchmaker of Thurso, produced what is known as a “chemical telegraph.” The words to be transmitted were set up in large metal type, all capitals, connected with the positive pole of a battery, the negative pole of which was connected to earth. A metal brush, divided into five points, each terminating a wire, was passed over the metal type. As often as a division of the brush touched metal it completed the electric circuit in the wire to which it was joined, and sent a current to the receiving station, where a similar brush was passing at similar speed over a strip of paper soaked in iodide of potassium. The action of the electricity decomposed the solution, turning it blue or violet. The result was a series of letters divided longitudinally into five belts separated by white spaces representing the intervals between the contact points of the brush.
The receiving instrument used by Messrs. Pollak & Virag in their high-speed system of telegraphy. This instrument is capable of receiving and photographically recording messages at the astonishing speed of 50,000 words an hour.
The Bain Chemical Telegraph was able to transmit the enormous number of 1500 words per minute; that is, at ten times the rate of ordinary conversation! But even when improvements had reduced the line wires from five to one, the system, on account of the method of composing the message to be sent, was not found sufficiently practical to come into general use.
Its place was taken by slower but preferable systems: those of duplex and multiplex telegraphy.
When a message is sent over the wires, the actual time of making the signals is more than is required for the current to pass from place to place. This fact has been utilised by the inventors of methods whereby two or more messages may not only be sent the same way along the same wire, but may also be sent in different directions. Messages are “duplex” when they travel across one another, “multiplex” when they travel together.
The principle whereby several instruments are able to use the same wire is that of distributing among the instruments the time during which they are in contact with the line.