Part 2 (1/2)

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 pa.s.sed 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 pa.s.sing at similar speed over a strip of paper soaked in iodide of pota.s.sium. 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 s.p.a.ces representing the intervals between the contact points of the brush.

[Ill.u.s.tration: _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 astonis.h.i.+ng 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 pa.s.s 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.

Let us suppose that four transmitters are sending messages simultaneously from London to Edinburgh.

Wires from all four instruments are led into a circular contact-maker, divided into some hundreds of insulated segments connected in rotation with the four transmitters. Thus instrument A will be joined to segments 1, 5, 9, 13; instrument B to segments 2, 6, 10, 14; instrument C with segments 3, 7, 11, 15; and so on.

Along the top of the segments an arm, connected with the telegraph line to Edinburgh, revolves at a uniform rate. For about 1/500 of a second it unites a segment with an instrument. If there are 150 segments on the ”distributor,” and the arm revolves three times a second, each instrument will be put into contact with the line rather oftener than 110 times per second. And if the top speed of fifty words a minute is being worked to, each of the fifteen signals occurring in each second will be on the average divided among seven moments of contact.

A similar apparatus at Edinburgh receives the messages. It is evident that for the system to work satisfactorily, or even to escape dire confusion, the revolving arms must run at a level speed in perfect unison with one another. When the London arm is over segment 1, the Edinburgh arm must cover the same number. The greatest difficulty in multiplex telegraphy has been to adjust the timing exactly.

Paul la Cour of Copenhagen invented for driving the arms a device called the Phonic Wheel, as its action was regulated by the vibrations of a tuning-fork. The wheel, made of soft iron, and toothed on its circ.u.mference, revolves at a short distance from the pole of a magnet.

As often as a current enters the magnet the latter attracts the nearest tooth of the wheel; and if a regular series of currents pa.s.s through it the motion of the wheel will be uniform. M. la Cour produced the regularity of current impulses in the motor magnet by means of a tuning-fork, which is unable to vibrate more than a certain number of times a second, and at each vibration closed a circuit sending current into the magnet. To get two tuning-forks of the same note is an easy matter; and consequently a uniformity of rotation at both London and Edinburgh stations may be insured.

So sensitive is this ”interrupter” system that as many as sixteen messages can be sent simultaneously, which means that a single wire is conveying from 500 to 800 words a minute. We can easily understand the huge saving that results from such a system; the cost of instruments, interrupter, &c., being but small in proportion to that of a number of separate conductors.

The word-sending capacity of a line may be even further increased by the use of automatic transmitters able to work much faster in signal-making than the human brain and hand. Sir Charles Wheatstone's Automatic Transmitter has long been used in the Post-Office establishments.

The messages to be sent are first of all punched on a long tape with three parallel rows of perforations. The central row is merely for guiding the tape through the transmitting machine. The positions of the holes in the two outside rows relatively to each other determine the character of the signal to be sent. Thus, when three holes (including the central one) are abreast, a Morse ”dot” is signified; when the left-hand hole is one place behind the right hand, a ”dash”

will be telegraphed.

In the case of a long communication the matter is divided among a number of clerks operating punching machines. Half-a-dozen operators could between them punch holes representing 250 to 300 words a minute; and the transmitter is capable of despatching as many in the same time, while it has the additional advantage of being tireless.

The action of the transmitter is based upon the reversal of the direction or nature of current. The punched tape is pa.s.sed between an oscillating lever, carrying two points, and plates connected with the two poles of the battery. As soon as a hole comes under a pin the pin drops through and makes a contact.

At the receiving end the wire is connected with a coil wound round the pole of a permanent bar-magnet. Such a magnet has what is known as a north pole and a south pole, the one attractive and the other repulsive of steel or soft iron. Any bar of soft iron can be made temporarily into a magnet by twisting round it a few turns of a wire in circuit with the poles of a battery. But which will be the north and which the south pole depends on the _direction_ of the current.

If, then, a current pa.s.ses in one direction round the north pole of a permanent magnet it will increase the magnet's attractive power, but will decrease it if sent in the other direction.

The ”dot” holes punched in the tape being abreast cause first a positive and then a negative current following at a very short interval; but the ”dash” holes not being opposite allow the positive current to occupy the wires for a longer period. Consequently the Morse marker rests for correspondingly unequal periods on the recording ”tape,” giving out a series of dots and dashes, as the inker is s.n.a.t.c.hed quickly or more leisurely from the paper.

The Wheatstone recorder has been worked up to 400 words a minute, and when two machines are by the multiplex method acting together this rate is of course doubled.

As a speed machine it has, however, been completely put in the shade by a more recent invention of two Hungarian electricians, Anton Pollak and Josef Virag, which combines the perforated strip method of transmission with the telephone and photography. The message is sent off by means of a punched tape, and is recorded by means of a telephonic diaphragm and light marking a sensitised paper.

In 1898 the inventors made trials of their system for the benefit of the United Electrical Company of Buda-Pesth. The Hungarian capital was connected by two double lines of wire with a station 200 miles distant, where the two sets were joined so as to give a single circuit of 400 miles in length. A series of tests in all weathers showed that the Pollak-Virag system could transmit as many as 100,000 words an hour over that distance.

From Hungary the inventors went to the United States, in which country of ”records” no less than 155,000 words were despatched and received in the sixty minutes. This average--2580 words per minute, 43 per second--is truly remarkable! Even between New York and Chicago, separated by 950 odd miles, the wires kept up an average of 1000 per minute.

The apparatus that produces these marvellous results is of two types.