Part 2
But when a Hertzian wave flying through the ether strikes the coherer, the particles suddenly press hard on one another, and make a bridge through which a current can pass. The current works a “relay,” or circuit through which a stronger current passes, opening and closing it as often as the coherer is influenced by a wave. The relay actuates a tapper that gently taps the tube after each wave-influence, causing the particles to _de_cohere in readiness for the succeeding wave, and also a Morse instrument for recording words in dots and dashes on a long paper tape.
The coherer may be said to resemble an engine-driver, and the “relay” an engine. The driver is not sufficiently strong to himself move a train, but he has strength enough to turn on steam and make the engine do the work. The coherer is not suitable for use with currents of the intensity required to move a Morse recorder, but it easily switches a powerful current into another circuit.
Want of space forbids a detailed account of Marconi’s successes with his improved instruments, but the appended list will serve to show how he gradually increased the distance over which he sent signals through space.
In 1896 he came to England. That year he signalled from a room in the General Post-Office to a station on the roof 100 yards distant. Shortly afterwards he covered 2 miles on Salisbury Plain.
In May, 1897, he sent signals from Lavernock Point to Flatholm, 3-1/3 miles. This success occurred at a critical time, for Sir W. Preece had already, as we have seen, bridged the same gap by his induction method, and for three days Marconi failed to accomplish the feat with his apparatus, so that it appeared as though the newer system were the less effective of the two. But by carrying the transmitting instrument on to the beach below the cliff on which it had been standing, and joining it by a wire to the pole already erected on the top of the cliff, Mr. Marconi, thanks to a happy inspiration, did just what was needed; he got a greater length of wire to send off his waves from. Communication was at once established with Flatholm, and on the next day with Brean Down, on the other side of the Bristol Channel, and 8-2/3 miles distant. Then we have--
Needles Hotel to Swanage 17-1/2 miles. Salisbury to Bath 34 " French Coast to Harwich 90 " Isle of Wight to The Lizard 196 " At Sea (1901) 350 " Dec. 17, 1901, England to America 2099 "
A more pronounced, though perhaps less sensational, success than even this last occurred at the end of February, 1902. Mr. Marconi, during a voyage to America on the s.s. _Philadelphia_ remained in communication with Poldhu, Cornwall, until the vessel was 1550 miles distant, receiving messages on a Morse recorder for any one acquainted with the code to read. Signals arrived for a further 500 miles, but owing to his instruments not being of sufficient strength, Mr. Marconi could not reply.
When the transatlantic achievement was announced at the end of 1901, there was a tendency in some quarters to decry the whole system. The critics laid their fingers on two weak points.
In the first place, they said, the speed at which the messages could be transmitted was too slow to insure that the system would pay. Mr. Marconi replied that there had been a time when one word per minute was considered a good working rate across the Atlantic cable; whereas he had already sent twenty-two words per minute over very long distances. A further increase of speed was only a matter of time.
The second objection raised centred on the lack of secrecy resulting from signals being let loose into space to strike any instrument within their range; and also on the confusion that must arise when the ether was traversed by many sets of electric waves.
The young Italian inventor had been throughout his experiments aware of these defects and sought means to remedy them. In his earliest attempts we find him using parabolic metal screens to project his waves in any required direction and prevent their going in any other. He also employed strips of metal in conjunction with the coherer, the strips or “wings” being of such a size as to respond most readily to waves of a certain length.
The electric oscillations coming from the aerial wires carried on poles, kites, &c., were of great power, but their energy dispersed very quickly into space in a series of rapidly diminishing vibrations. This fact made them affect to a greater or less degree any receiver they might encounter on their wanderings. If you go into a room where there is a piano and make a loud noise near the instrument a jangle of notes results. But if you take a tuning-fork and after striking it place it near the strings, only one string will respond, _i.e._ that of the same pitch as the fork.
What is required in wireless telegraphy is a system corresponding to the use of the tuning-fork. Unfortunately, it has been discovered that the syntony or tuning of transmitter and receiver reduces the distance over which they are effective. An electric “noise” is more far-reaching than an electric “note.”
Mr. Marconi has, however, made considerable advances towards combining the sympathy and secrecy of the tuning system with the power of the “noise” system. By means of delicately adjusted “wings” and coils he has brought it about that a series of waves having small individual strength, but great regularity, shall produce on the receiver a _cumulative_ effect, storing, as it were, electricity on the surface of the receiver “wings” until it is of sufficient power to overcome the resistance of the coherer.
That tuned wireless telegraphy is, over moderate distances, at least as secret as that through wires (which can be tapped by induction) is evident from the fact that during the America Cup Yacht Races Mr. Marconi sent daily to the _New York Herald_ messages of 4000 total words, and kept them private in spite of all efforts to intercept them. He claims to have as many as 250 “tunes”; and, indeed, there seems to be no limit to their number, so that the would-be “tapper” is in the position of a man trying to open a letter-lock of which he does not know the cipher-word. He _may_ discover the right tune, but the chances are greatly against him. We may be certain that the rapid advance in wireless telegraphy will not proceed much further before syntonic messages can be transmitted over hundreds if not thousands of miles.
It is hardly necessary to dwell upon the great prospect that the new telegraphy opens to mankind. The advantages arising out of a ready means of communication, freed from the shackles of expensive connecting wires and cables are, in the main, obvious enough. We have only to imagine all the present network of wires replaced or supplemented by ether-waves, which will be able to act between points (_e.g._ ships and ships, ships and land, moving and fixed objects generally) which cannot be connected by metallic circuits.
Already ocean voyages are being shortened as regards the time during which passengers are out of contact with the doings of the world. The transatlantic journey has now a newsless period of but three days. Navies are being fitted out with instruments that may play as important a part as the big guns themselves in the next naval war. A great maritime nation like our own should be especially thankful that the day is not far distant when our great empire will be connected by invisible electric links that no enemy may discover and cut.
The romantic side of wireless telegraphy has been admirably touched in some words uttered by Professor Ayrton in 1899, after the reading of a paper by Mr. Marconi before the Institution of Electrical Engineers.
“If a person wished to call to a friend” (said the Professor), “he would use a loud electro-magnetic voice, audible only to him who had the electro-magnetic ear.
“‘Where are you?’ he would say.
“The reply would come--‘I am at the bottom of a coal mine,’ or ‘Crossing the Andes,’ or ‘In the middle of the Pacific.’ Or, perhaps, in spite of all the calling, no reply would come, and the person would then know his friend was dead. Let them think of what that meant; of the calling which went on every day from room to room of a house, and then imagine that calling extending from pole to pole; not a noisy babble, but a call audible to him who wanted to hear and absolutely silent to him who did not.”
When will Professor Ayrton’s forecast come true? Who can say? Science is so full of surprises that the ordinary man wonders with a semi-fear what may be the next development; and wise men like Lord Kelvin humbly confess that in comparison with what has yet to be learnt about the mysterious inner workings of Nature their knowledge is but as ignorance.
HIGH-SPEED TELEGRAPHY.
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 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.
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 circumference, 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 pass 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 passed 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 passes 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 snatched 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. The one type records messages in the Morse alphabet, the other makes clearly-written longhand characters. The former is the faster of the two, but the legibility of the other more than compensates for the decrease of speed by one-half.
The Morse alphabet method closely resembles the Wheatstone system. The message is prepared for transmission by being punched on a tape. But there is this difference in the position of the holes, that whereas in the Wheatstone method two holes are used for each dot and dash, only one is required in the Pollak-Virag. If to the right of the central guiding line it signifies a “dash,” if to the left, a “dot.”
The “reversal-of-current” method, already explained, causes at the receiver end an increase or decrease in the power of a permanent magnet to attract or repel a diaphragm, the centre of which is connected by a very fine metal bar with the centre of a tiny mirror hinged at one side on two points. A very slight movement of the diaphragm produces an exaggerated movement of the mirror, which, as it tilts backwards and forwards, reflects the light from an electric lamp on to a lens, which concentrates the rays into a bright spot, and focuses them on to a surface of sensitised paper.
In their earliest apparatus the inventors attached the paper to the circumference of a vertical cylinder, which revolved at an even pace on an axle, furnished at the lower end with a screw thread, so that the portion of paper affected by the light occupied a spiral path from top to bottom of the cylinder.
In a later edition, however, an endless band of sensitised paper is employed, and the lamp is screened from the mirror by a horizontal mantle in which is cut a helical slit making one complete turn of the cylinder in its length. The mantle is rotated in unison with the machinery driving the sensitised band; and as it revolves, the spot at which the light from the filament can pass through the slit to the mirror is constantly shifting from right to left, and the point at which the reflected light from the mirror strikes the sensitised paper from left to right. At the moment when a line is finished, the right extremity of the mantle begins to pass light again, and the bright spot of light recommences its work at the left edge of the band, which has now moved on a space.
The movements of the mirror backwards and forwards produce on the paper a zigzag tracing known as syphon-writing. The record, which is continuous from side to side of the band, is a series of zigzag up-and-down strokes, corresponding to the dots and dashes of the Morse alphabet.
The apparatus for transmitting longhand characters is more complicated than that just described. Two telephones are now used, and the punched tape has in it five rows of perforations.
If we take a copy-book and examine the letters, we shall see that they all occupy one, two, or three bands of space. For instance, _a_, between the lines, occupies one band; _g_, two bands; and _f_, three. In forming letters, the movements of the fingers trace curves and straight lines, the curves being the resultants of combined horizontal and vertical movements.
Messrs. Pollak and Virag, in order to produce curves, were obliged to add a second telephone, furnished also with a metal bar joined to the mirror, which rests on three points instead of on two. One of these points is fixed, the other two represent the ends of the two diaphragm bars, which move the mirror vertically and horizontally respectively, either separately or simultaneously.
A word about the punched paper before going further. It contains, as we have said, five rows of perforations. The top three of these are concerned only with the up-and-down strokes of the letters, the bottom two with the cross strokes. When a hole of one set is acting in unison with a hole of the other set a composite movement or curve results.
The topmost row of all sends through the wires a negative current of known strength; this produces upward and return strokes in the upper zone of the letters: for instance, the upper part of a _t_. The second row passes _positive_ currents of equal strength with the negative, and influences the up-and-down strokes of the centre zone, _e.g._ those of _o_; the third row passes positive currents _twice_ as strong as the negative, and is responsible for double-length vertical strokes in the centre and lower zones, _e.g._ the stroke in _p_.