In the receiving apparatus already described a filings coherer was used to detect the ether waves, and, by means of a local battery, to translate them into audible signals with a sounder, or printed signals with a Morse inker. This coherer however is unsuitable for commercial working. It is not sufficiently sensitive, and it can be used only for comparatively short distances; while its action is so slow that the maximum speed of signalling is not more than about seventeen or eighteen words a minute. A number of different detectors of much greater speed and sensitiveness have been devised. The most reliable of these, though not the most sensitive, is the Marconi magnetic detector, [Plate XIII.b]. This consists of a moving band made of several soft iron wires twisted together, and passing close to the poles of two horse-shoe magnets. As the band passes from the influence of one magnet to that of the other its magnetism becomes reversed, but the change takes a certain amount of time to complete owing to the fact that the iron has some magnetic retaining power, so that it resists slightly the efforts of one magnet to reverse the effect of the other. The moving band passes through two small coils of wire, one connected with the aerial, and the other with a specially sensitive telephone receiver. When the electric waves from the transmitting station fall upon the aerial of the receiving station, small, rapidly oscillating currents pass through the first coil, and these have the effect of making the band reverse its magnetism instantly. The sudden moving of the lines of magnetic force induces a current in the second coil, and produces a click in the telephone. As long as the waves continue, the clicks follow one another rapidly, and they are broken up into the long and short signals of the Morse code according to the manipulation of the Morse key at the sending station. Except for winding up at intervals the clockwork mechanism which drives the moving band, this detector requires no attention, and it is always ready for work.
Another form of detector makes use of the peculiar power possessed by certain crystals to rectify the oscillatory currents received from the aerial, converting them into uni-directional currents. At every discharge of the condenser at the sending station a number of complete waves, forming what is called a “train” of waves, is set in motion. From each train of waves the crystal detector produces one uni-directional pulsation of current, and this causes a click in the telephone receiver. If these single pulsations follow one another rapidly and regularly, a musical note is heard in the receiver. Various combinations of crystals, and crystals and metal points, are used, but all work in the same way. Some combinations work without assistance, but others require to have a small current passed through them from a local battery. The crystals are held in small cups of brass or copper, mounted so that they can be adjusted by means of set-screws. Crystal detectors are extremely sensitive, but they require very accurate adjustment, and any vibration quickly throws them out of order.
The “electrolytic” detector rectifies the oscillating currents in a different manner. One form consists of a thin platinum wire passing down into a vessel made of lead, and containing a weak solution of sulphuric acid. The two terminals of a battery are connected to the wire and the vessel respectively. As long as no oscillations are received from the aerial the current is unable to flow between the wire and the vessel, but when the oscillations reach the detector the current at once passes, and operates the telephone receiver. The action of this detector is not thoroughly understood, and the way in which the point of the platinum wire prevents the passing of the current until the oscillations arrive from the aerial is something of a mystery.
The last detector that need be described is the Fleming valve receiver. This consists of an electric incandescent lamp, with either carbon or tungsten filament, into which is sealed a plate of platinum connected with a terminal outside the lamp. The plate and the filament do not touch one another, but when the lamp is lighted up a current can be passed from the plate to the filament, but not from filament to plate. This receiver acts in a similar way to the crystal detector, making the oscillating currents into uni-directional currents. It has proved a great success for transatlantic wireless communication between the Marconi stations at Clifden and Glace Bay, and is extensively used.
The electric waves set in motion by the transmitting apparatus of a wireless station spread outwards through the ether in all directions, and so instead of reaching only the aerial of the particular station with which it is desired to communicate, they affect the aerials of all stations within a certain range. So long as only one station is sending messages this causes no trouble; but when, as is actually the case, large numbers of stations are hard at work transmitting different messages at the same time, it is evident that unless something can be done to prevent it, each of these messages will be received at the same moment by every station within range, thus producing a hopeless confusion of signals from which not a single message can be read. Fortunately this chaos can be avoided by what is called “tuning.”
Wireless tuning consists in adjusting the aerial of the receiving station so that it has the same natural rate of oscillation as that of the transmitting station. A simple experiment will make clearer the meaning of this. If we strike a tuning-fork, so that it sounds its note, and while it is sounding strongly place near it another fork of the same pitch and one of a different pitch, we find that the fork of similar pitch also begins to sound faintly, whereas the third fork remains silent. The explanation is that the two forks of similar pitch have the same natural rate of vibration, while the other fork vibrates at a different rate. When the first fork is struck, it vibrates at a certain rate, and sets in motion air waves of a certain length. These waves reach both the other forks, but their effect is different in each case. On reaching the fork of similar pitch the first wave sets it vibrating, but not sufficiently to give out a sound. But following this wave come others, and as the fork has the same rate of vibration as the fork which produced the waves, each wave arrives just at the right moment to add its impulse to that of the preceding wave, so that the effect accumulates and the fork sounds. In the case of the third fork of different pitch, the first wave sets it also vibrating, but as this fork cannot vibrate at the same rate as the one producing the waves, the latter arrive at wrong intervals; and instead of adding together their impulses they interfere with one another, each upsetting the work of the one before it, and the fork does not sound. The same thing may be illustrated with a pendulum. If we give a pendulum a gentle push at intervals corresponding to its natural rate of swing, the effects of all these pushes are added together, and the pendulum is made to swing vigorously. If, on the other hand, we give the pushes at longer or shorter intervals, they will not correspond with the pendulum’s rate of swing, so that while some pushes will help the pendulum, others will hinder it, and the final result will be that the pendulum is brought almost to a standstill, instead of being made to swing strongly and regularly. The same principle holds good with wireless aerials. Any aerial will respond readily to all other aerials having the same rate of oscillation, because the waves in each case are of the same length; that is to say, they follow one another at the same intervals. On the other hand, an aerial will not respond readily to waves from another aerial having a different rate of oscillation, because these do not follow each other at intervals to suit it.
If each station could receive signals only from stations having aerials similar to its own, its usefulness would be very limited, and so all stations are provided with means of altering the rate of oscillation of their aerials. The actual tuning apparatus by which this is accomplished need not be described, as it is complicated, but what happens in practice is this: The operator, wearing telephone receivers fixed over his ears by means of a head band, sits at a desk upon which are placed his various instruments. He adjusts the tuning apparatus to a position in which signals from stations of widely different wave-lengths are received fairly well, and keeps a general look out over passing signals. Presently he hears his own call-signal, and knows that some station wishes to communicate with him. Immediately he alters the adjustment of his tuner until his aerial responds freely to the waves from this station, but not to waves from other stations, and in this way he is able to cut out signals from other stations and to listen to the message without interruption.
Unfortunately wireless tuning is yet far from perfect in certain respects. For instance, if two stations are transmitting at the same time on the same wave-length, it is clearly impossible for a receiving operator to cut one out by wave-tuning, and to listen to the other only. In such a case, however, it generally happens that although the wave-frequency is the same, the frequency of the wave groups or trains is different, so that there is a difference in the notes heard in the telephones; and a skilful operator can distinguish between the two sufficiently well to read whichever message is intended for him. The stations which produce a clear, medium-pitched note are the easiest to receive from, and in many cases it is possible to identify a station at once by its characteristic note. Tuning is also unable to prevent signals from a powerful station close at hand from swamping to some extent signals from another station at a great distance, the nearer station making the receiving aerial respond to it as it were by brute force, tuning or no tuning.
Another source of trouble lies in interference by atmospheric electricity. Thunderstorms, especially in the tropics, interfere greatly with the reception of signals, the lightning discharges giving rise to violent, irregular groups of waves which produce loud noises in the telephones. There are also silent electrical disturbances in the atmosphere, and these too produce less strong but equally weird effects. Atmospheric discharges are very irregular, without any real wave-length, so that an operator cannot cut them out by wave-tuning pure and simple in the way just described, as they defy him by affecting equally all adjustments. Fortunately, the irregularity of the atmospherics produces correspondingly irregular sounds in the telephones, quite unlike the clear steady note of a wireless station; and unless the atmospherics are unusually strong this note pierces through them, so that the signals can be read. The effects of lightning discharges are too violent to be got rid of satisfactorily, and practically all that can be done is to reduce the loudness of the noises in the telephones, so that the operator is not temporarily deafened. During violent storms in the near neighbourhood of a station it is usual to connect the aerial directly to earth, so that in the event of its being struck by a flash the electricity passes harmlessly away, instead of injuring the instruments, and possibly also the operators. Marconi stations are always fitted with lightning-arresters.
The methods and apparatus we have described so far are those of the Marconi system, and although in practice additional complicated and delicate pieces of apparatus are used, the description given represents the main features of the system. Although Marconi was not the discoverer of the principles of wireless telegraphy, he was the first to produce a practical working system. In 1896 Marconi came from Italy to England, bringing with him his apparatus, and after a number of successful demonstrations of its working, he succeeded in convincing even the most sceptical experts that his system was thoroughly sound. Commencing with a distance of about 100 yards, Marconi rapidly increased the range of his experiments, and by the end of 1897 he succeeded in transmitting signals from Alum Bay, in the Isle of Wight, to a steamer 18 miles away. In 1899 messages were exchanged between British warships 85 miles apart, and the crowning achievement was reached in 1901, when Marconi received readable signals at St. John’s, Newfoundland, from Poldhu in Cornwall, a distance of about 1800 miles. In 1907 the Marconi stations at Clifden and Glace Bay were opened for public service, and by the following year transatlantic wireless communication was in full swing. The sending of wireless signals across the Atlantic was a remarkable accomplishment, but it did not represent by any means the limits of the system, as was shown in 1910. In that year Marconi sailed for Buenos Ayres, and wireless communication with Clifden was maintained up to the almost incredible distance of 4000 miles by day, and 6735 miles by night. The Marconi system has had many formidable rivals, but it still holds the proud position of the most successful commercial wireless system in the world.