We have not space to give a description of the other commercial systems, but a few words on some of the chief points in which they differ from the Marconi system may be of interest. We have seen that an ordinary spark gap, formed by two metal balls a short distance apart, becomes overheated by the rapid succession of discharges, with the result that the sparking is irregular. What actually happens is that the violent discharge tears off and vaporizes minute particles of the metal. This intensely heated vapour forms a conducting path which the current is able to cross, so that an arc is produced just in the same way as in the arc lamp. This arc is liable to be formed by each discharge, and it lasts long enough to prevent the sparks from following one another promptly. In the Marconi system this trouble is avoided by means of a rotating spark gap, but in the German “Telefunken” system, so named from Greek tele, far off, and German Funke, a spark, a fixed compound spark gap is used for the same purpose. This consists of a row of metal discs about 1/100 inch apart, and the spark leaps these tiny gaps one after the other. The discs are about 3 inches in diameter, and their effect is to conduct away quickly the heat of the discharge. By this means the formation of an arc is prevented, and the effect of each discharge is over immediately, the sparks being said to be “quenched.” The short discharges enable more energy to be radiated from the aerial into the ether, and very high rates of sparking are obtained, producing a high-pitched musical note.
The “Lepel” system also uses a quenched spark. The gap consists of two metal discs clamped together and separated only by a sheet of paper. The paper has a hole through its centre, and through this hole the discharge takes place, the discs being kept cool by water in constant circulation. The discharge is much less noisy than in the Marconi and Telefunken systems, and the musical note produced is so sensitive that by varying the adjustments simple tunes can be played, and these can be heard quite distinctly in the telephone at the receiving stations.
In the three systems already mentioned spark discharges are used to set up oscillatory currents in the aerial, which in turn set up waves in the ether. Each discharge sets in motion a certain number of waves, forming what is known as a train of waves. The discharges follow one another very rapidly, but still there is a minute interval between them, and consequently there is a corresponding interval between the wave-trains. In the “Goldschmidt” system the waves are not sent out in groups of this kind, but in one long continuous stream. The oscillatory currents which produce ether waves are really alternating currents which flow backwards and forwards at an enormous speed. The alternating current produced at an ordinary power station is of no use for wireless purposes, because its “frequency,” or rate of flow backwards and forwards, is far too low. It has been found possible however to construct special dynamos capable of producing alternating current of the necessary high frequency, and such dynamos are used in the Goldschmidt system. The dynamos are connected directly to the aerial, so that the oscillatory currents in the latter are continuous, and the ether waves produced are continuous also.
The “Poulsen” system produces continuous waves in an altogether different manner, by means of the electric arc. The arc is formed between a fixed copper electrode and a carbon electrode kept in constant rotation, and it is enclosed in a kind of box filled with methylated spirit vapour, hydrogen, or coal gas. A powerful electro-magnet is placed close to the arc, so that the latter is surrounded by a strong magnetic field. Connected with the terminals of the arc is a circuit consisting of a condenser and a coil of wire, and the arc sets up in this circuit oscillatory currents which are communicated to the aerial. These currents are continuous, and so also are the resulting waves.
The method of signalling employed in these two continuous-wave systems is quite different from that used in the Marconi and other spark systems. It is practically impossible to signal by starting and stopping the alternating-current dynamos or the arc at long or short intervals to represent dashes or dots. In one case the sudden changes from full load to zero would cause the dynamo to vary its speed, and consequently the wave-length would be irregular; besides which the dynamo would be injured by the sudden strains. In the other case it would be extremely difficult to persuade the arc to start promptly each time. On this account the dynamo and the arc are kept going continuously while a message is being transmitted, and the signals are given by altering the wave-length. In other words, the transmitting aerial is thrown in and out of tune alternately at the required long or short intervals, and the receiving aerial responds only during the “in-tune” intervals.
The various receiving detectors previously described are arranged to work with dis-continuous waves, producing a separate current impulse from each group or train of waves. In continuous wave systems there are of course no separate groups, and for this reason these detectors are of no use, and a different arrangement is required. The oscillatory currents set up in the aerial are allowed to charge up a condenser, and this condenser is automatically disconnected from the aerial and connected to the operator’s telephones at regular intervals of about 1/1000 second. Each time the condenser is connected to the telephones it is discharged, and a click is produced. These clicks continue only as long as the waves are striking the aerial, and as the transmitting operator interrupts the waves at long or short intervals the clicks are split up into groups of corresponding length.
Continuous waves have certain advantages over dis-continuous waves, particularly in the matter of sharp tuning, but these advantages are outweighed to a large extent by weak points in the transmitting apparatus. The dynamos used to produce the high-frequency currents in the Goldschmidt system are very expensive to construct and troublesome to keep in order; while in the Poulsen system the arc is difficult to keep going for long periods, and it is liable to fluctuations which greatly affect its working power. Although all the commercial Marconi installations make use of dis-continuous waves exclusively, Mr. Marconi is still carrying out experiments with continuous waves.
There are many points in wireless telegraphy yet to be explained satisfactorily. Our knowledge of the electric ether waves is still limited, and we do not know for certain how these waves travel from place to place, or exactly what happens to them on their journeys. For this reason we are unable to give a satisfactory explanation of the curious fact that, generally speaking, it is easier to signal over long distances at night than during the day. Still more peculiar is the fact that it is easier to signal in a north and south direction than in an east and west direction. There are also remarkable variations in the strength of the signals at certain times, particularly about sunset and sunrise. Every station has a certain normal range which does not vary much as a rule, but at odd times astonishing “freak” distances are covered, stations having for a short time ranges far beyond their usual limits. These and other problems are being attacked by many investigators, and no doubt before very long they will be solved. Wireless telegraphy already has reached remarkable perfection, but it is still a young science, and we may confidently expect developments which will enable us to send messages with all speed across vast gulfs of distance at present unconquered.
Wireless telephony, like wireless telegraphy, makes use of electric waves set up in, and transmitted through the ether. The apparatus is practically the same in each case, except in one or two important points. In wireless telegraphy either continuous or dis-continuous waves may be used, and in the latter case the spark-frequency may be as low as twenty-five per second. On the other hand, wireless telephony requires waves which are either continuous, or if dis-continuous, produced by a spark-frequency of not less than 20,000 per second. In other words, the frequency of the wave trains must be well above the limits of audibility. Although dis-continuous waves of a frequency of from 20,000 to 40,000 or more per second can be used, it has been found more convenient to use absolutely continuous waves for wireless telephony, and these may be produced by the Marconi disc generator, by the Goldschmidt alternator, or by the Poulsen arc, the last named being largely employed.
In wireless telegraphy the wave trains are split up by a transmitting key so as to form groups of signals; but in telephony the waves are not interrupted at all, but are simply varied in intensity by means of the voice. All telephony, wireless or otherwise, depends upon the production of variations in the strength of a current of electricity, these variations being produced by air vibrations set up in speaking. In ordinary telephony with connecting wires the current variations are produced by means of a microphone in the transmitter, and in wireless telephony the same principle is adopted. Here comes in the outstanding difficulty in wireless transmission of speech. The currents used in ordinary telephony are small, and the microphone works with them quite satisfactorily; but in wireless telephony very heavy currents have to be employed, and so far no microphone has proved capable of dealing effectively with these currents. Countless devices to assist the microphone have been tried. It was found that one of the causes of trouble was the overheating of the carbon granules, which caused them to stick together, so becoming insensitive. To remedy this the granules have been cooled in various ways, by water, gas, or oil, but although the results have been improved, still the microphones worked far from perfectly. Improved results have been obtained also by connecting a number of microphones in parallel. The microphone difficulty is holding back the development of wireless telephony, and at present no satisfactory solution of the problem is in sight.