Brown's Signalling, 18th Edition, February, 1916, pages 213-223:

WIRELESS  TELEGRAPHY.
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    Any book dealing with signalling in general is incomplete without a reference to wireless telegraphy which, for mercantile signalling, offers so many advantages over other methods of signalling, by flags, lamps and semaphores, which are so dependent on natural conditions such as fog, are slow, open to error and of limited range.

The  Quenched  Spark  System.

    The quenched spark system is one of the most important and successful systems of wireless telegraphy, as can be gauged from the fact that upwards of 1250 ship stations and 398 land stations have been equipped upon this system. gap diagram
    This system dates from 1903, but its introduction into the British Mercantile Marine by Messrs. Siemens Brothers & Company, Ltd., only dates from 1911, since when no less than 150 British ships have been fitted and its advantages are being quickly recognised by British shipowners.
    The theory of the "quenched spark" is somewhat intricate, although the apparatus is exceedingly simple. It may be briefly summarised as follows:--
    All modern wireless stations are compelled by International Regulations to generate the wireless oscillations in a circuit other than that by which the oscillations are radiated into space. This regulation is necessary since with only a single generating and radiating circuit, the waves emitted have a very indefinite wave length and would so cause unnecessary mutual interference when a number of stations are working at the same time.
    The generating circuit having the necessary electrical properties of inductance and capacity is best described as a primary or "excitation" circuit, since it excites oscillations in the "aerial" circuit which radiates the energy into space. Fig. 1.
    When two such circuits are electrically combined, it can be shown both by mathematical reasoning and practically, by use of a testing instrument, known as a "wavemeter," that two distinct oscillations having different wave lengths are present.
    As the energy in the aerial circuit approaches a maximum value, the energy in the excitation circuit is reduced to a minimum. After this point the energy in the aerial circuit becomes reduced and that in the closed or excitation circuit attains the maximum value again. In this way the energy surges from one circuit to the other.
    Only the energy in the aerial circuit can be radiated, however. By means of the quenched spark gap, at the moment when the energy in the excitation circuit is at its lowest point and that in the aerial circuit at maximum the spark is quenched and the excitation circuit is thereby interrupted. There is no possibility of the energy in the aerial circuit surging back into the excitation circuit, and the energy is therefore radiated by the aerial circuit oscillating in its own natural periodicity.
    Approximately a wireless installation in which the spark-discharge in the exciting circuit is quenched will be twice as efficient as one in which the spark is not quenched so that, with the use of any given primary energy, the range of the station will be approximately doubled when the excitation circuit oscillation is quenched. Fig. 2.
    There are various methods of quenching this exciting oscillation in a greater or less degree. The particular method used in the quenched spark system is characterised by the completeness and rapidity of the quenching and the extreme simplicity of the apparatus employed.
    This apparatus is a special form of spark gap consisting of a number of circular metal electrodes separated by thin mica discs. The section between the plates being of the form:--
    Sparking takes place across a number of small gaps such as AB. As soon as a spark forms at the point A, the ionised metallic particles volatilised from the electrodes, the movement of which constitutes an electrical current, are, owing to the presence of a transverse magnetic field, urged outward at a very great velocity towards the periphery of the circular electrode. In a very short time the spark arrives at the circular groove in the electrode at CD, over which the pressure used is insufficient to bridge, and the oscillation, therefore, immediately dies out.
    A number of these small spark gaps are used clamped together in a metal stand shown in fig. 1. The spark gaps (with the exception of those for very small stations)--have between each pair of electrodes, a copper plate which serves to conduct and radiate away the small amount of heat formed by the passage of the spark.
    Although this gap is so simple in design, it is exceedingly effective and requires no attention during working. The importance of this action may be judged by the fact that with an ordinary unquenched spark gap only 25 per cent. to 33 per cent. of the energy supplied is radiated, whereas with the quenched spark gap the percentage of energy radiated rises to as much as 50 per cent. to 75 per cent.
    Still more important is the fact that the spark gaps, only having a small distance of separation, require a small pressure to break down their resistance, and are practically noiseless in action. No expensive sound insulation of the spark gap and the wireless apparatus is required to deaden the noise which always accompanies the passage of a long spark. The most notable feature of the quenched spark installation is, therefore, the complete absence of sound insulation. Fig. 3.
    In the quenched spark system the exciting spark is caused by the use of a special high frequency alternator, to take place 1000 times per second, so that 1000 distinct electrical wave trains per second are radiated by the aerial and these, on receipt at a receiving station, are converted by a well-known device into sound waves, which naturally have the same frequency of 1,000 vibrations per second, and a distinct musical note is therefore produced. Such a musical note is far easier to distinguish from noises due to natural electrical disturbances known technically as "atmospherics" than is the case with a non-musical note. Hence, irrespective of the increased range obtained by the use of the "quenched spark," the range of a wireless station installed on this principle is also greatly increased owing to the use of this musical note, which can be heard at a much greater distance and distinguished from disturbing atmospherics even when these are from twenty to thirty times as intense, in which case it would be totally impossible to distinguish a non-musical note and communication would so be interrupted. For this reason, the adoption of the quenched spark system in tropical countries, where such atmospherics are always present and very intense, has been very extensive.
    This increased rate of the formation of the exciting spark again permits the employment of a much smaller pressure, and the apparatus can hence be made much smaller and more compact. This can be realised from fig. 2, which shows a complete station manufactured by Siemens Brothers & Company, Ltd., for use on small yachts and vessels not requiring a permanent wireless service.
    All the wireless apparatus is seen enclosed in a wooden roll front cabinet, the dimensions of which are 2' 2" X 2' 2" X 4' 8". This exceedingly small station has such a high efficiency that with masts 100 feet high, a range of 90 miles by day and 125 miles by night can be guaranteed.
    The manufacturers in giving such a range must, as a matter of commercial precaution, fix a range which can be guaranteed under adverse conditions. Under favourable conditions this range is exceeded twice or threefold, and such installations have even carried on communication at ranges exceeding 800 miles.
    For passenger and cargo vessels larger installations are manufactured, a typical example being the station illustrated in fig. 3, which is installed on the S.S. Chindwin, of Messrs. P. Henderson & Co.
    The source of energy is the direct current supply from the ship's mains, which is led through the fuses 1 and switch 2 to the starter 5 of the motor 6. The speed of this motor is regulated by the resistance 7. The ammeter 3 and the voltmeter 4 are installed for its control. The speed of the motor is normally 1500 r.p.m., but is capable, for the regulation of the transmitting radiating signal, of a variation of 30 per cent, above or below the normal speed of revolution. This motor drives a high frequency alternator 8 built upon the inductor principle, which delivers current through the fuses 10 and switch 11 to a small highly laminated high frequency transformer not visible in the illustration. A second ammeter 12 and voltmeter 13 are installed upon the switchboard to control the alternator, the excitation of which is varied by means of a regulating resistance 9 and fine adjustment 9a.
Fig. 4.
A 500 frequency alternator. H     Excitation and coupling inductance.
B Switchboard controlling alternator. J     Aerial hot wire ammeter.
C Morse Key. L     Aerial shortening capacity.
D Resonance choke coil. M     Aerial lengthening inductance.
E1 E2 High frequency transformer. N     Protective lightening switch--
F Quenched spark gap.                 Position a aerial to earth k.
G Excitation capacity.                 Position b aerial to apparatus.
O          Aerial leading in insulator

    To protect the insulation of the windings in the motor and alternator from damage due to surges of high frequency current, a special high frequency device 14 consisting of resistance lamps and condensers is connected across the armature of the motor and the D.C. and A.C. terminals of the alternator. The duty of these devices is to conduct all high frequency surges by an easy path to earth in preference to passing through and damaging the winding of the machine.
    The excitation circuit, fed from the transformer already mentioned, consists of an inductance 15, a battery of Leyden jar condensers 16 and a "quenched" spark gap 17.
    The excitation circuit is direct coupled to the aerial circuit by means of plugs and sockets upon the exciting inductance. The aerial circuit goes to earth through a high frequency hot wire ammeter 18 and passes to the aerial through the variometer inductance 19, shortening capacity 20, protective lightning switch 21 and leading-in insulator 22.
    The inductance 19 is made variable in order to permit the aerial circuit to be tuned to the excitation circuit, such tuning being shown by the maximum reading of the hot wire ammeter already referred to. The arrangement of the various apparatus of the transmitter is shown diagramatically in figure 4.
    The apparatus for the reception of wireless signals is seen at 23. This is directly connected to the aerial and to earth.
    It consists of a primary inductance and variable condenser for tuning purposes in the aerial. This primary inductance induces upon a secondary inductance which can be connected to either of two detectors. Signals are given in an ordinary head telephone, two of which can be used with the receiver, in order that two persons can listen to signals at the same time.
    The receiver has a switch at the back which, when reception is required, interrupts the transmitting circuit so that danger to the apparatus or operator cannot be caused by the accidental tapping of a key. Similarly, when transmission is taking place this switch interrupts all the receiving circuits.
    By means of a small switch upon the back plate, the internal connections can be changed as required, so as to be most favourable for the reception of either long or short waves. A larger illustration of this particularly compact receiver which has a range of from 200 to 2000 meters is shown in fig. 5. Fig. 5.
    Reception by this receiver is extremely simple. By means of a plug lead and plug socket any desired inductance is put into the aerial circuit. By means of a second plug a tapping is taken off the secondary coil for the required wave length. An exact tuning to this wave length is then obtained by means of the variable condenser at the base.
    In order to comply with present and forthcoming regulations, which make it compulsory for vessels carrying over 50 persons to be equipped both with a wireless installation and a means by which transmission can be carried on even if any accident occurs to the ship's electrical generating plant, a battery of accumulators 26 (fig. 3) is provided. These are charged through a resistance 25 which reduces the pressure of the ship's supply to that required by the accumulators. The current, passing through a cut-out 24 which breaks the supply circuit should the vessel's dynamos stop when the operator is not present, and so prevents the cells from discharging. The current from these cells is delivered through the fuses 27, and switch 28 to the induction coil 30, the current and pressure being controlled by indicating instruments upon the switchboard by means of a small switch beneath. The pressure to the induction coil is regulated by the resistance 29 and has a special heavy current hammer break. The high voltage from the induction coil is supplied to the main excitation circuit composed of 15, 16, 17, the number of spark gaps being reduced to two by means of short circuiting plugs. In order to change from transmission on the main installation to the emergency set just described, a change over switch 31 is provided; transmission by either circuit taking place by means of one of the Morse keys 32 which are inserted in the low tension circuits. For a larger station employing a heavier current, interruption is not carried out directly by a Morse key, but by a special relay worked by means of key.
    At 33 and 34 testing instruments known as a buzzer and an aperiodic circuit may be seen which serve to test receiving and transmitting circuits respectively.
    It will be noted from fig. 3, which illustrates the complete station, that it is most compact and that sound insulation is absent.
    The range guaranteed with such an installation depends necessarily upon the height and position of the masts of the vessel. With masts 115 feet high, the range is 250 miles by day and 375 miles by night, but with the installation in question and similar installations ranges of 1500 miles are frequently obtained.
    Messrs. Siemens Brothers & Company, Ltd., the manufacturers of this station, also manufacture land stations and a number of useful accessories for wireless transmitting, such as a wireless direction finder which when installed at a land station upon the coast enables ships to determine their bearings from the shore, the only addition to the ship station being a small stop watch.
    Another apparatus is the duplex relay which enables transmission and reception of messages to take place at the same time, the double receiving switch which enables signals to be received from two stations at the same time, various forms of intensifiers to magnify weak received wireless signals, a call signal apparatus to enable a particular station to be called up by a ringing of a bell when the operator is not on watch, and various controlling and testing apparatus such as wave-meters, buzzers, aperiodic circuits, spark-control apparatus, etc.