|Original article by
W.H. Marchant, from the book Wireless Telegraphy (1914)
The article is found on three pages:
As an example of the Marconi system we have chosen the 1½ Kw. installation as it is perhaps in more general use than any other set manufactured by that company. The general principles of the transmitter and the receiver have already been dealt with and this chapter will therefore be mainly descriptive of the apparatus. A rotary converter is used to convert the direct current from the ship's dynamo into an alternating current. The machine has four poles and runs at a speed of 1500 revolutions per minute, the frequency of the alternating current will therefore be 50. Across the brushes on both the alternating and the direct-current side, a special form of lamp resistance is connected. These are known as the guard lamps and provide a path of low self-inductance to shunt out any high-frequency currents which might be set up in the circuit by induction from the oscillatory circuits of the transmitter and cause damage to the machine.
The transmitter of this set consists of five circuits namely, the direct-current circuit, the low- tension alternating - current circuit, the high-tension alternating current circuit and the primary an secondary high-frequency circuits. The direct-current circuit consists of the motor starting switch, the field regulating resistance and the D.C. side of the rotary converter connected together and to the direct-current supply of the ship as shown in Fig. 61 .
The internal connections of the starter are shown in Fig. 62, and it will be seen that it differs some what from the ordinary pattern. As the arm of the switch is moved over the contacts, the resistance which at the start was in series with the armature coils is transferred to the field circuit, as this weakens the field it enables the motor to start quickly. The small electro-magnet on the face of the starter is termed a no volt release and serves while the machine is running to hold over the arm of the switch and should the power supply fail or the field circuit be broken the arm will be released and move over to the off position, and cut the motor out of circuit. The low-tension alternating-circuit consists of the primary of the transformer, Morse key, magnetic key, and impedance coil which together with the ampere meter and fuses are connected in series and to the alternating current side of the rotary converter, as in Fig. 61.
Fig. 63 shows the internal connections of the alternating-current transformer. It will be seen that it really consists of two open core transformers placed side in the same case and that the primaries and secondaries can be connected in series or parallel as necessity requires. The case is filled with oil to improve insulation. The construction of the magnetic key and its action has already been described in an earlier chapter. The morse key is of the ordinary heavy contact type, but connection between the lower contact and its terminal is made through a side lever which provides a means of breaking the alternating-current circuit ready to the hand of the operator in case of an emergency. The lever of the Morse key also carries a small ebonite arm which closes a pair of contacts are so arranged and connected as to short-circuit the telephone receiver before the main contacts of the key close.
The volt and ampere meters, the fuses, the double-pole quick-break switch, and the pilot lamp are mounted together on a slate panel (Fig.66). The high-tension alternating-current circuit consists of the secondary of the transformer connected through two air core chokes to the spark-gap and so to the condenser. The purpose of the chokes is to prevent the high-frequency currents in the closed oscillatory circuit from flowing back into the secondary of the transformer. The primary or closed oscillatory circuit consists of the condenser, spark-gap, primary of oscilation transformer and a variable inductance all connected in series. The condenser, the plates of which are zinc and the dielectric of which is glass, is so constructed that by changing the positions of the connecting strips between its terminals its capacity. Can be reduced to one-fourth of its maximum capacity. This provides a means of changing from the 600 to the 300 metre wave. Figs. 67,68 and 69 show the internal connections of the condenser and the positions of the connecting strips for the two wave-lenghts. The numbers on the terminals indicate the number of plates connected to each terminal. When working on a 600 metre wave the secondaries of the transformer should be connected in parallel and when working on the 300 metre wave they should be in series and the spark-gap doubled in length. The purpose of making these changes is to keep the spark frequency and the energy stored in the condenser before discharge constant. The energy stored in a condenser is proportional to the product of its capacity and the square of the voltage to which it is charged and therefore as the capacity of the condenser has been reduced to one-fourth in order to get the 300 metre wave, we must charge it to double the voltage, if we wish to keep the energy constant. The variable inductance consists of two brass rods with a metallic bridge, to the position of which determines the amount of inductance included in the circuit: the purpose of this inductance is to secure the exact adjustment to the circuit to a given wave-length after the main adjustment has been made on the condenser. The spark-gap which is enclosed in a muffling chamber is shown in Fig. 70.
It consists of two steel hemispheres, the length of the gap can be adjusted by turning either of them. In parallel with the main spark-gap is another formed of two pointed brass rods; the length of this gap, which is kept constant, is determined by the safe voltage to which the condenser can be charged and its purpose is to protect the condenser from excessive voltaged, which might puncture the dielectric. Connection between the various pieces of apparatus in this circuit is made by means of flat copper strips and not ordinary round sectioned conductors. These copper strips have large surface and therefore their resistance to high-frequency currents is low, also they are placed parallel and near together (about 1/8 " apart) and the space between them filled with strips of hard rubber. It will thus be seen that they are in effect a condenser and form part of the capacity of the circuit. The inductance of the circuit therefore consists almost entirely of the primary of the oscillation transformer and practically the whole of the inductance can therefore be utilised for coupling pruposes. The secondary or open oscillatory circuit is made up of variable tuning inductance, secondary of oscillation transformer, the inductance of which is also variable, and a special form of spark-gap called by the Marconi Company an earth arrester terminal all connected together is series and to antenae and earth. The tuning of the circuit is effected by varying the inductance and the coupling can be varied by sliding the secondary over the primary. The earth arrester terminal Fig.71 consists of two brass plates separated exept for a small space round the edge by a mica washer. The receiving apparatus is connected across the spark-gap so formed and its purpose is to obviate the necessity for a change-over swich and so permit the receiving operator to interrupt the sending operator during the course of transmission if necessity requires.
Its action is as follows: During the time the Morse key is closed the receiver is shorted out by the sparks which pass between the plates, but the moment the key is opened the path of incoming oscillations will be through the primary circuit of the tuner as the received oscillations are not of sufficient intencity to jump the gap. Shunted across about a yard of the earth lead is a small incandescent lamp in series with a variable choking coil; the purpose of the lamp is to indicate when the primary and secondary circuit of the transmitter are in tune. The brilliancy of the lamp will of course be a maximum when resonance is obtained. The function of the variable choke is to regulate the current through the lamp and thus to protect it from excessive currents which might burn out its filament. If a high-frequency current has two paths open to it, it splits up in inverse proportion to the inductance of the paths and therefore if we insert inductance in the branch in which is the tuning lamp we shall diminish the current through it.
In addition to the above-discribed transmitter it is the practice of the Marconi Company to provide an emergency set which is quite independent of the ship's machinery. The adventage of this provision will be apparent and has justified the extra cost involved on many occasions. The set of a 10-inch induction coil, a battery of 8 accumulator cells and a special type of switchboard designed to facilitate of changing the coil to or from the cells to the current-supply of the ship. Fig. 73 is a diagram of the coil connections; it will be seen that the condenser in the base of the coil, being across the contact-breaker and the Morse key in series as well as serving its usual purpose of accelerating the rate at which the lines of force cut the secondary winding when the circuit is broken, also serves to eleminate the sparking at the key contacts. Copper pins form outside connections between the will isolate the condenser for testing pruposes.
Fig. 78 is the charging switchboard. To charge the accumulators the double-pole switch is placed first in one position, then in the other: the correct position for charging being that in which the lamp resistances are dim. In this position the voltage of the cells is opposing that of the charging dynamo and the positive pole of the dynamo is therefore connected to the positive pole of the accumulators. The voltage of the lamps used as resistances is chosen to suit the voltage of the ship's supply and the candle power to permit the correct charging current being supplied to the accumulators. The single-pole switch in one position enables the coil to be worked from the cells and in the other position from the ship's dynamo, the coil resistances at he top of the board being in series in order to cut down the current to the correct value.
Fig. 79 will
enable the composition of the circuits for different
positions of the switch to be seen. It should be noted
that when working off the dynamo the lamp resistance must
be removed and the double-pole switch may then be placed
in either position.
THE MULTIPLE TUNER
The Marconi multiple tuner, which
is of the three circuit type described in the chapter on
receiving apparatus, and the magnetic detector are
usually used as the receiving apparatus with the 1½ Kw.
set,although on some stations the Fleming valve and the
special tuner with which it is incorporated are in use. Fig. 80 shows
the internal connections of the multiple tuner. It will
be seen that the three coupled switches are moved round
the effect is to add capacity (and in the case of the
primary circuit inductance) to the three circuits and so
to increase the wave-lenght range. For instance, when the
switch is in position one small condenser of fixed
capacity is put in series with the variable condenser in
each of the three circuits and only half the turns an the
primary coupling coil are in circuit, when the switch is
in position two the small condensers are cut out of
Position three increases the inductance of primary coil and puts condensers of fixed capacity in parallel with the variable condensers in the intermediate and detector circuits. Position four puts still larger condensers in parallel with the variable condensers in the intermediate and detector circuits and increases the inductance of the primary coil to its maximum value.
Fig. 82 Shows
the compositions of the three circuits for the various
positions of the switch. It will be noticed that the
inductance of the secondary circuit consists of two
coils, this is to facilitate the coupling of the circuit
to the primary and to the detector circuit. Both the
coils are carried on a shaft which terminates in a handle
seen in Fig. 83 and the coupling between the
circuits is thus varied simultaneously. On top of the
tuner (Fig.83) is a change-over switch which enables the
detector to be connected in the third circuit for tuned
reception or places it directly in connection with the
antennae circuit, which is the respond to a comparatively
large range of wave-lengths. When working on the standby
posistion the multiple tuning-switch should be in No.1
position, the smaal fixed capacity condenser will then
act as a blocking condenser to the primary of the
magnetic detector. Referring to the diagram of the tuner,
1 is a self-inductive shunt to earth; its purpose is to
prevent the accumulation of an electro-static charge on
the antennae: 2 is a micrometer spark-gap and serves the
same purpose as a lightning arrester and protects the
primary circuit from the transmitter when it is in
action. Terminals A and E are connected to the top and
bottom plates of the earth arrester, when the transmitter
is in action sparks pass across the gap and the rpimary
circuit of the tuner is thus shorted out. When the
transmitter is not in use the circuit from antennae to
earth is complete through the primary coil of the tuner.
It will thus be seen that the receiver can be left
connected to the antennae and a charge-over switch is not
needed, also it permits the receiving operator to break
in or interrupt the sending operator during the course of
transmission if necessary. The terminals marked D, D are
connected to the terminals marked A and E on the
detector. Fig. 84 shows
connection between tuner and detector. The magnetis
detector has been fully described in the chapter on
detectors and it only remains to say that the telephone
receiver is short-circuited before the power circuit of
the transmitter is closed by means of an extra pair of
contracts which projects from the lever of the Morse key.
The telephone condenser shown in Fig.
85 is variable in seven
steps by means of three plugs. This condenser, together
with the telephones which have inductance and resistance
form an oscillatory circuit, and the purpose of the
condenser is to enable us to tune the circuit to the
spark frequency of the transmitter or to one of the
harmonics of the frequency.
FLEMING VALVE TUNER
The action of the Fleming valve has already been described in a previous chapter, but the connections of the special tuner with which it is incorporated may be of interest. Like the multiple tuner, it is of the three circuit type, but the charge-over switch simply substitutes dufferent primary coils. The primary coil in use on the stand-by position is closely coupled to the detector circuit and when the switch is put over to the circuit, the coupling of which can be varied, is substituted. The high self-inductive shunt and the micrometer gap are connected in the same position and serve the same purpose as in the multiple tuner. Fig. 86 is a diagram of the connections. It will be seen that two valves are provided and a switch which charges the battery from one to the other. The purpose of the variable resistance is to regulate the current supplied to the filament of the valve.