DETAILS OF APPARATUS.
A full description of the details of the apparatus would occupy too much space, but the following particulars include the salient points about the more important parts of the transmitter.
Spark Discharger.The mushroom-shaped electrodes, which are of cast iron, are mounted on ebonite insulated brass rods and placed in a teak silence box lined with asbestos and lead. (Fig. 4.) Lime is placed in a removable zinc tray to absorb the acid produced by the spark. The gap is adjusted by turning the electrodes on their threaded brass supports. A protective point spark-gap shunting the main gap is set for a fixed gap to protect the condensers against excessive voltage.
Half-plate Condenser.This is in two parts, each with 36 glass plates interspaced with 35 sheets of zinc. Ninety-four plates are active, the others being guard plates. They are 1/10 in. thick, and are individually tested to 27,000 volts. The container is filled with high-flash insulating oil. For the short wave the two parts of the condenser are connected in series, and have a capacity of about 0.016 mfd. With the long wave, in parallel, the capacity is about 0.065 mfd. The condenser is seen in Figs. 5 and 6.
Variable Coupling Oscillation Transformer or Transmitting Jigger.The primary and secondary windings are provided respectively by two coils of heavily insulated wire, the former of one turn and the latter of seven turns. It is at this point that the wave-generating and wave-radiating circuits are coupled together. For the purpose of adjusting the coupling between the circuits the secondary is made to slide laterally over the primary. On each oscillation transformer is a scale which gives approximately the percentage coupling between the two windings.
Aerial Tuning Inductance.Tappings are made at various points on 20 turns of insulated wire and brought to eight insulated terminals in front of the box.
Aerial Lead-in.This is a 1/2 in. metal rod in a long ebonite tube, which passes through a cast-iron gland fixed on the roof of the cabin. A zinc cone is fitted to the external end of the rod to ensure that a considerable length of the ebonite will remain dry in all circumstances. A shackle head on the cone takes the strain of the aerial, and prevents the connection being worked loose by the vibration of the aerial wire. Three ebonite discs spaced along the external part of the ebonite tube prevent surface sparking in event of heavy weather causing the insulator to be wet with salt spray. (Fig. 7.)
Earth Arrester Terminal.This consists of two round brass plates separated generally to about 0.01 in. by discs of mica. The wires to the ship's earth connect with the bottom plate. The top plate (seen in Fig. 8) is connected to the earth of the oscillation transformer and the aerial terminal of the tuner. When transmitting, the top plate sparks on to the bottom plate and connects direct to earth. When receiving, the top plate remains insulated and the aerial oscillations find their way to earth through the tuner. The terminal, while serving as an automatic switch, also provides protection for the aerial against lightning.
Multiple Tuner.A means of tuning to the incoming signals of one given wave-length, and eliminating the signals of all other wave-lengths, is provided by the three inductively coupled adjustable circuits, the first connected to the aerial and the third to the magnetic detector. The full range of 100 to 2,500 metres is obtained in four steps by a triple four way switch, which simultaneously alters the amount of fixed inductance and capacity in each circuit. Accurate tuning is obtained by an adjustable tuning inductance and variable condenser in all circuits. Injury to the instrument when transmitting is prevented by a micrometer spark-gap, which automatically shunts the aerial terminal to earth.
Magnetic Detector.—This instrument has been described so often that particulars would be superfluous. It may be mentioned that for the greatest intensity of signals the two permanent magnets should be placed with like poles together over the coils. This arrangement, however, introduces a low noise in the telephone termed “breathing.” If the magnets are placed with unlike poles together, one of them well down on the coils and the other with its inside limb at a certain distance up the inside limb of the first magnet (the correct position being found by test), the “breathing” becomes negligible, at the expense of a very slight decrease in the intensity of the sounds. The detector is seen in Fig. 9.
Wavemeter.A wavemeter is provided to enable the transmitting circuit to be correctly adjusted to the international waves of 300 and 600 metres. It consists of an inductance coil with a fixed number of turns connected to an adjustable condenser. Across the condenser is shunted a high resistance asymmetrical crystal in series with a double-head telephone. The condenser is adjusted until signals are loudest in the telephone. A table affixed to the lid of the wavemeter gives the wave-length corresponding to each condenser reading. The range of the standard apparatus is 200 to 800 metres.
Aerials and Aerial Insulators.Bare silicon bronze wires are used for the aerials. The overall dimensions depend on the design of the ship. In shape the aerials are either a twin L or a twin T, the spacing between the twins being 12 ft. A 12 ft. 6 in. ash spreader usually supports the aerial at each end. The spreader has two bands, each with two lugs, one taking an aerial strain insulator and the other one leg of the spreader bridle, the bight of the bridle being in turn made fast to the halyard. The strain insulator commonly in use for short aerials is 3 ft. long and circular in section, composed of Palmer cord encased in rubber and vulcanised. It is designed for a working load of 10 cwt. and a breaking load of 30 cwt. Long aerials are supported on larger insulators in the same manner. Each insulator is covered with a special insulating preservative compound to protect the rubber and furnish a non-hygroscopic surface. The arrangement is seen in Fig. 10.
Fig. 11 is a diagram showing the connections of wireless equipment on board ship.