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telegraphist being affected by undue noise in the telephone during transmission. The receiving apparatus to be permanently connected to the aerial and earth across the spark gap in the earth terminal, so that whenever the transmitting key is lifted, the operator is in a position to receive signals from the corresponding station and be interrupted if desired during the actual transmission of the message.
RECEIVING INSTRUMENTS.
22. Two separate receivers to be furnished, each to be calibrated to permit the instruments to be set to any pre-arranged wave lengths to be received, also to be provided with means to permit of the changing of the receiving circuits from a highly syntonised condition, when receiving a particular set of signals from a corresponding station, to a comparatively untuned condition such as is desired when the operator is standing-by and watching for any station which may be calling. A double pole two-way switch to be provided to enable either receiver to be connected to the aerial, as desired.
To comprise :-
(A) One receiver of the magnetic detector pattern working in conjunction with the Marconi multiple tuner, the com- bination being capable of syntonising the receiving circuits to all wave lengths between 100 and 2,500 metres.
The magnetic detector to consist generally of a soft iron band moving at a uniform rate in the magnetic field provided by two permanent magnets; this band to pass through a glass tube carrying a primary winding directly coupled to the aerial circuit through the multiple-tuner, a secondary winding connected to the headgear tele- phones to be carried on an ebonite bobbin placed over the centre of the primary. The band to be composed of a large number of strands of fine gauge silk-covered soft iron wire, loosely twisted together and covered with vaseline; to pass as an endless band round two ebonite pulleys, one driven by clockwork and the other running free. The clockwork to be mounted on the baseboard of to the instrument, and designed when fully wound up drive the band for 14 hours with only a slight diminution of speed. The arrangement of primary and secondary windings with their corresponding terminals to be duplicated on the receiver, so that in the event of damage to the windings on one side the opposite set can be employed. A movable wooden cover with a glass top to be securely fitted on the case to protect the band, windings, and other cxternal parts, from dust or damage.
23.
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(B) One Marconi balanced crystal receiver, pattern No. 16, having a range of adjustment for the reception of all wave lengths between 300 and 2,500 metres, and fitted with two crystals which can be used for balanced or independent working as desired.
The receiver to be provided with an adjustable aerial tuning inductance, having a total value of 4,000 micro- henries, regulated by means of a 21-stud control switch with one "off" position. An adjustable disc condenser to be connected in series with the aerial tuning induct- ance. A safeguard to be provided against high potential static charges by means of a micrometer spark gap and protecting coils.
The jigger to have a fixed primary coil wound on a spherical former. The value of the coupling between the primary and secondary circuits to be varied by rotating the primary inside the secondary as indicated by the degrees marked on the intensifier handle which is placed on the right of the aerial tuning inductance switch. The jigger secondary to be divided into three portions, forming practically a continuous winding and controlled by a three-way switch with platinum contacts, by means of which the amount of inductance in the jigger secondary circuit can be varied. A condenser with a wide range of adjustment to be provided for tuning the jigger secondary circuit and, from a scale fixed to the instrument, the jigger circuit to be capable of being syntonised to waves of any length within the limits of the range of the instrument.
The detector circuit to contain double potentiometers and duplicate adjustable crystal clips, with a switch to open each crystal circuit when required. The potentio- meters to be connected with the zero points at the centre so that either the positive or negative characteristics of the crystals can be brought into use without the necessity of reversing the position of the crystal in the holder.
A battery box with three terminals and a pair of low resistance telephones to be furnished with the receiver, the telephone transformer being mounted inside the detector case.
TESTING INSTRUMENTS AND SUNDRIES.
A portable type of wavemeter to be provided, by means of which the various transmitting circuits can be tuned quickly and easily to the required wave length and a constant check kept on the condition of the radiating circuits.
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