GENERAL SERVICE TYPES
N.2124 AND N.2206
G. R. GUNSON, Circuit Development Department
Magneto telephones can be operated under difficult conditions which make C.B. or automatic systems impracticable. The magneto instrument came into use at an early stage in the development of telephone communication, and many existing installations employ telephones which do not take advantage of the improved techniques and materials now available. The new N.2124 and N.2206 instruments are designed on modern principles to fulfil the requirements of magneto systems in an efficient manner.
|Fig 1||Fig 2|
THE magneto telephone is still the most reliable means of communication in many parts of the world where conditions are unsuitable for C.B. or automatic systems. Generally speaking, magneto working is indicated where no skilled maintenance is available, where lines are situated in difficult terrain and may be long or of poor quality from the transmission point of view, or where there is no power supply for charging central batteries. The advantages of the magneto telephone in such situations arise mainly from the fact that the functioning of the instrument is less dependent on line and exchange conditions than is the case with other systems. For instance, the microphone current is obtained from local dry cells instead of from a central battery, so that the sending level is always adequate regardless of the line length. Similarly, the inclusion of a hand generator in the instrument ensures that ample ringing current is available independently of external factors. The simplicity and robustness of magneto systems ease the maintenance problems caused by lack of skilled staff, and adverse climates.
Since magneto working lends itself to communication service under arduous conditions, it is natural that instruments should sometimes be required in teak or other hardwood cases built mainly for durability. However, the majority of new installations are in premises where such cases would be out of place and a modern design is needed. For this reason magneto telephones were made available some years ago in borrowed finery, as it were, by housing them in the moulded cases designed for automatic telephones. The new instruments, depicted in Figs. 1 and 2, have cases specially designed for magneto telephones, separate models being available for wall and table mounting. The cases are polished one piece mouldings of modern form with the generator handle included as a pleasing part of the design. Their shapes, with the ribbed front, are interesting but simple enough to avoid the collection of dust. The handset is of the new curved type and its contours match those of the cases. Both wall and table models are made in black, ivory, jade green and Chinese red.
To ensure satisfactory service in tropical atmospheres ranging from extremely humid to dry and dusty, the instruments are dust-screened and insect proofed, and nylon covered cords, p.v.c. insulated wire and varnish impregnated coils are used. The table model has cup-shaped ventilation grilles fitted in the sides of the case; these grilles also form convenient lifting holds.
The handset is fitted with an inset transmitter and the new rocking armature inset receiver, which is substantially more sensitive than the type previously used. The cord conductors are taken through the bore of the handle and are attached to screw terminals. At the instrument end the cord passes through a protective grommet, which in the table model has separate entry holes for the handset and desk cords.
|Fig 3||Fig 4|
The internal apparatus, illustrated in Figs. 3 and 4, is mounted on a die-cast base. The induction coil is of the closed iron circuit type in which the core is made up of grain-oriented silicon iron laminations forming a complete loop round the windings. This coil is designed for use with the rocking armature
receiver in the handset, and is much more efficient than the older type of coil with open iron-wire core. A rotating magnet generator with laminated pole pieces is employed, this type of generator having a particularly smooth and easy action. A light-duty generator is normally fitted but a heavy-duty version is available to meet requirements for additional ringing power. A change-over contact operated by the rotation of the generator handle disconnects the bell during ringing out. When wired to the standard circuit, shown
to the right, the instruments are not fitted with capacitors in the receiver or bell circuits; however, on certain party or omnibus lines it is desirable to introduce a capacitor to guard against shunting
effects caused by failure to restore handsets at the end of conversation. Also, some magneto systems have auto clear facilities which necessitate a capacitor in the bell circuit to break the d.c. loop during conversation. Alternative models are available, in both wall and table patterns, to meet these requirements, one with a 0.5
uF capacitor in the receiver circuit and another with a 2.0 uF capacitor in the bell circuit. The bell normally fitted is the
B.P.O. No.59A type with two 500 ohm coils, but higher resistance coils can be fitted if required. Cradleswitch operation is effected by removal of the handset from the rest, the contacts being actuated through a virtually frictionless and dustproof mechanism of simple design.
The instruments are designed to permit plan number types to be provided in addition to direct exchange line telephones. Plan number instruments are fitted with buttons, key springsets, terminals and cords in accordance with the needs of each particular installation.
The picture to the right shows the induction coil circuit in schematic form. It will be seen to have the character of a bridge in which the line forms arm A. The line impedance is matched, under average conditions, by the network making up arm B, which contains resistive and capacitative elements. During reception the speech current passes from A through arm C of the bridge and the receiver R; at the same time an antiphase current is induced in arm D. This current passes through arm B and aids the current in the receiver, as shown by the dotted arrows in the figure. During transmission, current is induced in coils C and D from the microphone winding 1-2. In this case the currents are in opposition in the receiver, thus producing an anti-sidetone effect, but are additive in the line, as shown by the solid arrows. Since the bridge is designed to operate over a wide range of line lengths, exact balance is not obtained and some sidetone is always present, but the level does not exceed the value usually considered desirable to denote a “live“ telephone.
The transmission performance of the new instruments is significantly better than that of previous magneto local battery telephones. Tests made with the standard circuit, gave results 1.5 dB better than SFERT for sending and 3.0 dB better than SFERT for reception. This represents an improvement over the N.2l21 type telephone, formerly regarded as the standard table set, of 2.0 and 3.0 dB for sending and transmission respectively. Figures for the alternative instruments with capacitors would not be appreciably different. The bridging loss for the new type is 0.4 dB less than for the older instrument.
The improved performance and appearance of the N.2l24 and N.2206 telephones, together with the choice of materials and techniques used in their manufacture make them suitable for use in modern installations where circumstances of the kind previously indicated demand the use of a magneto telephone system.
The new instruments available are as follows:-
Magneto telephones N.2124, table type, or N.2206, wall type:-
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