LOCAL BATTERY (LB)
|All the original telephones were powered by a local battery. This was because the
transmitter needed power and the local battery provided the right power irrespective of
the length of line or bad external wiring. But as the batteries were installed in
customers premises this was a major expense for the maintenance team and many of
the original batteries
were wet cells which could leak. These were were latter superseded by dry
cells, but the expense of replacement mounted as the telephone population rose.
The LB system was superseded by the Central Battery system which used a large set of batteries at the local exchange and this became possible because of savings made by economies of scale.
Local Battery Telephones for Use on Long Lines
The transmission efficiency of a local battery telephone connected by a line of high resistance to an exchange is greater than that of a Central Battery telephone. since in the latter case the amount of current supplied to the transmitter is very small on account of the high resistance. Accordingly, it is the practice to employ a local battery instrument when the requisite transmission cannot be given by a Central Battery telephone. The instrument used is a Telephone No. 154 in conjunction with a Bell Set No. 21. The telephone is of the table type; it is fitted on a shelf in lieu of a wall instrument, should this arrangement be preferred. A special three-winding induction coil (Coil Induction No. 15) is used, and is accommodated in the bell set. The primary winding is wound with 400 turns of No. 23 S.W.G. wire to a resistance of about 1 ohm; the secondary coil has 1,000 turns of No. 32 S.W.G. wire giving a resistance of about 19 ohms, while the tertiary winding consists of 1,500 turns of No. 32 S.W.G. wire to a resistance of about 38 ohms. It will be seen that, when the receiver is removed from the rest, the three coils are connected in series, i.e. on the auto-transformer principle; the primary is in the local circuit of the transmitter, the secondary is connected via the 2 F condenser and the gravity switch to the receiver, while the tertiary, in series with the other two windings, is connected across the lines. In speaking, the fluctuations of current in the ordinary winding cause alternating current to be generated in the whole of the transformer, whence they pass to line. Incoming speech currents pass through the transformer, and the current induced in the secondary winding passes via the condenser, the receiver, and the gravity switch. Incoming ringing currents pass through the condenser and magneto bell, so giving the calling signal. The induction coil has been designed as a result of careful experiments to determine the optimum turns required for each of the windings. The amount of direct current passing through the of the lines will not interfere with the efficiency of the instrument. This telephone can also be used on Central Battery Signalling Nos. 2 and 3 systems and, by interposing a generator between the instrument and the line, on magneto systems.
Previously, a Telephone No. 4 and a Bell Set No. 15 were used for local battery telephones connected to Central Battery exchanges. In these conditions it will show that the direct current from the central battery flows through the receiver during the whole of the time that the receiver is off the rest. This is a condition which also occurs during the transition ease when a local battery system is being converted to Central Battery working. Since the reversal of the current flowing through the receiver coils results in a magnetic field being set up in opposition to the main magnetic field of the permanent magnet, there will be a considerable loss in efficiency under these conditions.
Last revised: November 03, 2020