BELLSET No. 20
This BPO wooden bell set was originally introduced to replace Bell Set No. 4 on Plan No's 5, 5A, 7 and 7A installations connected to automatic exchanges.  The Bellset is shown to the right and the picture is dated 1932.  As it was found to give better transmission conditions than Bell Set No. 4, its use was extended to Central Battery manual circuits.  The Bell Set consists of a 4-position switch, an indicator-relay (Relay 257A of 50 ohms resistance), a generator No. 4C, a Bell No. 1A, a 2F condenser, and a 1 mF condenser.  A two-cell battery is provided to furnish speaking current on main to extension calls.  The connections of the telephones resulting in the four positions of the switch are similar to those of the Bell Set No. 4.  The internal connections of the Bell Set No. 20 are shown in diagram N520.

It will be seen that the indicator relay is shunted by a 2 mF condenser instead of by a non-inductive resistance as in the Bell Set No. 4.  Also the magneto bell of the main telephone is disconnected by the contact of the indicator relay when the switch is in the through position.  By these means, a circuit clear of any bridged apparatus is provided, with the result that, under automatic conditions, there is no distortion of the dialled impulses on account of the introduction of the bell set.

The generator used at the extension instrument is connected in series with a 2 mF condenser, this combination being known as Generator No. 4CP.  The necessity for the condenser arises from the fact that should the generator handle be turned when the switch is in the through position, the indicator relay would be operated by the current flowing round the loop provided by the armature of the generator when brought into circuit by the operation of the cut-out spring, whereas it is necessary that this relay should be released in order to connect the magneto bell at the main for the reception of the ring.  The indicator-relay is of the pendant armature type, and is provided with a copper sleeve on the core.  The armature actuates a single change-over spring set and an indicator attachment, consisting of a balanced aluminium lever which moves an aluminium flag into position behind a rectangular window.

The indicator relay was designed by Messrs. Siemens Bros. to meet the following conditions:-

  • The relay contacts shall not break when an extension is being rung from the exchange over a line of zero loop resistance with maximum generator E.M.F. and lowest resistance ringing circuit in use.
  • The signal shall operate with the current over a line of maximum loop resistance with a minimum P.D. on the main exchange battery.
  • The relay having operated as in 2 above shall retain during dialling at minimum speed, i.e. the bell in the bell set shall not tinkle.
  • The relay shall release after operation, as in 2 or 3, on a line of 10 ohm loop resistance and with maximum P.D. on the main exchange battery.
  • The relay contacts shall not break when the extension station rings the main by hand generator in series with a condenser, the switch being in the through position.  Since the bell of the main station telephone is disconnected when the switch is in the through position, it is necessary that any extension bell required at the main station shall be connected in series with the magneto bell of the bell set.  Terminals are provided to enable this to be done.

Taken from "Telephony - a detailed exposition of the telephone system of the British Post Office - volume 1 - manual switching systems and line plant"
Herbert T E, Proctor W S, Pitman, 1934 (1946)
ISBN 0-9523654-1-3

Connecting to the UK telephone system

Superseded by the Bellset No. 39

Produced by GEC, their Catalogue List No. 871.

Diagram - N520.

Diagram for 1 extension working (Plan 7) - N4310

Diagram for 2 extension working (Plan 5) - N4308

Adjustment of Magneto bells

Click here for the difference between a Bell and a Bellset

Additional Pictures

Front of Bellset.  Note that everything is painted black
 
Internal view of a later variant.  Note the terminal strip at the bottom.
 
Rear View
 
Internal view showing an early Bellset No 20.  The terminals are all single terminations.
EB is where an extension bell would be wired.  Z (for Zinc) and C (for carbon) is where the battery would be connected.

 

 
 
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Last revised: December 29, 2018

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