BELLSET No. 20
This
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. 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 2 F condenser, and a 1 F 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 F 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 F 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
Superseded by Bellset No. 39
Diagram N520 - click here for the circuit diagram - Web
users - CD users
Diagram for 1 extension working (Plan 7) - N4310
Click here for the circuit diagram - Web
users - CD users.
Diagram for 2 extension working (Plan 5) - N4308
Click here for the circuit diagram - Web
users - CD users.
Adjustment of Magneto bells
Click here for the difference between a Bell and a
Bellset
Last revised: June 22, 2007
FM