GOWER-BELL TELEPHONE
How the telephone works

Taken from an article about the Gower-Bell telephone, but the same system worked with the Crossley (Blakey & Emmott) set and the GPO Telephones No. 17 and 19.

This is just an overview and does not give full details of how the exchange worked.  These ttelephones worked on the Permanent Current Exchanges that were in general use by the Post Office from 1880 until replaced by the more advanced Magneto, C.B. and CBS systems.  All these exchanges were manual.

Click here for a general diagram of the permanent current system

Click here the Permanent Current System

Introduction
The system adopted by the Post Office was designed to work on the automatic call and ring off principle.  But originally most telephones used a press button was used to call the exchange and this disconnected the telephone battery, thus dropping the indicator at the exchange.

The telephone is connected so that when on hook a current is sent to the exchange.  This current passes through an indicator bridged permanently across the line, so arranged that the current through it's electromagnet holds up an iron shutter.  When the current is stopped the shutter falls, calling the operators attention to the circuit.  A small magnetised needle is also added and this indicates when a current is flowing through the electromagnet.  This indicates the ring off or finished signal.  The placing of the telephone receiver/receivers on rest automatically restores the permanent current.

The customers apparatus contains an adjustable relay and the permanent current passes through this relay which is biased against it.  The ring is thus produced by augmenting the current through the relay by joining the telephone battery with the exchange battery, thus operating the relay.  If the telephone was only a short distance from the exchange, no relay would be used, with the bell directly connected in circuit.

Gower-Bell circuit explained
It can be seen, from the diagram below, that the battery employed consists of four cells, two of which are used for the microphone and two for the local bell.  The permanent current, usually about 7 milliamperes, is sent out by the four cells.  The resistance of the exchange indicator is 1000ohms, hence for reasonable distances the resistance of the line is of little account.  The current flows from the positive pole of the battery to terminal 1, through the relay and the right hand switch hook spring, through the back contact of the press button to the A line, through the exchange indicator along the B line to terminal 5 and back to the negative pole of the battery.

The removal of the receiver from the switch hook stops the permanent current, thus dropping the exchange indicator.  This arrangement was known as Automatic Signalling.

The relay within the telephone is biased against the permanent current passing through it and in order to effect a ring, the exchange clerk joins a battery to the line, which combines with the subscribers battery, thus overcoming the bias upon the relay and closing the local circuit.  The path of this local circuit is from the the split of the battery to terminal 6, thence through the bell to terminal 8, to the contact screw of the relay and then via the other contact screw back to the negative pole of the battery.

The Gower-Bell telephone circuit is shown below, with the bell shown to the right at BE.  The relay is in the centre of the diagram.

Last revised: February 25, 2024