GOWER-BELL TELEPHONE
The Permanent Current Exchange

This is just an overview and does not give full details of how the exchange worked. These permanent current exchanges 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.

The following telephones would be used on this system:-
Gower-Bell telephone, Crossley (Blakey & Emmott) and Telephones No. 17 and 19. These were all universal telephones and used in most cases.

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

The telephone is connected so that when on hook a current can flow to the exchange. This current passes through an indicator that is permanently across the line, so arranged that the current through it's electromagnet holds up an iron shutter. When the telephones goes off hook, the current is stopped and the indicator 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.

Click here for a general diagram of the permanent current system

Gower Bell telephones

Gower-Bell telephone circuit explained
It can be seen, from the diagram below, that the Daniell 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 sourced by all 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 or Gower tube 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.

Subscriber line operation
Each subscribers line circuit has a jack comprising of two springs, one on the A wire and one on the B wire. They are normally in contact with each other and the circuit is thus made for the subscribers indicator to be connected across the line. The subscribers battery current deflects the indicator to the left, which visually indicates that the line is available for use. In the diagram below, Subscriber 2 is shown in this condition. The GPO normally used 2 wires but if single wire working was in use then the B wire would be connected to earth.

When the telephone goes off hook the permanent current from the battery is disconnected and the indicator at the exchange drops to a vertical position indicating the telephone is off hook. The operator plugs a 2 wire cord into the subscribers jack, which parts the jack contacts and connects the plug to the two contacts. The indicator remains in the vertical position and the operator ascertains what number is required.

The operator then plugs into the required numbers jack. These plugs have a projection on one side that locates in the jack, ensuring that they can only be placed the correct way up. The calling indication from the exchange is effected by means of a calling battery, of sufficient electromotive force to send a current of 40 milli-amperes through the longest line on the exchange system.

The ''permanent current" passes through the coils of a relay at the subscriber's telephone, and this relay, which is for actuating the subscriber's call-bell, is ''biased" against the permanent current (about 20 milliamperes); but when the permanent current is strengthened by the calling current acting in the same direction, the bias of the relay is overcome, and the call is effected. The relay has a contact that operates a direct current bell by means of two cells of the subscribers battery

As the permanent currents from all lines flow in the same direction, if two communicating lines were joined straight through, "A line to "A" line and "B" line to "B" line, the two currents would neutralise each other.

In order that this may not be so the switch-plug cords are crossed, that wire in the cord which goes to the upper side of one plug being connected to the lower side of the other plug. This is shown in the figure (Nos. 3 and 4). By this means, when, at the end of a communication, the two corresponding subscribers replace their hearing tubes, the currents on either side of the exchange combine, and so, by deflecting the magnetic needles of the two indicators at the exchange, give notice to the exchange clerk that the conversation is at an end. The plugs are then removed, and the indicators show the lines in their normal condition.

Subscribers 3 and 4 are shown connected to each other and the indicator is vertical to show the lines are in use.

Beyond the primary advantages of the permanent current system, such as the automatic indication at the exchange of the conclusion of a conversation between two subscribers, an incidental advantage also arises in the fact that the permanent current acts as a test for the line, and at once indicates when a disconnection, a contact, or any other fault comes on. Further, it shows when a subscriber omits to replace the tubes in the switch levers, and so prevents waste of time in calling.

In order to gain the attention of any subscriber who has inadvertently left off the tubes, a "buzzer," or ''howler," is placed in the circuit of the calling battery at the exchange. This is an induction coil which sends rapid intermittent currents to the subscriber's line, which, acting upon his telephone receiver, make a loud buzzing sound and so attracts their attention.

Switchboard operation
The general arrangement of a medium-sized exchange (from 100 to 300 subscribers) is shown in the picture below.

Each switchboard of one hundred jacks is placed in charge of two clerks, and the corresponding indicators are fixed upon tablets in two groups, one on each side of the switchboard. Each switch-clerk is provided with a telephone by which to communicate with subscribers, and there are other telephones which are used for sending and receiving telegraphic messages. These telephones are joined up to a "speaking tablet" fixed beneath each switchboard jack field, with two connection-screws for each instrument. To these two screws a flexible cord is attached; this is taken down through the shelves, of the switch-frame, round a light pulley, back through the shelf, and then connected to a switch-plug. Each of these speaking telephones can thus be placed in communication with any one of the subscribers on the switchboard, and when not in use the cords of the speaking plugs are drawn beneath the shelf by the pulleys, and so kept clear of the switchboard.

Where more than one hundred subscribers are connected to an exchange, provision is made for the communication of subscribers fixed on different switchboards by means of "through tablets" fixed above the switchboard. These consist of jacks similar to those of an ordinary switchboard, connected in sets: that is, (say) A, B, C of No, 1 switch are connected respectively to A, B, C of No. 2 switch; D, E, F of No. 1 to D, E, F of No. 3; and G, H, I of No. 2 to G, H, I of No. 3. Thus, if subscriber 58 wishes to speak to subscriber 223 the switch-clerk of No. 1 switch places a pair of plugs in holes 58 and (say) E, and instructs the switch-clerk of No. 3 switch to similarly connect E and 223.

Permanent Current System
Switchboards

Finding pictures and information on the Permanent Current systems is quite hard and the pictures below were taken from the BT archive, but they were not that well described. The descriptions put against them are as accurate as possible.

What must be remembered is that these are the early days of using a telephone as a form of communication and the people who may use a telephone were already in communication with others by telegram or their personal use of an ABC instrument. There were other telephone companies operating as well and these, especially the National Telephone Company (NTC), had connections to the GPO exchanges, so that long distance calls could be progressed.

The lines between the Post Office and the NTC exchanges were called "Junctions" whereas the lines between Post Office exchanges were called "Trunks".

The system also used electromagnetic indicators as miniature lamps had not been invented and Galvanometers were two large to fit in the switchboards. The indicators were of different types depending on how they were used.

Another interesting point is that the UK tried some horizontal or table jack panels with a gantry above the panels which housed cords and indicators. As these jacks could collect dust very easily a suction pump was used to clean them, but after a short period they went out of favour and vertical mounted jacks were used due to ease of maintenance and access.

THE "A" SWITCH SECTION

These are used in very small offices where there are less than 3 trunk lines, a section of "A" type is used.

Upon an "A" switch section three pairs of pegs, with their corresponding three speaking keys, six ringing keys, and ring-off indicators, are fitted. The ring-off indicators are placed at the top of the section, the self-restoring indicators being placed uppermost. Beneath the exchange galvanometers come the polarised indicator relays corresponding to the trunks, the corresponding switch-springs for which are at the bottom of the board. Next to the trunk relays comes a space for ten polarised drop indicators of the form previously described. The lines from the subscribers are connected to the outer springs, whilst the inner springs are joined to the indicator. The shutter is held up by the permanent current, which also deflects the needle to the right. When the permanent current is stopped by the removal of the receivers, the shutter drops, and calls the attention of the operator.

There are three self-restoring indicators attached to the three circuits between the switch and the National Company's Exchange.

'A' Section Switchboards

THE "B" SWITCH SECTION

Where there are from three to five trunks, one B switch section is used. The "B" section is of exactly the same size and shape as the "A" section previously described; but a strip of five five-point switch-springs is added to accommodate the additional junctions. In addition to this six pairs of cords, together with the necessary ringing and speaking keys, are furnished. In certain cases two visual indicators are added; but this is a matter which will be dealt with later.

The junctions all terminate upon five-point switch springs, and no indicators whatever are fitted upon them. The local contacts are precisely as described in the case of the "A" section.

The arrangements as show in the figure below, save that the service wire to the Company terminates in one of the self-restoring indicators. In the event of anything going wrong a junction may be used by leaving a peg in at either end, and thus leaving the ring-off indicators in circuit.

'B' Section Switchboards

THE "C" SWITCH SECTION

At all the larger offices switch sections of the "C" type are fitted. Where it is thought that the number of switch sections is likely to grow to four, i.e., sixteen or more trunks, it is not desirable to fix "B" sections, as to change a "B" into a "C" section is somewhat difficult and expensive. To take a case in point, a certain office at the time of the transfer had only some twelve or thirteen trunks ; but here "C" sections were fitted, and the wisdom of this policy is shewn by the fact that an additional section has recently been fixed. In another case the number of trunks was about the same, but their growth was so improbable that "B" sections were fitted. Thus it will be seen that fixed and immutable rules are not wise, and that each case should be taken upon its merits.

The chief point of difference between a "B" section and a "C" section lies in the transfer circuit provision ; indicators and switch-springs, for ten transfer circuits being fitted instead of two, as in the "B" sections. These are fitted in four strips. The top strip consists of five visuals, underneath which is placed the strip of five switch-springs corresponding to them ; then comes the second five visuals and switch-springs.

Two transfer circuits are provided from Section 1 to Section 3 and two from 1 to 4. Section 2 can reach all save Section 4, between which two circuits are provided.

'C' Section Switchboards

LINE TRANSFER BOARD

It will be quite obvious that where a large number of trunk switch sections have to be provided with transfer circuits, direct circuits from each section to every other section within the exchange would be utterly unfeasible. Where it is possible to adopt this system it is obviously desirable to do so, as it saves switching operations, which, however smartly performed, occupy time. It is somewhat difficult to say how many sections may be dealt with in this way, as conditions vary so much. For example, an office may have forty trunks, twelve of which go to one town. Again, by carefully considering the arrangement of the trunks upon the sections, the transfer work may be reduced to a minimum. Generally we may say that where the number of sections exceeds six or eight a transfer board will be essential.

The transfer board is the connecting link between the various sections. There are circuits from each section in the room to the transfer board, and means for connecting them together in any prescribed manner are provided. In order to render the working as rapid as possible the circuits between the sections and the transfer board are divided into two groups, known as A and B circuits. The A circuits are those upon which the section operator passes forward a call to the transfer board operator. The B circuits are those upon which the latter again passes forward the call to the required switch section. The A circuits then are used outwards from and the B circuits inwards to the sections. To each section there are allotted three A circuits and two B circuits i.e., the section operator can pass forward three calls and receive two simultaneously if necessary.

The A circuits are those upon which calls come inwards to the transfer board, and these are placed upon the slanting desk (Figure 93). The visuals in connection with these circuits are at the top of the slanting desk, and immediately below them come the combination keys, the object of which is to enable the transfer operator to speak and signal upon any circuit. The loop of each circuit terminates in a peg, and upon this circuit there is absolutely no apparatus. These pegs are to be seen above the desk. Each A circuit is provided with a peg, a visual indicator, and a combination key. It will be noticed that the board shown is only half fitted; but, it is, of course, a very simple matter to add more apparatus when required.