BPO Dials, Automatic, No's 1 - 9

Lettering on dials

Click here for an explanation of how dials work

Information on later dials - Dial No's 10 to 54.

Some of the text below is taken from:-

ENGINEERING INSTRUCTION, TELEPHONES, Automatic, B1003, Issue 1 & Issue 2, Oct 1955 & May 1965.
P.O. Engineering Dept. Technical Instructions XXV. Part 24 - Description and Adjustment of Dial Mechanisms, May 1928.


BPO early standard dials

Detailed descriptions of the BPO’s standard dials can be found in Herbert & Procter’s Telephony and in Atkinson’s Telephony. Here is a summary:-

  • No. 1               Automatic Electric, used at Epsom and the Official Switch (London).
  • No. 2               Variant on No. 1 used only on the test desk at Epsom.  Obsolete by 1928.
  • No. 3               Siemens Bros, used at Grimsby, Stockport and Southampton.
  • No. 4               Never issued.
  • No. 5               Western Electric, used at Dudley and Darlington.
  • No. 6               Redesigned version of No. 5 and also used at Dudley.
  • No. 7               Similar to No. 3, modified to give an impulse ratio suitable for A.T.M. systems.
  • No. 8               Prototype of  Dial No 10 - standard design.  Four terminals, small dial label.
  • No. 9               As No. 8 but with an extra contact - prototype of Dial No. 11.

The dials below are covered in more detail in another document - click on the links.

  • No. 10             First standard type. Five terminals, slipping cam mechanism.
  • No. 11             As No. 10 but with extra contact for payphones.
  • No. 12             Modification of Type No. 10, trigger type mechanism.
  • No. 13             As No. 12 but with extra contact for payphones.
  • No. 14             As No. 12 but intended for use in exchanges.
  • No. 15             Used on testers, has additional break contact.
  • No. 17             Small figure dial for use on telephone No 280.
  • No. 19             Telex dial, has changeover pulsing contacts.
  • No. 20             As No. 13 but allows free 1-level calls as well.
  • No. 21             Redesigned dials for 700-type telephones.
  • No. 22             Version for prepayment payphones, non-director.
  • No. 23             Ditto, director areas.
  • No. 24FA        Ditto,  with changeover contacts for telex use.
  • No. 25             Used on testers.
  • No. 26             Used on Flameproof telephones.
  • No. 28FA        These can dial the digit 9 only; used for emergency telephones intended to call 999 only.
  • No. 30             Similar to Dial No. 21 but redesigned for Trimphones, fitted with a betalight.
  • No. 51             Standard dial of GEC design (alternative to dial No. 21).
  • No. 52             Alternative to dial No. 21.
  • No. 53             STC design (alternative to dial No. 30 used on telephone No. 722).
  • No. 54A          Standard dial of EMI design.

Other Dial Information
The GPO Rate Book number of each dial is followed by characters which indicate the type of number plate/ring and pulse wheel fitted.  The first letter indicates the number plate, namely:-

B suffix (e.g. 10BA) denotes special Brighton area dial (see below).

F suffix (e.g. 21F) denotes figures-only (FS - Figures with special impulse wheel for Siemens exchanges at Grimsby, Stockport, Southampton, Hurley and Ramsey.  FW for Western Electric Rotary exchanges (S.T. & C. Co) at Dudley and Darlington).

L suffix (e.g. 10L) denotes letters as well as figures.  Lettered dials were used in and around Director areas (where letter dialling codes were used)  and were introduced nationally in preparation for the introduction of Subscriber Trunk Dialling (STD) in 1958.  With the abolition of letters in dialling codes number-only dials were fitted to newly issued telephones, starting  1969.  Letter dials were also used on London Transport’s private internal system, which retained letter codes for some time after they were abandoned by the BPO.

C suffix has neither letters nor figures.

Label No. 155
Label No. 149
Label No. 150

Brighton Dial

The second letter and any following number refer to the type of pulse wheel, shown below. 

  • Type A = 66.6/33.3 - used on standard GPO Strowger exchanges.
  • Type S = 33.3% break for Grimsby, Stockport, Southampton, Hurley and Ramsey (Siemens Bros.)
  • Type W = 18.5% break plus final break of not less than 92.5% of the total impulse period, for Dudley and Darlington (STC Rotary).

 Click here for more information on the Brighton Dial

Early British dials

Prior to the introduction of the BPO No. 8 and 10 dials, each supplier had its own pattern and these were given numbers by the BPO as follows:-

  • No. 1               Automatic Electric, used at Epsom and the Official Switch (London).
  • No. 2               Variant on No. 1 used only on the test desk at Epsom.
  • No. 3               Siemens Bros, used at Grimsby, Hurley, Ramsey, Stockport and Southampton.
  • No. 4               Never issued.
  • No. 5               Western Electric (S.T. & C. Co), used at Dudley and Darlington.
  • No. 6               Redesigned version of No. 5.
  • No. 7               Flameproof version of No. 3.
  • No. 8               Prototype of the Dial No. 10.
  • No. 9               Prototype of the Dial No. 10 when used on coin boxes.

History of early dial progression (by Jack Ryan)
In 1920 there were several trial exchanges operating on the public telephone network.  Each of these exchanges made use of proprietary subscriber equipment which had been accepted for use by the GPO.

In November, 1922, the Engineer-in-Chief of the Post Office, Colonel Purves, recommended the adoption of the Step-by-Step system with the addition of the director for use by the Post Office.

Once adopted, this recommendation would standardise all switching equipment on the public network.  The requirement of the GPO for standard interchangeable and interworking equipment was now clearly documented.

By this time the Standard Telephone was fairly well established but each exchange type used its own dial.

  1. Dial No 1 - introduced by ATM in 1912.
  2. Dial No 3 - introduced by Siemens Brothers in 1916.
  3. Dials No 5 & 6 - introduced by Western Electric from 1914.

The GPO required a Standard Dial for logistical if for no other reason.  None of the existing dials could be made to emulate the operation of all the others so a new Standard Dial was required.

This Standard Dial was to supersede the four dials listed above and to achieve that, the following was required:-

  • It had to conform to the established mounting (ATM), form (Siemens Brothers) and operation (all).
  • The pulse cam had to be the low speed (approx. 1 revolution per second) type with interrupter teeth around its circumference to give the maximum flexibility in timing.
  • The pulse cam had to be interchangeable to support ATM (‘A’ cam), Western Electric (‘W’ cam) and Siemens Brothers (‘S’ cam) exchanges.
  • The inter-digit period was to be adjustable to take advantage of the shorter setup time between digits required on Western Electric exchanges (the finger stop location was to be adjustable).

The first Standard Dial, the Dial No. 8, was released circa. 1921 on the Telephones No's 105 and 124.  The Telephone No. 124 superseded the Telephone No. 72 (ATM), the Telephone No. 76 (BWE), the Telephone No. 82 (Siemens Brothers) and others so that all manufacturers could now use the same pattern telephone and dial.  It featured changeable pulse cams and an adjustable finger stop location.

Technical issues necessitated another change to the Standard Dial.  To support telephone circuit changes designed to address problems of contact arcing, coherence and acoustic shock, the Dial No 10 was released.  The only change from the Dial No. 8 to the Dial No. 10 was the number and configuration of the off normal contacts.

The Telephone No. 150, fitted with Dial No. 10, was released around 1929.

With the introduction of the No. 10, most suppliers adopted this for telephones supplied to all customers, not just the BPO.  ATM, however, favoured the Type 24 dial designed by Automatic Electric Inc of Chicago (introduced 1926) and supplied this to Hull Corporation and other private customers well into the 1960s.  A variant of this was the ‘dimple dial’, using the same mechanism but a plastic finger wheel having moulded dimples instead of proper finger holes.  These ATM dials were never adopted by the BPO, although many type 24 dials, taken from Hull Corporation Telephones 162, were ‘brassed up’ in the 1970's and fitted to otherwise genuine BPO Telephone 150's.

The stainless steel fingerplates were brought out around 1936 (first GPO picture showing a stainless dial is dated 1936).  The stainless steel finger wheel also replaced the Bakelite finger wheel on the coloured Telephones 162.  The Bakelite finger wheel was just not durable enough for every day use.

The first Dial No.10 had oxidised finger plates matching the switch hook lever.  The oxidised finger wheel was heavy, made of brass which was copper plated and oxidised black.  The dial label centre was small.  The finger plates were later just painted black until the introduction of the chrome finger plate.


Dial No. 1
The first dial used in this country (known as Dial, Automatic, No. 1) is shown below.  This was used on the ATM Co's Auto switching system.  A minimum pause is not provided in this dial. The impulse springs normally rest in contact due to their own resilience instead of being pressed into contact by an impulse wheel.  Impulsing is effected by causing the wings of a fibre butterfly cam to pass between the springs, so separating them.  When the wing of the cam leaves the springs they fall together to form the “make” portion of the impulse.  A ratchet, which can be seen in the side view, prevents the rotation of the cam while the finger plate is being pulled.  The return motion of the dial is effected by a spiral spring (i.e., not a clock spring). It will be seen from fig. 31 that the method of operating the switching spring combination is not so robust as that of Dial, Automatic, No. 10 and it was the undesirability of operating more than one spring set in this way which precluded the use of an instrument circuit such as the present standard (Telephone No. 150).  This dial was used on Telephone No. 55 and 72.  Superseded by the Dial No. 8FA.

Adjustment instructions for these dials are contained in circular A.T. 6 dated July, 1919.

Front Rear


Dial No. 3
The Dial, Automatic, No. 3, which provides minimum pause and which was used with Messrs. Siemens earlier equipments at Grimsby, Stockport and Southampton.  This dial resembles the No. 1 dial in that a fibre cam is used to interrupt the impulse circuit and also in the method of operation of the switching springs.  Used on Telephones No's 77 or 82.  Superseded by the Dial No. 8FS.

Adjustment instructions for these dials are contained in a mimeographed circular dated March, 1924.

Click here for the Canadian Patent No. 229759

Front Rear


Dials No's 5 & 6
Dial, Automatic, No. 6 used in association with the S. T. & C. Company’s rotary system at Dudley and Darlington.  The mechanism is totally enclosed.  Externally the dial is unique in that the number ring rotates with the finger plate.  This dial is similar to No. 5 which it superseded.

Dial No. 6 was used on Telephones No's 65 (Mark 235) and 76 (Mark 235).  Superseded by Dial No. 8FW.

Dial No. 5


Dial 5 - Front view Dial 5 - Rear view

Dial No. 6 Front Dial No. 6 Rear


Dial No. 7
Similar to the No. 3 dial but was modified to give an impulse ratio suitable for the A.T.M. Company’s system, and was enclosed to exclude gas.  It was intended for use in mines, or where petrol was stored.  Obsolete by 1928.

Dial No's 8 & 9

Dial No. 8
The Dial, Automatic, No. 8 is the prototype of Dial, Automatic, No. 10 and its construction is essentially the same.  The necessity for the introduction of the No. 10 dial arose when an improved telephone instrument circuit was developed (see Technical Instruction XXV, Part 3).  This circuit, which is the present standard, involves the use of two pairs of off normal springs instead of the single change over which may be seen in the illustration of the No. 8 dial shown below.  The No. 8 dial is easily identified from a Dial No. 10 as it only has four terminals on the springset.

This dial has a British Patent in the name of Siemens Brothers - PAT 178936 - 1921.
Ericsson made some of these dials - their N4380 was the Dial, Automatic, No. 8FA.
GEC also made these dials, which had the PAT178936 stamped on the finger stop or finger plate.  Australian variants had the Australian Patent number on them.
GEC Dials
C104A - UK variant - Chassis, fingerplate, number holder and finger stop all painted black.
C104C - Australian variant - Chassis, fingerplate, number holder and finger stop all painted black.
C104G - UK variant - Black chassis with fingerplate, number holder and finger stop in Stainless.
C106A - NZ variant - Chassis, fingerplate, number holder and finger stop all painted black.
C106B - NZ variant - Black chassis with fingerplate, number holder and finger stop in Stainless.

The adjustments of Dial, Automatic, No. 10 and the instructions regarding the replacement of parts are applicable to No. 8 except in respect of the switching springs for which the following requirements are specified.  The three switching springs must never be simultaneously in contact.  The contact opening must not he less than 10 mils and the pressure exerted by the springs must be sufficient to ensure a distinct follow on either side.

Click here for Slipping Cam Dial adjustment information - Dial No's 10 & 11

Dial No. 8 was available in FA, FS and FW variants.

The Dial No. 8 uses a Label No. 138B as the paper dial label.  This paper disc is 111/64 inches in diameter.

Dial No. 9

Until the introduction of the No. 10 dial the telephone used with pre-payment type coin collecting boxes was equipped with Dial, Auto., No. 9FA (Telephone No. 119).  This dial was the immediate forerunner of the Dial, Automatic, No. 11.  Dial, Automatic, No. 9 differs from No. 11 in those particulars which distinguish the No. 8 dial from No. 10 and also differs slightly in the design of the auxiliary cam springs and impulse control cam.

The Dial No. 9 uses a Label No. 138B as the paper dial label.  This paper disc is 111/64 inches in diameter.

Dial No. 9 was supplied in variants FA and FS.

The adjustments specified for Dial, Automatic, No. 9 are the same as those for No. 8, and in addition the same specification is laid down for the auxiliary springs as for those on No. 11.

Should it be necessary to replace the main spring, remove the securing ring, celluloid label protector, and instruction card from the finger plate and loosen the screw in the gear wheel to release the tension in the main spring.  In addition the finger stop, finger plate, securing ring for number ring and number ring should be removed.  The following parts should then be removed from the back of time dial in the order named:-

  1. Spring set assembly.
  2. Screw for securing auxiliary impulse control cam.
  3. Auxiliary impulse control cam.
  4. Spindle for auxiliary impulse control cam.
  5. Switching lever.
  6. Spring washer.
  7. Slipping cam.
  8. Bush.
  9. Bracket with auxiliary spring set attached.
  10. Impulse wheel.
  11. Spring box with spring.

Dial No. 8 rear view


Parts for Dials No's 8 & 9


GEC Dial No. 10 broken down


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Last revised: July 13, 2020