BPO Dials, Automatic, No's 10 - 54


Trigger dial adjustment information - Dials No's 12 - 19

Slipping Cam dial adjustment information - Dial No's 10 & 11

An explanation of how dials work

Siemens Dial, Switch No. 18

Early GPO Dials, Automatic, No's 1 to 9

Dial number rings

Dial labels - paper labels


Text below is taken from the following documents and other sources

TELECOMMUNICATIONS INSTRUCTION, C MARKETING, INSTALLATION, 3 Internal, M0015, Issue 1, Sept 1971.
&
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.

 

DIALS, AUTOMATIC, No's 10 to 54

Scope
This page describes the Dials, Automatic, No. 10 - 54 and gives general information relating to their operation, use and maintenance.  Details of the dials are given in Table 1 .  Dials stocked by the Supplies Dept. and the installations at which they are used are shown in Table 2.  The number, date and manufacturer of the dial can generally be found on the underneath of the dial, on the rim.  See the Dial, Automatic, No. 10 - rear view as an example.

General
For a long time all the dials used by the British Post Office were of the slipping-cam type which were superseded by a trigger type.  However, a large number of the slipping cam dials are still in use and will remain so for a number of years.  Dials of the trigger type are fully interchangeable with their counterparts of the slipping-cam type.

Dial Identification
Like all GPO equipment dials can be identified by labelling which is located under the rim of the dial.  The dial will have to be removed from it's mounting to see this labelling.  The label will be in the format - dial number - manufacture - year.  e.g.21 EET 65/1 is a Dial No. 21 made by Ericsson in 1965.  Early Dial No. 10's may also have 236 as well but this is just a Mark number.

Dial terminal arrangements
The arrangement of the terminal connections on each dial is similar but there are slight differences in physical dimensions.  The electrical characteristics are identical. Diagrams N 581 to N 584, N 588, N 610, N 612 and N 613 show the terminal numbering and spring-set connections.  Dials, Automatic, No's 51 to 54 have differing spring and terminal layouts.  In these cases the terminal numbers are printed on the dial case or dust cover.

Terminal Connections

Dial No. 10, 12 & 21 Dial No. 22 Dial No. 23 Dial No. 24 Dial No. 28

TABLE 1

Dials, Automatic, No.Type of mechanism Finger plate Speed (P.P.S.) Ratio % break Remarks
10Slipping cam   Stainless-steel   10+/-1   63 - 70   Superseded by No. 12
11Slipping cam Stainless-steel 10+/-163 - 70 Superseded by No. 13
12Trigger   Stainless-steel   10+/-1   63 - 70   Superseded by No. 21
13Trigger   Stainless-steel   10+/-163 - 70   Superseded by No. 22
14Trigger Matt Black Enamel10+/-1 63 - 70    
14LA/2 V.F.TriggerMatt Black Enamel10+/-0.5 64.6 - 68.6Use on positions at exchanges with 2VF (Signalling)
15FT3Trigger   Stainless-steel   11.75+/-0.5   76 - 80   Superseded by No. 25LT3
15FT4Trigger   Stainless-steel   11.75+/-0.5   48 - 52   Superseded by No. 25LT4
16FATrigger   Stainless-steel   10+/-0.5   64.6 - 68.6   Superseded by No. 25LA
16FT3Trigger   Stainless-steel   11.75+/-0.5   76 - 80   Superseded by No. 25LT3
16FT4Trigger   Stainless-steel   11.75+/-0.5   48 - 52   Superseded by No. 25LT4
17LATrigger   Aluminium alloy   10+/-163 - 70 Used on Tele. No. 280
19FATrigger   Stainless-steel   10+/-1   See Note   Superseded by No. 24FA
20LATrigger   Stainless-steel   10+/-1   63 - 70   Superseded by No. 23LA
21CA Trigger P.V.C. (Colour) 10+/-1 63 - 70 
21CA Trigger   Stainless-steel   10+/-1   63 - 70 
21LA Trigger   P.V.C. Black /Grey 10+/-1   63 - 70 
21FA Trigger Clear plastic10+/-163 - 70 
21FA Steel TriggerStainless-steel 10+/-163 - 70Used in public coin boxes
1/21FATrigger Clear Plastic 10+/-163 - 70Used on Tele. 776
22FATrigger   Stainless-steel   10+/-1   63 - 70Coin box use
23LA   Trigger   Stainless-steel   10+/-1   63 - 70 
24FA   Trigger   Stainless-steel   10+/-1   See Note Telegraph use
25CATrigger   P.V.C., Grey   10+/-1   63 - 70 
25FT3 TriggerClear 11.75+/-0.5   76 - 80Used on testers
25FT4Trigger Clear11.75+/-0.5   48 - 52Used on testers
25LA Trigger   P.V.C., Black   10+/-0.5   64.6 - 68.6 Used on testers
25LT3 Trigger   P.V.C., Black   11.75+/-0.5   76 - 80Used on testers
25LT4 Trigger   P.V.C., Black   11.75+/-0.5   48 - 52Used on testers
26LA Slipping cam   Stainless-steel   10+/-1   63 - 70   Modified Dial, Automatic, No. 10 in flame-proof enclosure
27CA Trigger P.V.C. (Colour)     Unknown
28LA Trigger   Stainless-steel   10+/-1   63 - 70999 only
30FA Trigger   Clear   10+/-1   63 - 70   Fluorescent tube beneath number ring
35FA Trigger   Clear   10+/-0.5   64.6 - 68.6   Similar to No. 25FA but specially adjusted for testers
51Trigger   Clear   10+/-1   63 - 70   GEC proprietary design. Used in lieu of No.  21
52ATrigger   Clear   10+/-1   63 - 70   STC proprietary design. Used in lieu of No. 21
53ATrigger   Clear   10+/-1   63 - 70   STC proprietary design. Used in lieu of No. 30FA
54ATrigger   Clear   10+/-1   63 - 70   EMI proprietary design. Used in lieu of No. 21

NOTE: Change-over unit, nominal operate to normal ratio of 3:2 within the limits of 56-64% operate.

TABLE 2

Apparatus or equipment   Notes Dial, Automatic, No.
Subscribers' apparatus  700 type telephones   21CA, (Colour), 21F, 25CA, 28LA
  700 type post 196921FA, 28FA, 51 - 54 (clear)
  Pre-700 type telephones   10LA, 12LA, 28LA, 21LA, Black
  P.B.X. switchboards   21LA, Black
Coin-box installations  Pay-on-answer   21CA, Steel
  Pre-payment, director   23LA
  Pre-payment, non-director   22FA
Exchange manual switchboards   21LA, Black
Testers  50% break   25LT4, 25FT4
  66.6% break   25LA
  80% break   25LT3, 25FT3
Telegraphs  Telex installations   24FA
  T.A.S. installations   21LA, Black
Telephone No. 280 Test telephone17LA
Telephone No. 722Trimphone 30
Telephone No. 149  26LA
Telephone No. 776Compact 1/21FA Clear

The Rate Book number of each dial is followed by characters which indicate the type of number ring and pulse wheel fitted.  The first letter indicates the number ring, namely, "B" special dial for the Brighton area, "F" has figures only, "L" letters and figures, "C" has neither letters nor figures.

B F L

Number Ring
Early number rings were enamelled and were seated on a rubber ring (Part No. 1/DWA/77).  These enamelled number rings were later superseded by a plastic type, which was thicker and did not require the rubber ring. 

The table below gives more details of the Number Rings:-

  Dial, Auto, No. 10 Dials, Auto, No's 12, 13, 14, 15, 16, 19 and 20
Enamel type F Type Label No. 149 Label No. 304
L Type Label No. 150 Label No. 305
Plastic type F Type Label No. 349 Label No. 314
L Type Label No. 350 Label No. 315

Click here for more information on the Brighton Dial

The second letter and any following number refer to the type of pulse wheel, shown below.  Dials stocked by the Supplies Dept. and their use at various installations are shown in Table 2 .

  • Type A = 66.6/33.3 - used on normal GPO Strowger exchanges (Dial No. 10).
  • Type A1 = 66.6/33.3 - used on normal GPO Strowger exchanges (Dial No. 12 onwards).
  • Type S = 33.3% break for Grimsby, Stockport, Southampton, Hurley and Ramsey (Siemens Bros.).
  • Type T = T1 & T3 - 80% break and T2 & T4 - 50break (Used on Testers AT....).
  • 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).
A and A1 S W


 

Dial No. 10 (slipping-cam type)
Invented by Siemens Bothers, The Dial, Automatic, No. 10 consists of a circular case in the centre of which is a brass bush in which the main spindle and gear-wheel turn.  On the main spindle, at the rear of the dial, are mounted a main spring in a spring box, a pulse wheel, a slipping cam between washers, a spring washer and an off-normal lever.  The outer end of the main spring is anchored in the spring box, which is keyed to the spindle, and the inner end of the spring engages in a slot in the brass bush.  When the spindle is rotated clockwise, as viewed from the dial front, the spring is wound up.  The dial mechanism is held in its rest position against the tension of the main spring by a screw in the main gear-wheel engaging with a post in the case.  Removal of the screw allows the main spring to unwind.  Within the case are mounted a centrifugal governor and worm on a common spindle, and a gear-wheel which drives the worm.  A pinion on the gear-wheel shaft meshes with the main gear-wheel on the main spindle so that the governor spins rapidly when the mechanism runs.  The governor weights move outwards under centrifugal force and, at the required speed, bear on the inner surface of the governor cup to exert a braking influence on the spindle.  This keeps the speed of the mechanism within the required limits.

Original Dial No. 10 front view Dial No. 10 rear view

The governor gear-wheel assembly includes a simple 'free-wheel' device consisting of a coiled spring coupling which slips when the dial is wound up, so that the governor does not resist this motion.

The front of the case is enclosed by the number ring, which is held in the case by a spring ring.

The finger-plate assembly is attached to the main spindle boss which just protrudes through the number ring.  This assembly comprises the finger plate, label-holder, and a backing plate to give rigidity to the finger plate.

The pulsing mechanism consists of the pulse wheel and slipping cam on the main spindle and the pulse lever and spring-set mounted on the rear of the case.

The spring-set assembly also contains two auxiliary contacts which are normally held open by an insulating bush on the off-normal lever.  When the finger plate is moved from its normal position, these contacts make in sequence and re-open in the reverse order on its return.

This dial could also be obtained in FT1 and FT2 variants.  These were used on Testers AT... and adjusted for 12 pulses per second.  The impulse wheels were T1 and T2.



Early Dial, Automatic, No. 10 - (GEC picture taken around 1928)
 

.

The above picture depicts a Dial, Automatic, No. 10 with an Australian dial label

Operation of the slipping-cam mechanism
The pulse wheel is keyed to the main spindle and the slipping cam is frictionally coupled to the wheel by steel washers under the pressure of the phosphor-bronze spring washer.  Movement of the cam is restricted by its two lugs engaging a forked stop mounted on the dial case.

When a digit is selected, the pulse wheel and slipping cam rotate together at first and the raised sector of the cam moves under the set of the pulse lever.  The pulse contacts are thus kept closed.  Then, the longer lug of the slipping cam strikes the forked stop to prevent any further rotation of the cam.  The pulse wheel continues to rotate and the cut-away sectors are exposed from under the cam.  The number of sectors that appear is determined by the digit selected.

When the finger plate is released, the mechanism rotates back to its normal position with the governor controlling the speed.  During the first part of the return motion, the slipping cam and pulse wheel move together, so that the pulse lever slides over the cam.  Thus, the pulse contacts remain closed for 220 milli-seconds (at 10 p.p.s.) to give the minimum inter-digital pause.  This period ends when the slipping cam has rotated sufficiently for its raised sector to clear the set of the pulse lever, which then drops on to the lower sector to open the pulse contacts.  Further movement of the slipping cam is prevented by its shorter lug striking the forked stop.  As the pulse wheel continues to rotate, each 'tooth' raises the pulse lever and closes the contacts until the contacts are finally closed at the normal position of the dial.

Parts for Dials, Automatic, No's 10 & 11

GPO Variants
1928 Rate Book advises:-
The Dial, Automatic No. 10 was available in variants BA, FA, FS, FW, LA and a LA/2VF.  The LA/2VF was used on manual switchboards, always in black and adjusted to 9.5 - 10.5 i.p.s with a 65-67% break..

1946 Rate Book advises:-
The Dial, Automatic, No. 10 was available in variants BA, FA, FS, FW and LA.  The FA and LA variants were available with finger plates coloured Black, Ivory, Jade Green and Chinese red.

A Dial, Automatic, S.S., No. 10 was also made available around 1934 and this had a Stainless Steel finger plate.  This was supplied in variants BA, FA, FS, FW and LA.  This dial is fitted with the following Stainless Steel parts:- Parts Nos. 2/SCA/18, 2/SLA/34, 2/SPL/306, 2/SSP/487 and 2/SST/338.

Another Stainless Steel model was also offered, the Dial, Automatic, S.S., No. 10FT1 and No.10FT2.  These were for use on Testers, AT ...

  1. For "long line" testing. 80% break.  Adjusted to 12 i.p.s. and fitted with Wheel, Impulse, T1 (Part No. 1/SWH/27). Otherwise as S.S., No. 10FA.

  2. For "short line" testing. 50% break.  Adjusted to 12 i.p.s. and fitted with Wheel, Impulse, T2 (Part No. 1/SWH/26). Otherwise as S.S., No. 10FA.

1956 Rate Book advises:-
The Dial, Automatic, No. 10 was available in variants FA and LA in the colours Black, Ivory, Jade Green and Chinese red.
The Dial, Automatic, S.S., No 10 was also available.

Dial, Automatic, No. 10 Additional Information
Variations in Dial, Automatic, No. 10 - The first issue of Dial, Automatic, No. 10 dial had the same type of slipping cam as Dial, Automatic, No. 8, i.e., a cam without a long projection, and the dial was also provided with a finger stop having slotted fixing holes instead of round fixing holes.  The slotted fixing holes were provided to facilitate compliance with the condition prescribed in para. 9, viz., that when the finger plate is rotated so that the finger rests against the finger stop, a corner of an impulse wheel tooth appears from behind the slipping cam.  Since the correct travel of the slipping cam cannot be obtained by bending the long projection, it is obtained by adjustment of the position of the finger stop.

It follows from the above that if an old pattern cam or an old pattern finger stop requires replacement, both these parts must be substituted by the standard items.

The Dial, Automatic, No. 10 was drilled to take the finger stop in two positions.  Normally the finger stop is further (clockwise) around the dial circumference, so as to allow the standard interdigital pause (see picture below).

Early Dials, Automatic, No. 10 (Mark 234 and 235) also used the small paper dial label that was used on the Dial, Automatic, No. 8.  These were a Label No. 138A in Director Areas and a Label No. 138B in Non-Director areas.  Both these paper discs are 111/64 inches in diameter.  The Mark 236 on the other hand used the larger dial label called the Label No. 160.

Drawing 62888 (Mk 4A).

Other Manufacturers
Ericsson Telephones Ltd (ETL) was a producer of this dial and used them on telephones supplied for home and overseas markets.  They can be identified as they may be marked as follows:-
The dial is the N8381 and was produced in many variants, which can be identified by a lettered suffix.
No Suffix - 10FA Mark 235, with Phosphor Bronze finger plate - English.
A - Stainless Steel finger plate - Egypt.
B - 10LA Mark 235, with Phosphor Bronze finger plate - English.
C - 10FA with Stainless Steel finger plate - English.
D - Stainless Steel finger plate - New Zealand (0 to 9).
E - 10LA Mark 236A with Stainless Steel finger plate - English.
F - 10FA Mark 4A with Stainless Steel finger plate - English (Dial, Automatic, SS No. 10FA/4A).
H - 10LA Mark 4A with Stainless Steel finger plate - English (Dial, Automatic, SS No. 10LA/4A).
J - War time substitutes.
K - War time substitutes.
M - 10FA with Stainless Steel finger plate - Australia.
P - 10FA with moulded finger plate.
Q - 10LA with moulded finger plate - Egypt.
R - 10FA with moulded finger plate.
S - Stainless Steel finger plate - Chinese.
U - Moulded finger plate - Chinese.
W- 10LA with a moulded finger plate.

Click here for more ETL dial information

GEC Dials
Their No. 10 dial was coded the Dial No. 104G.  The number plate type had to be specified when ordering.

ATM produced this model of dial.  Models available:-
T4213 - British Post Office standard.  Brass base with two positioning and one tapped lug for mounting.  Black finish.  Black figures on white ground "1" to "0".   Instruction card protected by transparent disc.
T4214 - As above but numbered and lettered for use in Director areas.

Siemens Brothers made also produced the Dial, Automatic No. 10.  Models available:-
Dial, Switch No. 8 was their equivalent to the Dial, Automatic No. 10 (All black).
Dial, Switch No. 18FA/SS was their equivalent to the Dial, Automatic S.S. No. 10FA.
Dial, Switch No. 18LA/SS was their equivalent to the Dial, Automatic S.S. No. 10LA.

Laurence Rudolf writes:-
A publicity shot for the Telephone No. 162 and photo of the registered design shows this telephone was launched in October 1929 with a solid brass blackened finger wheel that had a small centre label holder.  However the pasted diagram shows that there are five contacts on the spring set.  As the Dial, Automatic, No. 8 only had four contacts on the spring set, it can be concluded that the Dial, Automatic, No. 8 was superseded by the Dial, Automatic, No. 10 in October 1929.

I uncovered an order at BT archives for 200,000 No. 8 dials. Then a GPO official had stamped on the order that it had been up-graded to the new No. 10 dial.

Following on from the above was a publicity advert that showed the new stainless steel dial with the introduction of Chinese red Telephone No. 162 in 1936.

When the GPO introduced coloured No. 162's in 1930 they also introduced coloured Bakelite finger wheels to match the coloured telephones. Siemens Brothers made a few coloured samples for the GPO to approve.  I have a Mahogany coloured No. 162 dated 1930 that has a mahogany finger wheel.  The GPO rejected the solid coloured Mahogany No. 162 in favour of a mottled walnut telephone No. 162.  This was produced with a matching Bakelite walnut brown finger wheel.

The coloured matching Bakelite finger wheels were produced in Walnut, Chinese Red, Jade Green and Ivory.  They were also produced in the painted lacquered colours of Old Gold and Oxidised Silver.  The Bakelite finger wheels were not strong enough for every day use and quickly discontinued after a couple of years.  As they are fragile only a few have survived and they are not a common item.  The finger stop was always painted gold and the middle of the finger stop was snapped off to accommodate the thicker Bakelite finger wheel.  The edge of the No. 10 dial was also painted gold.  The metal clip to hold the label cover was made of brass.
 

Andy Emmerson writes:-
Were other varieties ever available for the Dial No. 8 and No. 10 and how early with the Stainless Steel fingerplate?
There were Stainless Steel (SS) dials in the late 1920s (with black brass rear parts) made by GEC.  These dials were No. 10's.
Stainless dials - some marked "SS 10" to distinguish them from the black types (for part number reasons as the difference is blatantly obvious!).
It must be noted that for some time all GEC Dials No. 10 had black stove-enamelled brass rear parts.  Only the finger wheel was in stainless steel.

When was the word "operator" dropped?
I have put an "0 operator" number ring and small label on my dial marked C27 but my gut feeling says late 1930's.

Was the standard size label holder introduced with the Stainless Steel dial, or did it happen earlier?
Earlier, I would say.  I have an undated all-black No.10 with the full-size dial label.
Note that not all black Dials No. 10 were marked with the date of manufacture.  For example, dials made by Ericsson for fitting to telephones sold to private systems users carried only a patent number and the letter E.  I have also seen ones with a patent number only (these were fitted to telephones assembled for the Relay Automatic Telephone Company (RATCO) by Sterling Telephone & Electric.  Another variant has the patent numbers and the letter G (Salford Electrical Instruments, a GEC subsidiary).

Oxide-blacked with solid copper finger rings seems to be the norm, why is this?
These finger wheels are brass, flashed with a thin layer of copper and then chemically blackened.  Copper is too soft and ductile to be viable for finger wheels, probably also to expensive.  Brass is considered too 'greasy' to retain nickel plating or chemical blackening, whereas copper sticks tenaciously to brass and takes plating and blackening well.

Maybe a little later, but only a few years, possibly around 1932, pressed brass painted finger rings come in, essentially the same for as the stainless types but made in brass.  A ring is pressed into many, possibly a later strengthening measure, inside the standard label area but would be outside a small label holder so must have been in the standard label holder era.  These eventually become the norm, and are even used on type No. 14's from the fifties: these were apparently for exchanges and had black finger rings on stainless Dial No. 12-style bodies.

The  Dials No. 14 were fitted with black finger wheels as sunshine glinting on a Stainless Steel finger wheel dazzled the GPO Switchboard operators (apparently).  Dials No. 10 with black 'thin' finger wheels were also specified on telephones made by Ericsson for Telephone Rentals in the mid-1950s.  One other oddity: GEC Dials No. 10 fitted to telephones exported to Argentina had not only SS finger wheels but Ivorine (while celluloid) number plates. The numbers were engraved in the plastic and filled with black paint.  The number 3 has a flat, not rounded, top.


Ron Kay in New Zealand writes:-
When the coloured Siemens Brothers Neophones first arrived in New Zealand in 1933 they were pictured in advertisements in the Telephone Directories with the finger stop in the 4 o'clock position.  This was to suppress the minimum pause which is not needed with Rotary as the register only has to step its in-pulsing control sequence switch a small distance between digits.  This alternative position was on all BPO dials until about 1951 when the first trigger dials started to appear.  By this stage I would say all Rotary registers had been changed to standard pulsing.  I notice the two options for the finger stop pictured in a dial in "Telephony" by Atkinson, so it looks as though all dials produced at the time had this feature.

Dials with MNO
Dials with the three letters MNO above the figure 6 and OPERATOR in the zero position were used in all of North America (including USA, Canada and the Caribbean). 

If the dial MNO dial is a British dial, it could have been used in any of these three territories, Canada being the most likely because it was a British dominion and carried on buying British telephones after it was no longer a dominion.  Cable & Wireless operated in the Caribbean and may well have used British-made telephones (not confirmed).  British-made telephones were used in the USA by Telephone Rentals Ltd, who had a subsidiary in New York (a two-tone grey Telephone 706 with an MNO dial ring has been seen in the USA).  

Dial No. 10 - Australian version

 

Dial No. 10 rear view Dial No. 11 rear view

 

Dial No. 11
This dial is similar to the Dial, Automatic No. 10, except that it has an additional set of springs that are operated by a raised cam fitted to the dial spindle.  These are used in Coinbox installations.  The upper cam has a number of drillings that will accept a rod and this rod is moved to correspond with the digit that operates the springset (see pictures below).

This dial was available in variants BA and FA.

Early Dials, Automatic, No. 11 (Mark 234) also used the small paper dial label that was used on the Dial, Automatic, No. 8.  These are called Label No. 138B which is 111/64 inches in diameter.  Later Dials, Automatic, No. 11 (Mark 235) used the larger Label No. 160.

Drawing 62888 (Mk 4A).
 

Dial, Automatic No. 11 Rear View
The additional springset is to the right

GPO models
1928 Rate Book advises:-
The Dial, Automatic, No. 11 was available in variants BA and FA.

1946 Rate Book advises:-
The Dial, Automatic, No. 11 was available in Stainless Steel only with variants BA, FA, FS, FW and LA.

Other versions of the Dial, Automatic, No. 11 were also offered and these were the Dial, Automatic, S.S., No. 11FT1 and No.11FT2.  These were for use on Testers, AT ...

  1. For "long line" testing. 80% break.  Adjusted to 12 i.p.s. and fitted with Wheel, Impulse, T1 (Part No. 1/SWH/27). Otherwise as S.S., No. 10FA.

  2. For "short line" testing. 50% break.  Adjusted to 12 i.p.s. and fitted with Wheel, Impulse, T2 (Part No. 1/SWH/26). Otherwise as S.S., No. 10FA.

The Dial, Automatic No. 11 was superseded by the Dial, Automatic, No. 12 and is not mentioned in the 1956 Rate Book.

Other Manufacturers
Siemens made also produced the Dial, Automatic No. 11.  Models available:-
Dial, Switch No. 19FA/S was their equivalent to the Dial, Automatic No. 11FA.
Dial, Switch No. 19LA/S was their equivalent to the Dial, Automatic No. 11LA.

Dial No. 11 - This picture shows two rods in the upper cam

 

Dial No. 12 (trigger type)
This dial was initially manufactured by GEC and introduced circa 1952.

The general construction of this trigger type dial is similar to that of the slipping-cam type.

The trigger dial differs from the slipping-cam dial on the following major points:-

(a) An articulated trigger assembly is used for the pulsing mechanism.  This assembly has two main parts: a swinging lever which pivots about a screw in the main case, and a trigger which pivots on a pin at the free end of the swinging lever.  A flat spring, which is part of the spring-set assembly, bears on the trigger to ensure its set follows the contour of the pulse wheel.

(b) The pulse contacts comprise a lever spring tensioned against a stiff buffer spring and they are moved apart by an insulating bush on the trigger striking an extension of the lever spring.

(c) The dial mechanism is held in its rest position by a spring stop plate which is fastened by the screws that hold the bearing bracket for the governor gear wheel.  A lug on the stop plate engages with a pin on the underside of the main gear-wheel when the plate is sprung upwards by a screw inserted through the back of the case.  Slackening the screw allows the stop plate to spring away from the gear-wheel and let the main spring unwind.

(d) The number ring has a smaller hole in the centre so that it is more difficult for dust to get into the dial mechanism.

Dial No. 12 front view Dial No. 12 rear view

Operation of the trigger mechanism
When a digit is selected, the end of the trigger engages with the first tooth of the pulse wheel and is flipped backwards.   Further rotation of the pulse wheel causes the end of the trigger to ride over the 'teeth' and the number of teeth that pass equals the number selected.

When the finger plate is released, the mechanism rotates in the opposite direction and the trigger is moved back to the pulsing position.  This movement occurs m approximately 240 milliseconds, giving the minimum inter-digital pause.  Having reached the pulsing position, the trigger rides over the teeth of the pulse wheel and the insulating bush breaks the pulsing contacts at each tooth.

The number of pulses sent equals the number of teeth over which the trigger passed when the dial was wound up.

Purpose of the slipping-cam and trigger mechanisms.
Both mechanisms provide the minimum inter-digital pause before pulses are sent out so that:-

(a) If a subscriber commences to dial without waiting for the receipt of dialling tone the delay in sending the first pulse provides additional time to allow a free outlet to be seized.

(b) the dial mechanism is able to attain its governed speed before it sends out pulses.

Neither of these advantages could be achieved with P.O. dials if the pause occurred after the pulse train, instead of before it.

Advantages of the trigger mechanism

(a) Its operation does not rely on frictional contact between surfaces, as in the slipping-cam design.  Frictional contact can vary considerably during the years of service without attention that are expected of a dial, and departure from the correct value can cause false pulsing or stalling.

(b) The position to which the finger plate must be rotated to select the digit required is less critical than with the slipping-cam design, in which a small variation can cause a shortened first pulse or a false one.  With the trigger design the correct number of pulses will be obtained if the finger plate is moved short of, or past, the finger stop by half the diameter of the finger hole.

(c) Wear of the pulse contacts and wear of the trigger set or pulse wheel affect the pulse ratio in opposite ways, so that any changes tend to neutralise one another.

(d) With the slipping cam, wear tends to increase the break to make ratio of the pulses.

Diagram 90158.

GPO models
1956 Rate Book advises:-
The Dial, Automatic, No. 12 was available in variants FA and LA.  No coloured finger plates were offered - Stainless Steel and black only.

Other Manufacturers
Siemens made also produced the Dial, Automatic No. 12.  Models available:-
Dial, Switch No. 20FA/S was their equivalent to the Dial, Automatic S.S. No. 12FA.
Dial, Switch No. 20LA/S was their equivalent to the Dial, Automatic S.S. No. 12LA.
Click here for more information

Ericsson Telephones Ltd (ETL) also produced this dial and used them on telephones produced for the home and overseas markets.  They can be identified as they are marked N4383 and were produced in many variants which can be identified by a lettered suffix.  Click here for more ETL dial information

AEI named the Dial, Automatic, No. 12FA as their Dial, Switch 20.  Click here for more information

GEC named the Dial, Automatic, No. 12 as their DL 1100 series (standard) or DL 1200 series (tropical finish). Click here for more information

STC named the Dial, Automatic, No. 12 as their Dial 4004B.

Click here for a time line of the roll out of the Dial, Automatic, No's 12 and 13

Guernsey Dial No. 12

 

Dial No. 13 (trigger type)
Dials for some types of coin collecting box circuits require auxiliary contacts which break when certain digits are selected and remain open until the pulsing stops.

The general construction of the Dial, Automatic, No. 13... is similar to the standard trigger dial but an auxiliary spring-set and associated parts are mounted on the back of the dial.

The contacts of the auxiliary spring-set are normally held in the 'made' position by a cam mounted on an extension of the main spindle.  A small control cam, loosely coupled to the main spindle by a spring clip, is mounted between the contact cam and the off-normal lever, and its movement is limited in both directions by a stop plate which is fixed to the supporting bracket of the main spring-set assembly.  A masking disk is fitted over the pulse wheel to reduce the effective height of every tooth except the first one, and the insulating bush on the trigger is longer than that on the normal trigger dial.

Operation of the coin-box auxiliary contact
The auxiliary cam is free to turn on the main spindle and has a light helical spring attached to it from a bracket on the auxiliary spring-set assembly.  Two control pins are riveted into the cam and project on the underside so that the off-normal bush can strike them.

When any digit up to and including '8' is selected, the spindle moves independently of the cam, which is held in its normal position by the pressure of the auxiliary spring-set.

If '9' or '0' is selected, the insulating bush on the off-normal lever strikes the operating pin (that furthest from the anchor screw for the helical spring) and rotates the cam to allow the auxiliary contacts to break.  During the return motion the contacts remain open because the helical spring is not strong enough to turn the cam and lift the lever spring up the cam step.  After the last pulse has been sent, the off-normal bush strikes the resetting pin (nearest to the anchor screw) and resets the cam and contacts to their normal position.

When free dialling of digit '8' is required, another control pin can be screwed into the tapped hole in the cam.

Prevention of fraudulent calls
The control cam, the masking disk, the longer trigger bush and the light helical spring in No. 13FA dials function to prevent certain forms of fraudulent calls being made.

GPO models
1956 Rate Book advises:-
The No. 13 was available in variants FA and LA (Using Label No. 314 and 315).

Other Manufacturers
Siemens made also produced the Dial, Automatic No. 13.  Models available:-
Dial, Switch No. 21FA/SS was their equivalent to the Dial, Automatic S.S. No. 13FA.
Dial, Switch No. 21LA/SS was their equivalent to the Dial, Automatic S.S. No. 13LA.

 

Dial No. 14
This dial was used on the GPO switchboards and always came with black finger plates.
The dial was available in three variants (1956 Rate Book advises):-

  1. No. 14FA  for use in Non-Director areas for use on manual positions other than 2VF.

  2. No. 14LA  for use in Director areas for use on manual positions other than 2VF.

  3. No. 14LA/2VF for use on positions at exchanges with 2VF. Adjusted to 9.5 to 10.5 pps and 66.33 +/- 2% break. 

The 14FA used number ring No. 304 whilst the 14LA & 14LF/2VF used number ring No. 305 (same rings that were used on Dial, Automatic, No 12).

Dial, Automatic, No. 14 is shown pictured to the right.

Variants
Siemens made also produced the Dial, Automatic No. 14.  Models available:-
Dial, Switch No. 22FA was their equivalent to the Dial, Automatic No. 14FA (All black).
Dial, Switch No. 22LA was their equivalent to the Dial, Automatic No. 14LA (All black).

 

Dials, Automatic, No. 14FA - Front Dials, Automatic, No. 14FA - Rear

 

Dials No. 15 and 16
These dials are for testers and are of the trigger type.  Details of their speeds and ratios are included in Table 1.  The standard governor mechanism is used, specially adjusted to give the required speeds, and the special pulse ratios are obtained by the use of different pulse wheels.

The Dial No. 16 dials are fitted with an extra spring-set and off-normal lever.  The spring-set is similar to the auxiliary one on the Dial No. 13FA dial and is operated directly by the additional off-normal lever which is mounted on an extension of the main spindle.  The angular position of this lever can be varied so that the spring-set is operated and the contacts made at different positions of the finger plate.

It is thus possible to utilise the contacts as make contacts for any particular instant in the travel of the dial, or as break contacts for the duration of the operation of the dial.

 

Dial No. 17LA
This dial is used only on the Telephone No. 280.  It has the same pulsing mechanism as the Dial, Automatic, No. 12 described above but its mechanism is mounted on a smaller case than that of the Dial, Automatic, No. 12.  The number ring fits closely over the case and is printed with figures only.  The finger plate is also smaller than that used on the Dial, Automatic, No. 12 and is printed with letters in positions corresponding to the figures on the number ring.

Drawing 90214.

Introduced in the mid 1950's.

Click here for more information

 

Dial No. 19FA
This is a trigger type dial having a similar form of construction to the Dial, Automatic, No. 12.  It is fitted with one pair of off-normal contacts and. the pulsing action is effected with a change-over unit.  This dial is used for signalling on automatic telegraph switching systems and the double pulse of the change-over contacts is adjusted to a nominal 3:2 (operate to normal) ratio.  A Dial, Automatic, No. 19 (rear view) is pictured to the right.

 

Dial No. 20LA
This is a trigger type dial having a similar form of construction to the Dial, Automatic, No. 12.  This dial has been introduced to enable users of existing type coin-boxes (Boxes, Coin-collecting No. 14.... ) in a director area to dial the new assistance code "100" without charge. Dials, Automatic, No. 13LA which they supersede, and Dials, Automatic, No. 13FA, permit only 9 or 0 to be dialled free.

The dial permits free dialling of digits '1', '9' and '0' and is used on prepayment coin-collecting boxes in director areas.  The operation of the auxiliary spring-set for the dialling of '9' and '0' is the same as that of the Dial No. 13 dial.  To give the free dialling of digit '1' the masking disk is also used as a cam which, in conjunction with an operating lever, controls the auxiliary spring-set assembly.  It is arranged that the auxiliary contacts are opened, or remain open for the duration of the last pulse of each pulse train.

In the normal position of the finger plate, the auxiliary contacts are opened by the operating lever which is resting on its cam.  As the finger plate is rotated clockwise, the cam releases the lever, and the contacts close.  The operation of the auxiliary spring-set is as previously described for the Dials, Automatic, No. 13FA dial until the make period of the pulsing springs immediately before the last pulse is transmitted.  During this make period the lever will again come under the control of the cam.  If the contacts have already been opened by the switching cam, when dialling '9' or '0', the lever will only increase the contact gap.  If a digit other than '9' or '0' is dialled, the auxiliary contacts will be closed, until they are opened by the lever during the make period immediately before the last pulse, and will remain open until the dial is re-operated.

Description of moulded body dial
The mechanism is essentially the same as that of the trigger dials previously described, except that it is mounted on a steel baseplate attached to a black moulding which forms the body.  The material of the pulsing contacts has been changed to palladium to avoid high resistance contacts which have occurred on earlier dials.  The number ring is located on two pillars between which its retaining spring is clipped.  The finger plate is formed from rigid P.V.C. sheet in the colour range to match 700-type telephones.  The label protector is moulded in clear polymethyl methacrylate.  It snaps into position in the finger plate, and is held by three pips which are equally spaced around its periphery.  A stainless-steel finger plate has been retained for some dials, e.g. those fitted at public call offices.  The dial mechanism is held in the rest position by a screw passing through a lug on the steel base plate and engaging with a pin screwed into the main gear-wheel.

Removal of the label protector
The label protector should be removed by one of the following methods:-

  1. Extractor No. 29.  This is a rubber suction disk with a finger grip.  The extractor should be firmly pressed on to the label protector and then sharply pulled away.  It can be released from the protector by sliding or by lifting one edge of the rubber with the finger nail.
     
  2. For the coloured finger plate use a small bladed flat screwdriver (Screwdriver, Instrument, No. 1).  This should be inserted radially between the finger plate and the number ring at the 3 o'clock position on the dial.  The screwdriver will then enter a slot in the finger plate, and by turning the screwdriver beneath the pip in a thin metal plate which is under the label protector the protector will be ejected.  Care must be taken to ensure that the end of the screwdriver is under the metal plate and not between the plate and the label protector.  It is advisable to hold two fingers over the protector to restrain it during ejection.
     
  3. For the clear finger plate, another method is to use a piece of Sellotape.  Roll the Sellotape into a circle about 2 inches diameter with the sticky side outwards.  Stick to the dial cover, covering as much of the cover as possible.  Pull the Sellotape towards you.

Click here for more information on the Dial No. 20.

 

Dials No. 21CA (Colour)
This moulded body dial is for use with 700-type telephones.  The number plate fitted is in matching colour and has printed arrow heads (called Chaplets) which appear beneath the finger holes.  These arrows pointed to the numbers which were printed on a ring that surrounded the dial.

Some earlier Dials, Automatic, No. 21CA, (Colour) will have number rings without arrow heads - very unusual.  Introduced in July 1959.  The dial shown on the right is a Dials, Automatic, No. 21CA Red.

Drawing numbers 91359 and 91475.

Made obsolete in 1969 and superseded by the Dial, Automatic, No. 21FA Clear.  The GPO had completed customer tests and decided that the dials with numbers on the finger plates were less prone to dialling errors.

Click here for more information on the Dial No. 21.

Dials, Automatic, No. 21CA - Front Dials, Automatic, No. 21CA - Rear

 

Dials No. 21CA Steel
This dial is for use on Telephone No. 705.  It is similar to the Dial, Automatic, No. 21CA, (Colour) but is fitted with a stainless-steel finger plate (Part No. 1/DWH/38).  The number ring is as fitted on Dials, Automatic, No. 21CA, Green.  Introduced in 1960 and made obsolete in 1969.  Superseded by the Dial, Automatic, No. 21FA Steel..

 

Dials No. 21LA Black and Grey
This dial is similar to Dial, Automatic, No. 21CA, Black with the exception that it is fitted with a number ring printed with figures and letters.  It was also supplied in grey for use with Mountings, Dial Automatic No. 21. 

The Black version supersedes Dial, Automatic, No. 12LA.

Other Manufacturers
ATM
L39511A - Director dial with moulded black finger plate.
L39511C - Director dial with moulded grey finger plate.

Dial No. 21LA Black - Front Dial No. 21LA Black - Rear



Dials, Automatic, No. 21FA Clear
Introduced in June 1967 and an improved version in September 1971.  The introduction coincided with the nationwide introduction of all figure numbering.  This dial was also fitted to all factory refurbished telephones that had a Dials, Automatic, No. 21CA.

The coloured finger plate (Label No. 468) was displaced by a clear plastic finger plate and the number plate, bearing arrowheads (Chaplets),  replaced by a number plate with black figures on a background, coloured, antique silver.

All Telephones No. 746 were fitted with this dial.  Drawing number 92526.

Later manufactured Dial No. 21's will be found with plastic impulse wheel/spring cover and trigger mechanism.

Dials, Automatic, No. 21FA Clear - Front Dials, Automatic, No. 21FA Clear - Rear

Dials Nos. 21FA Steel
This is a Dial, Automatic, No. 21FA fitted with a Stainless Steel finger plate.

Originally this dial was issued for use on Units, Telegraph No. 20 and 23 and had a standard label protector.

It was later modified with a Kit No. 269A.  The Kit No. 269A is a Perspex dial label cover with an Allen Key centre screw.  The dial was used in public telephone kiosks to reduce vandalism.  (Dial, Automatic, No. 21 Steel is shown pictured to the right).

Drawing 91475.

Dials, Automatic, No. 21FA Steel - Front Dials, Automatic, No. 21FA Steel - Rear

Variations
AEI used a Dial, Switch No. 34AF/A on the Centenary Neophone which was similar to the Dial 21FA Steel but used a dial number plate with a white background.
 

Dials Nos. 1/21FA Clear
This is a low inertia Dial, Automatic, No. 21 produced specially for the Compact telephone - Telephone No. 776.

Drawing 92526.

 

Other Manufacturers
GEC tried a design having a finger wheel with spokes rather than holes; later it also made a very cheap dial with many metal parts replaced by plastic.  Click here for the GEC spoked variant & Click here for the GEC cheapened Dial.  These were used on PAX telephones and were also supplied to the New Zealand Post Office.

ETL also produced this dial and used them on telephones produced for the home and overseas markets.  They can be identified as they may be marked as follows:-
N4394520A - (Dial No. 21CA). 
N439451A - Transparent finger plate (Dial No. 21FA).
N439452A - Transparent finger plate (Dial No. 21LA).

Many of the ETL dials were each produced in many variants, which can be identified by a lettered suffix.  Click here for more ETL dial information.

British manufacturers made versions of these dials for overseas use, including New Zealand numbering (0 to 9), Canadian letters and numbers, Egyptian numbers only, English and Egyptian numbers, also English and Chinese numbers.  Click here for pictures.

 

Dials No. 22FA and 22LA
This dial has a stainless steel finger plate and a mechanism similar to that of Dial, Automatic, No. 13, mounted on a moulded body.  The FA variant is fitted with a non-Director number ring bearing figures only (Label No. 427) whilst the LA variant has figures only.  It supersedes Dial, Automatic, No. 13FA.  Fitted with an additional springset which was normally made.  When the digits 9 or 0 are dialled, the springs remain open until the pulsing stops.  Used on coin collecting boxes and shown pictured below.

Dials, Automatic, No. 22 - Front Dials, Automatic, No. 22 - Rear

 

Dial No. 23LA
Used in Director areas, this has a mechanism similar to that of Dial, Automatic, No. 20, mounted on a moulded body.  It is fitted with a stainless-steel finger plate and a number ring bearing figures and letters.  Supersedes Dial, Automatic, No. 20LA.  Shown pictured above.

Drawing 91757.
 

Dial No. 23FA
As above but use in Non-Director areas, with figures only.

Dials, Automatic, No. 23 - Front Dials, Automatic, No. 23 - Rear


 

Dial No. 24F
This dial has a similar mechanism to Dial, Automatic, No. 19, but mounted on a moulded body.  It is fitted with a number ring bearing figures only and a black P.V.C. finger plate.  The dial is for use on Telegraph equipment and supersedes Dials, Automatic, No. 19FA.

Drawing 91762 and 92295.
 

Dial No. 25CA
This dial is similar to Dial, Automatic, No. 25FA except that it has a grey P.V.C. finger plate and a number ring bearing arrowheads.  It is for use on the operator's telephone of Switchboard, P.M.B.X., No. 2/... where inter-switchboard circuits are provided for dialling into P.A.B.X.s.  The dial is fitted with an additional springset with the contacts made when stationary and broken when off normal.


Dial No. 25FA Clear
This dial is similar to Dial, Automatic, No. 25CA except that the grey P.V.C. finger plate and number ring bearing arrowheads are replaced by a clear finger plate and a number ring with figures, coloured antique silver.  The dial is fitted with an additional springset with the contacts made when normal and broken when off normal.  This dial has a standard impulse wheel marked A1 and runs at 10pps.  Uses a label No. 468A.

Dials, Automatic, No. 25FA FT3 - Front Dials, Automatic, No. 25FA FT3 - Rear
   
Dials, Automatic, No. 25FA FT4 - Rear Dials, Automatic, No. 25FA FT4 - Rear

 

Dials No. 25FA FT3 and 25FA FT4
These have mechanisms similar to Dials, Automatic, Nos. 16LA, 16LT3 and 16LT4 respectively, mounted on a moulded body.  The dials are fitted with a number ring bearing figures and a clear finger plate. These dials are used in testers and supersede Dials, Automatic, No. 16.  The dial is fitted with an additional springset with the contacts made when normal and broken when off normal.

The FT3 has a 80% break and 12pps.  This dial is usually found with a dial label stating "ADB 80/12" or similar.  The impulse wheel is stamped "T3 80%B.

The FT4 has a 50% break and 12pps.  This dial is usually found with a dial label stating "ADA 50/12" or similar.  The impulse wheel is stamped "T4 50%B".

 

Dials No. 25LA, 25LT3 and 25LT4
These have mechanisms similar to Dials, Automatic, Nos. 16LA, 16LT3 and 16LT4 respectively, mounted on a moulded body.  The dials are fitted with a number ring bearing figures and letters, and a black P.V.C. finger plate.  These dials are used in testers and supersede Dials, Automatic, No. 16.  See the Dials, Automatic, No. 25FA for the specification.

 

Dial No. 26LA
This is a modified Dial, Automatic, No. 10 fitted in a flameproof enclosure and is used with the Telephone No. 149.  The flameproof construction requires it to have an additional spindle and to prevent this impeding the return of the pulsing mechanism the steel finger plate is coupled to the pulsing mechanism by means of a clutch.  At the moment of release of the finger plate it is declutched from the pulsing mechanism and returns to normal very quickly under the action of an additional coiled spring.  The return of the pulsing mechanism is therefore unimpeded.  When the finger plate returns to normal it is locked until the completion of pulsing.  Then when the mechanism returns to normal the finger plate is unlocked and the next digit can be dialled.  The finger plate should be locked by repositioning the screw in the flyback mechanism when the Telephone No. 149 is used on C.B. systems.

 

Dial No. 27CA
This dial is similar to the Dial, Automatic, No. 21CA and had finger plates with the standard colours.  It is fitted with a Pulse Wheel A3, so it is not a standard dial.

It is not known if this dial was ever used by the BPO.

Drawing 91853.

 

Dial No. 28LA
This dial has a mechanism similar to that of a Dial, Automatic, No. 22 but modified by the addition of a cam so that the contacts of the auxiliary spring-set are closed during the pulsing of any number other than '9'.  It is used on installations where outgoing access is required to be limited to the '999' emergency service only, either permanently or via a lockable switch.

Drawings 91928.

 

Dial No. 28FA Clear
As above but fitted with a clear finger plate and a number ring with figures, coloured antique silver.

Diagram 92524.

Dials, Automatic, No. 28 - Front Dials, Automatic, No. 28 - Rear


 

Dial No. 30
This dial is similar to Dial, Automatic, No 21 but has a fluorescent tube beneath the finger plate for illumination.  This dial was originally used in the Telephone No. 712 (Early Trimphone) and then on the Telephone No. 722 (Trimphone).  These were available in FA and LA variants.

The dial finger plate is tinted a very light Lilac colour and the dial body is slightly oversized, so as to locate under the telephone case.

Other Manufacturers
Ericsson (ETL) was one company that produced this dial and their part number for the Dial, Automatic, No. 30FA is N24591.

Click here for more information on the Dial No. 30.

Diagram 92808.

Dial No. 30 - Front View Front View with Dial Finger plate and number label removed, exposing Tritium tube. Dial No. 30 - Rear View

 

Dial No. 35FA Clear
Effectively a Dial, Automatic No. 25 that has been fitted with a Label No. 468A, Finger Plate 1/DPL/2145 and a protector Label 1/DPR/6.  It is a figures only dial used on testers.  Runs at 10pps and has a 66.6% break.  The dial is shown pictured below.

 

Dials, Automatic, No. 35 - Front Dials, Automatic, No. 35 - Rear

 

Dial No. 51A
Made by GEC this is an all plastic reduced cost dial.  Replacement for Dial, Automatic, No. 21.

Dials, Automatic, No. 51 - Front Dials, Automatic, No. 51 - Rear

 

Dial No. 54A
Later dial made completely of plastic.  Superseded Dials, Automatic, No. 21 for general use.

Dials, Automatic No. 54 - Front Dials, Automatic No. 54 - Rear

 

Dials for blind subscribers and subscribers with defective sight
The modification of dials for blind subscribers is described in Control C 1001.  For subscribers having in operating the dial due to defective sight, the procedure outlined in M 0025 should be followed.

 
 
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Last revised: April 19, 2022

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