These are extracts of minutes from the British Telephone Technical Development Committee (BTTDC), Subscribers Apparatus Development Sub-Committee meetings, which were held every 3 months.  These were run by the GPO and held on their premises.  The GPO contractors also attended these meetings.  The minutes started around 1942 and finished in December 1967 due to the re-organisation of the Post Office.

The GPO were looking at the failure rates of the Dials, Automatic Nos. 10 and 11 and obviously a more efficient replacement would over time save money.  Experience of the Dial No. 10 and 11 showed that excess of lubricant between the impulse wheel and the slipping cam or inadequate pressure could result in the cam slipping during the forward motion of the mechanism.  There was also a tendency for the oil to congeal, which was temperature dependant, causing drag on the return motion of the dial.

Dials No. 12 and 13 were introduced nationally around 1953.  It is questionable as to whether they totally replaced the Dial No. 10 and 11, as many refurbished telephones have been found with these dials dated later than 1953.

A US patent for the No. 12 dial was filed on 14th August 1939, but at this point in time the dial was most probably at the prototype stage.  The Dial No. 12 was produced by GEC and first appeared in the Committee minutes in 1944 and a picture of a dial No. 12 is shown below, dated 1944.  Diagram N612 is also dated 1944.

The Dials No. 12 shown are this page are all pre-production dials.  They all have a governor speed adjuster which is marked S - T on the dial body and are generally dated 1944.

Dial No. 10 to the left and Dial No. 12 to the right

There is also a drawing of a trigger dial in 'Atkinson' Volume No. 1 (1948) where reference is made to a 'new type of dial in which the slipping cam has been eliminated'.  This Dial is marked 12 C44/1, so is dated 1944 and made by GEC.  This Dial is most likely to be a prototype as it is fitted with an ingenious oil soaked washer between the conical washer and the impulse cam 'by which the impulse and the impulse wheel teeth are kept lubricated' and which also forms a buffer when the trigger falls into the impulse cam recess.  There is also a similar arrangement to lubricate the trigger mechanism.  In the Notes below you will see that the GPO requested these be removed. 

The pictures of the Dial No. 12 in the GPO picture library are nearly all dated 1953, which matches with the date of official introduction.

One has to consider that this was the end of the Second World War and that there was a shortage of resources.  In 1946 there was no satisfactory supply of English spring steel, so permission was sought to purchase from Sweden.  This issue continued until late 1953 as it was debatable as to whether British Steel springs were up to the job.

At the same time the GPO found that the governor worm drive and gearing varied between manufacturers and this debate continued until the middle of 1951, when a specification, agreeable to all parties, was drawn up.  Because the British worms were not suitable it was requested that worms be purchased from Switzerland as an expediency.

The Committee notes below tell the story and give precise timescales to the Dial No. 12 introduction.

The dial above looks like it has the oil soaked washer


Close up of the speed adjuster



9th February 1944
CPS 54 - Dials Auto-Trigger Type.
There are two trigger dials presented to the BPO that are supposed to be superior to the Dial No. 10 and Dial No. 11.

13th February 1946
Field Trials on Dials. Auto (C.P.S.54).
The Chairman said that 200 dials No.10 supplied by two contractors were being compared with 200 dials No.12 supplied by the G.E.C.

The dials had been on trial on operators positions at various London exchanges and the reports on dials No.10 were most unsatisfactory: of the 200 dials 37 had been reported faulty, 67 of the faults being attributed to sticking, due to faulty governor pivot bearings.  The Chairman appealed to all contractors to pay special attention to those points.

The report on dials No.12 is not yet ready.  A summary of faults on all dials tested would be prepared and distributed as early as possible after conclusion of the tests.

The Chairman asked whether the contractors would agree to the inclusion of all contractors dials in the field trial and this was agreed.  Several contractors representatives pointed out that the dials under test were of wartime (1944-45) production.

19th June 1946
The field trial of Dial Auto No.10 was still proceeding and 100 Dials Auto Nos. 12 and 13 had been returned, to the contractors, for removal of certain lubrication washers (see M.P.B. No.93).  Mr. Ritter (PO) said, that a field trial of 50 dials from each contractor and 50 produced by the P.O. factory was being arranged.

15th January 1947
CPS 54 is renamed SCP 54.

23rd July 1947
S.C.P. 54. Dials Auto G.E.C. Trigger Type.
Mr. N.D. Smith (STC) stated that as a result of the Circ. Lab. report, it had been decided to undertake a much more comprehensive field trial and, to this end, some 20,000 dials had been ordered for exclusive use of operators in Exchanges.  They were of the all-black type and would not be used for subscribers instruments.  In reply to Mr. Penhale (ET), Mr. Ritter (PO) said it was not proposed to try a black moulded dial as it was doubtful if it was as robust as the metal type.

6th October 1948
Mr. Barker (PO) said that 500, No. 13 Dials had been on field trial with 500, No.11 dials for about a year.  Up to the present a total of only 40 had been returned as faulty and it was clear that it would be some time before sufficient were returned to enable reliable fault liabilities to be deduced.  The P.O. had become concerned over the time which it now seemed might elapse before the performance of the trigger dial could be assessed and in an endeavour to shorten this time discussions had been opened with the manufacturer concerned with a view to increasing the recent trial order from 20,000 dials to 100,000.  Mr. Dewar (GEC) said that he thought this could be arranged.

28th September 1949
Mr. Barker (PO) said that the trials with the trigger dials in kiosks were continuing.  427 of each of Dials of the 11 and 13 types were installed in call offices in 1947 and up to date 13% of the No.11 type and 10% of the No.13 type had been recovered as faulty.  There was, therefore, a slightly lower fault liability with the No. 13 type and it had the advantage of being proof against a number of fraudulent practices to which the No.11 type was susceptible.

From the results of SCP 54 and SCP 75 (dimension specification of Dial No. 10/11) the P.O. had to decide whether to adopt the trigger type dial or whether to ask the manufacturers to change production of the No. 10 type to the drawings agreed under SCP 75.

In making the decision four points would be taken into consideration:-
(a) Which had the lower fault liability.
b) Which could the P.O. more easily maintain.
c) Which would the manufacturers prefer.
d) Which would be the more economical.

The P.O. was obtaining information from field trials on the relative fault liabilities and was looking into the maintenance question.  The views of the manufacturers on their preferences would be appreciated.  They should assume for the present that the trigger dial would, in any case, be adopted by the P.O. on coin box circuits and in exchanges.  Mr. Lewis (SB) asked what were the quantities of these dials as compared with those on subscribers' lines. Mr. Barker (PO) said that the approximate figures for dials in use were 3 million on ordinary subscribers circuits, 100,000 on exchanges and 100,000 on coin box circuits: it should be remembered, however, that the turnover rates varied greatly, the average life of an ordinary subscriber's dial being about 10 years, an exchange dial about 6 month and a coin box dial up to four years depending upon usage.

The manufacturers agreed to furnish comments and discussion followed on the possibility of finalising the design of the trigger dials before general manufacture commenced. Mr. Barker (PO) agreed that before production commenced the P.O. would decide which of the modifications now on trial would be adopted.

11th January 1950
Mr. Warren (TAC) said that the manufacturers had not yet decided whether they would prefer to change production to the trigger dial or to the standardised No. 10 dial.

Mr. Barker (PO) said that the Post Office was still hesitant to make a general decision as there was so far little experience of the operation of the trigger dial but the Post Office was satisfied that it should be adopted for use on all coin-box circuits and it was proposed to do this.  The manufacturer would be approached and the preparation of official drawings discussed.  Mr. Barker (PO) mentioned that, as already reported, a field trial of approximately 900 dials had been made and about 120 had been returned as faulty.  Since last reporting on this only ten more dials had been returned, and it was considered that the field trial could be discontinued.

13th April 1950
Mr. Warren (TAC) said that, on the question of whether the Manufacturers favoured production of the trigger dial or the standardised No. 10 dial, consideration had been given to the technical aspect: the commercial aspect was being considered at a meeting of the T.A.C.  He would report on any agreement reached.

12th October 1950
Mr. Barker (PO) recapitulated briefly on the question of the choice between going over to the trigger dial completely or manufacturing the No.10 dial to the new drawings being produced under SCP 75 and stated that a letter dated 7th October 1950 had been sent to the T.A.C. indicating that the P.O. wished to standardize the trigger dial for all purposes.

Mr. Warren (TAC) drew attention to Mr. Barker's agreement (Minute 348, Meeting.25) that the P.O. would decide upon all modifications to be incorporated before general manufacture of the trigger dials commenced.  Mr. Barker (PO) reiterated this agreement.  The Chairman inquired whether all manufacturers would be in a position to supply these dials when the next orders were placed.  Mr. Lewis (SB) thought this would be so but it was unlikely that it would be possible to supply them under current dial orders.

10th January 1951
Mr. Barker (PO) referring to the L.O.'s recommendation to discharge, said that the case could not be closed until the drawings were completed.  He inquired how work on the drawings was proceeding.  Mr. Dewar (GEC) replied they would be passed to the P.O. in a few weeks, Mr. Warren (TAC) stated that the Manufacturers felt that the larger main spring should be incorporated in the trigger dial. Mr. Barker (PO) said that such a change was fundamental and before any decision on its introduction could be taken a field trial would be necessary.  In any case such a modification was regarded as in the nature of a long term development and implementation would require 4 or 5 years.  He said that a prototype dial with the larger spring had not even been produced yet and for a change of so fundamental a nature it would be desirable to try out a few thousand.

Mr. Lewis (SB) pointed out that the present type of dial had been in use for about 30 years and there was nothing much wrong with it.  He considered that there was no reason, therefore, why the introduction of the trigger type dial could not be delayed until the question of whether the latter type of dial should have the larger spring was resolved.  He thought that it was better to implement any possible modifications before the item was introduced.

Mr. Barker (PO) replied that the P.O. regarded the spring question as a separate and non-urgent issue and were not convinced that a larger spring was necessary.  In fact, the P.O. would prefer that Manufacturers concentrate on acquiring better quality British springs of the present dimensions than make allowances for poor quality steel by pressing the P.O. to accept a larger spring.  He felt that efforts to induce steel manufacturers to improve the quality of British steel were in the national interest.  In course of further discussion Mr. Barker (PO) said that the other modifications mentioned in the report on SCP 91 would not be introduced piecemeal but would be held and introduced together after a number of years to allow manufacturers a fair run of production on the dials as now agreed.  Mr. Dewar (GEC) raised the question of whether a bigger impulse wheel should now be considered in view of the possibility of a larger spring being introduced at some later date. Mr. Barker (PO) replied that it was not proposed to consider that at the present stage.  Mr. Lewis (SB) stated that the Manufacturers had not had any information to show that the trigger type dial was better than the current types.  Mr. Barker (PO) replied that the standardization of the dial was agreed between the P.O. and Manufacturers and he did not see any reason now for going back on that decision.  Whilst the P.O. might have, at times, made available to the Manufacturers information upon which decisions were based, there was no obligation to do so.

11th April 1951
It was noted that Mr. H. J. C. Spencer (PO) was now P.O. Liaison Officer for this SOP, Mr. Combridge (PO) said that P.O. comments had been sent to C.E.O.  Mr. Warren (TAC) said his impression was that further comments were to follow after detailed check of the proofs.  He mentioned that agreement had been reached between the Manufacturers and their comments would be discussed between the L,O.s.  He added that there had been a suggestion that Dial No. 14 be adjusted to close limits and if this were done, did the P.O. wish to have two codes, i.e. Code No. 14 and Code No. 14. V.P.?

Mr. Dewar (GEC) said that the P.O. Liaison Officer had asked if all the No. 14. dials would be adjusted to close limits.  If this course were followed he considered that the V.P. dials could drop out, Mr. Combridge (PO) agreed and said that if all the dials could be adjusted to close limits there would be no need for two codes.  Mr. Bryan (SB) stated that the requisite adjustment could only be made at additional expense.  The Chairman said that the P.O. would look into the matter.

10th October 1951
Mr. Combridge (PO) said that comments on the acceptance drawings and specification had been sent to the Manufacturers.  Mr. Warren (TAC) replied that these were receiving consideration.  Mr. Combridge (PO) added that the draft drawings for parts particular to the No. 13 dial were awaiting P.O. check which would proceed upon receipt of an outstanding drawing.

16th July 1952
Mr. Combridge (PO) said that tracings had been received and were being checked against marked up prints.  Mr. Dewar (ET) said that the drawing for the dial main spring was in hand.  Mr. Warren (TAC) mentioned that comments on the specification would be furnished shortly.

15th October 1952
It was noted that the Post Office liaison officer was now Mr. B Haynes (PO).  Mr. Combridge (PO) said that the specification was in course of publication.

Mr. Warren (TAC) remarked that the manufacturers had further comments on the specification: he was sorry that it had not been possible to advise these earlier.  Mr. Combridge (PO) said he would have the printing of the specification held up until the comments had been received.  Mr. Dewar (GEC) would contact Mr. Combridge.  He added that the drawings for the No.13 dial had been sent to the Post Office.

15th April 1953
It was agreed to discharge this SCP.


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