HISTORY OF THE
DEVELOPMENT OF THE
extracts of minutes from the British Telephone Technical Development Committee
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
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
13th February 1946
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
contractors representatives pointed out that the dials under test were of
wartime (1944-45) production.
19th June 1946
15th January 1947
23rd July 1947
6th October 1948
28th September 1949
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
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. 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
12th October 1950
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. 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
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
16th July 1952
15th October 1952
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
Last revised: February 10, 2021