|These are 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, with contractors also
in attendance. The minutes started around
1942 and finished in December 1967 due to the re-organisation of the Post
The minutes on this page show the development of the Trimphone until just
before it's general release.
8th July 1959
877.6 Bedside Telephone
The Chairman said that an SCP would be issued before the next meeting for an item similar to the
American one but using British design and apparatus.
Mr. Harding (S Branch) gave a preliminary description of the telephone which should be a luxury item of distinctive appearance, small, dainty and deluxe. It should include lighting giving a dim glow at all times, increasing when the handset was lifted so that the numbers
could be seen. The volume of the bell or buzzer should be capable of being
adjusted by the subscriber. Details of the colour range were not yet settled but it
would probably be required in light pastel shades. The new handset would be used.
13th January 1960
889.14 Bedside Telephone (last minute 883.6) (SCP 177)
Mr. Warren (M.S.S.C.) asked. if the P.O. could clarify its requirements set down in 2.6
and 5.4 of the CP. Mr.
Harding (S Branch) replied that a magneto bell, with a gong struck by a hammer, was not required. Something quite new was wanted, with a pleasing tone.
One way to provide it might be to use the receiver as a tone transducer, but it was entirely open for the designer.
As regards the timing of, the development, the Chairman said that the Post Office was extremely
interested in forward looking in this field. Considerable development work was taking place in other countries and new ideas wore being brought
forward such, for example, as that described in the Bell Laboratories Record, July, 1959, page 257. The design would
need to place emphasis on novelty, smallness and neatness
and need not be inhibited by cost within reason. The quantities to be purchased
would, of course, depend upon the appeal and cost of the new instrument, but the Post Office was
prepared to place an order sufficient to justify initial tooling. It was interesting to note that
American experience was that the “boudoir” or “princess” telephone had been of sufficient appeal for it to be ordered in large quantities for use in other rooms. As the Post Office was asking for something entirely new, not
a copy of any existing design, a considerable amount of development work would be involved, and this would. influence the time taken to come into production. On the other hand, if only one manufacturer was interested then the time for development could be
reduced by comparison with that of the wall telephone since no selection of various
manufacturers designs would be necessary. Taking all these factors into account, the Chairman felt it reasonable to hope that first production would commence in 18 months time.
13th April 1960
894.4 Bedside Telephone (Last Minute 889.4) (SCP 177)
Mr. Warren (M.S.S.C.) said that A.E.I. and A.T.E. hoped to submit to the Post Office preliminary mock-ups to illustrate the appearance of their proposed designs in about four weeks; S.T.C. also hoped to submit information by that time,
and G.E.C. were considering whether they wished to make a claim for the allocation of the
Mr. Harding (S Branch) said that previously the quantity required had been expected to be a few thousand. The actual quantity, of course, depended upon the attractiveness
and to a certain extent, the price of the item produced, but the Post Office was now thinking in terms of 10,000 a year. It was thought that there would prove to be a demand for the instrument
for use not only as a bedside telephone but for other rooms as well.
Minutes of the meetings after 13th April 1960 & 12th July 1961 are
12th July 1961
922.l Telephone No. 712 (SCP's 177, 188, 190, 191 and 193) (last minute 915.1)
The Chairman recalled that it had been requested that revised proposals for the luxury
telephone should he submitted by the end of May. An extension to this date to the 20th June had been granted, and as a result, by that date the Post Office had. received four new
SCP's and four groups of models. The Chairman thanked the manufacturers for their
co-operation, and said that he was quite impressed with the new designs which were definitely a considerable advance. They had been seen by the
Engineer-in-Chief, and by the administration, and they had been shown to the Council of Industrial Design, who had undertaken to give comments on them and to indicate an order of preference. The new
SCP's would be numbered SCP 201, 202, 203 and 204, and in view of their impending issue it was agreed to discharge
SCP's 188, 190, 191 and 193.
11th October 1961
929.1 Telephone No. 712 (Luxury Telephone) (SCP's 177, 201, 202, 203, 204)
The Chairman reported that the revised models submitted by the manufacturers had been shown to the
C.O.I.D. Their reaction had been that all the models showed a considerable advance on the earlier submissions, and the Council were quite
impressed. They had given an order of preference indicating their first three
choices. These three were all models which employed novel forms of handset, and Research Branch would need to
carry out lengthy acoustical tests to confirm the transmission efficiencies.
The C.O.I.D. comments, together with Eng. Dept. views had been submitted to the Inland Telecommunications Department, and a
decision was awaited.
The Chairman mentioned that he had been asked by the Engineer-in-Chief how long it would take to
introduce a completely new design of telephone employing a non-standard handset, and his reply, based on experience of the 700 type telephone, had been that it would take between four and five years. This time was so long that the
P.O. might consider the introduction of a design using a standard handset as an interim
Mr.. Williams (R Branch) stated that acoustic tests on a new handset would take about three months, using two handsets and a sufficiently large number of subjects carrying out free
conversation tests. If two designs of hand set were tested simultaneously the total length of time required would be about four months.
The Chairman mentioned that he would like to retain the models for the time being but they were of course available if the manufacturers required
10th January 1962
938.1 Telephone No. 712 (SCP's 177, 201, 202, 203, 204) (Last minute 929.1)
The Chairman regretted that despite his best endeavours the position was unchanged since the last meeting as
regards the decision of the choice of the luxury telephone.
11th April 1962
945.1 Telephone No. 712 (SCP's, 177, 201, 202, 203, 204) (Last minute 938.1)
The Chairman reported that since the last meeting, approval had been given by the administration for transmission tests of two designs of
luxury telephone employing novel hand sets, Samples of these, designs had been received and were under test at
Dollis Hill, Mr. Williams (R Branch) hoped to have an interim assessment of the models available
shortly. He pointed out, however, that both of the models used the small carbon transmitter from the operator’s set, and he recalled that in the development of the operator’s set, trouble had been encountered with noise, and as a result the transmitter was restricted to a
current of below 70 milliamps. The two samples of luxury handset were sent to the Post Office as being intended for use with the telephone
No. 706 circuit, and this would mean their working at 100 milliamps on short lines, and indication were that this would not prove to be satisfactory. It would therefore be necessary to use some form of regulator circuit to restrict the current, Neither design would entirely meet
1000 ohm limit, so that it would not be possible to add any additional loss on long lines. Mr.
Towner (S.T.C.) replied that he was sure that this question must have received consideration, and he would look into
The Chairman recalled that the Post Office had written to the manufacturers, asking them for an estimate of the time that it would take to come into production once the decision had. been taken, and also asking them
for estimates of the likely price. Mr. Warren (M.S.S.C.) replied that the letter had been sent to the Manufacturers’ Commercial Committee, and he would follow the matter up and supply the information as soon as possible. Mr.
Towner (S.T.C.) enquired whether the price estimate should include provision for
electro luminescent lighting, and the Chairman replied that he thought the voltage required was likely to be inconveniently high and it would he better for this to be added to Mark II version rather than hold up matters.
11th July 1962
953.1 Telephone No. 712 (SCP's 177, 201, 202, 203, 204) (last minute 945.1)
Mr. Harding (S Branch) recalled that a short list had been agreed after the
C.O.I.D. had seen and commented upon the original models submitted by the manufacturers.
As a result of the comments made, new models had. been produced two by STC
and one by GEC and these were at present receiving the consideration of the P.O. administration. An addendum to
SCP 201 describing the new models had been submitted by STC and GEC would also submit an informative
Mr. Harding (S Branch) also mentioned that both sets under transmission test
used the Inset No. 15 which had previously been restricted to a feed current of about 70 milliamps and there were some doubts about serviceability of the Inset 15 when used under maximum line current conditions. A meeting
had been held between GEC, STC, R and S Branches to discuss the problem. One possible solution might he to devise a circuit limiting the current, but it seemed likely that this would. result in too high a loss to he acceptable for long lines. Another alternative was the use of a different type of
carbon and STC were investigating the use of pre-aged or finer granuled carbon. The P.O. had undertaken to try out Insets 15 in handsets subject to
full line current with the insets held by adapters in a plane similar to that which they would occupy in the luxury telephone.
GEC and STC were providing the P.O. with the necessary adapters. The Chairman confirmed that the P.O. did not rule out circuit changes if transmission limits were met.
In reply to a question from Mr. Warren (M.S.S.C.) regarding the 1000 ohm requirements, Mr.
Williams (R Branch) stated that tests showed the luxury telephones to be marginal and. further losses
could not be tolerated.
It was reported that Mr. Paren had written to the P.O. on the 1st May giving an estimate of the costs and production dates for various quantities of the luxury telephones.
10th October 1962
958.1 Telephone No. 712 (SCP's 177, 201, 202, 203, 204) (Last minute 953.1)
Mr. Harding (S Branch) said that the three models referred to earlier were considered by the Administration and later submitted to
C.O.I.D. In addition a further model described in SCP 203 Add 2 (now in process of duplication) was submitted by
GEC direct to the C.O.I.D. The C.O.I.D. comments on the various models had been circulated to the parties concerned. The I.T.D. comments on the first three models had. also been
forwarded but those for the latest GEC phone were still awaited.
On the transmission side tests were in hand on the Transmitter Inset No. 15 under maximum line current conditions. Arrangements had also been made for the supply of a sample number of Insets No.
15 with pre-aged carbon.
The latest GEC handset used the Transmitter Inset No. 13 or 16 and Receiver
4T, both at the correct nodal distance. It had. been noticed, however, that when holding this particular handset there was a tendency for the fingers to touch the cheek.
9th January 1963
964.1 Telephone No.712 (SCP's 177, 201, 202, 203 & 204) (Last minute 958.1)
Mr. Troke (S Branch) said that the further GEC model covered by SCP203 Add. 2
did not find great favour with the P.O. The various designs had continued to receive consideration by the P.O. administration who felt unable to make a decision without
having assistance from market research. This would take the form of showing static models to callers at sales offices and asking their opinions. For this purpose the P.O. would like to have three models each from
G.E.C. and STC. The P.O. would be writing to the manufacturers about this.
Trials on Transmitter Insets No. 15 in Handsets No. 3 in Telephones No. 706 were
continuing, and no complaints had been received with regard to noise. STC had. supplied some Insets No. 15 filled with
pre-aged. carbon and these were being assessed by Dollis Hill before being put into use in conditions of high line current.
A further STC design would. be the subject of an SCP addendum.
10th April 1963
973.1 Telephone No 712 (SCP's 201, 202, 203, 204.) (Last Minute 964.1)
Mr. Harding (S Branch) reported that market research would take place using 3 models of the S.T.C.
No. 6 version and. one of the S.T.C. No. 5 version, together with 3 models of the
G.E.C. design. The models were expected to be received. from both manufacturers within a month. A number of Insets 15 were on trial, some using ordinary carbon and some pre-aged carbon.
No complaints had so far been received about noise from Insets 15 mounted in adapters in the Handset No.
10th July 1963
983.1 Telephone No. 7 (SCP's177, 201, 202, 203, 204) (Last minute 973.1)
It was agreed. to discharge SCP's 202 and 204.
Mr. Harding (S Branch) said. that three non-working models of each of the two designs had
been received and were undergoing customer preference trials. Trials of Insets No. 15 using
normal and pre-aged carbon were proceeding with no complaints or reports of noise to date. Various
means of overcoming the higher signalling resistance of the Inset No. 15 were being considered by the
manufacturer. Mr. Sutton (S.T.C.) stated that S.T.C. would be forwarding another addendum shortly to
9th October 1963
990.1 Telephone No. 712 (SCP's 177, 201, 203) (Last minute 983.1)
Mr. Harding (S Branch) reported. that the customer preference trials were proceeding. Although the results were not yet conclusive there appeared to be a slight preference in favour of the
S.T.C. version. The Chairman commented that the two designs appealed to different tastes and. there might be a case for the adoption of more than one version. Mr.
Harding (S Branch) added. that trials of Insets No. 15 were continuing, and. noise complaints were being investigated.. Various means of overcoming the higher signalling resistance of the Inset No. 15 had been
considered, but the suggested. remedies were not suitable. Mr. Towner (S.T.C.) said that the results of tests on the last delivery of carbon had been satisfactory and.
samples would be sent to the P.O.
Mr. Harding (S Branch) said. that the P.O. required six handsets to test ,the acoustic performance, and also were considering the possibility of a field trial of 1,000. He asked
G.E.C. and S.T.C, to look into the question of whether these could be supplied.
8th January 1964
996.1 Telephone No. 712 (SCP's 177, 201 and 203) (Last Minute 990.1)
It was noted that G.E.C. was withdrawing from this field, and Mr. Harding (S Branch) thanked the Company for the work that it had carried out. Mr.
Harding (S Branch) went on to say that if the administrative decision was favourable it was proposed to place an order for 10,000 instruments; initially, 1,000 would be required for field trials. The trial instruments would
probably be supplied temporarily to existing subscribers without extra charge.
Mr. Towner (S.T.C.) confirmed that S.T.C. would be replying to the P.O. during the next week with regard to costs.
He said that the six handsets referred to at the last meeting could not be supplied in advance of production. They would, however,
be made as soon as the moulds were available. Mr. Harding (S Branch) reported that the P.O. had.
received 50 Insets No. 15 containing Morgan pre-aged carbon, and 50 containing ordinary carbon as controls. The Chairman expressed. concern that the production samples would not be available until August or September. He said that it was desirable to test the modified
Insets No. 15 in advance of production handsets for the purpose of investigating means of
overcoming the high signalling resistance.
8th April 1964
1003.1 Telephone No. 712 (SCPS 177, 201 and 203) (Last Minute 996.1)
The Chairman said that the public had shown considerable interest in the Telephone
No. 712 which had been on view at the Ideal Home Exhibition. He went on to say that the P.O. was anxious to investigate transmission problems, but this could not be done until moulded handsets were made available by the
manufacturer. Mr. Towner (S.T.C.) asked the P.O. to provide S.T.C. with interim results of the trials on Insets No. 15, Mr.
Harding (S Branch) agreed, although he thought that the results would be of only limited use because of the restricted conditions of the trial.
Mr. Towner (S.T.C.) remarked that S.T.C. would like some indication regarding the possibility of a firm order, or contract, for the Telephone No. 712. The Chairman replied that an administrative decision was still awaited. He pointed out, however, that the absence of a decision did. not imply that there was any fundamental fault with the instrument.
The Chairman said that interest had been aroused by the proposed illuminated dial. Referring to the radioactivity hazard presented by tritium, Mr.
Harding (S Branch) said that the original tritium tube had been accepted by the Ministry of Science, but the amount of tritium had since been reduced by
60%. Tests were at present being carried out by the P.O. but it appeared unlikely that the tritium would constitute. a hazard. Some precautions might, however, be
necessary when storing in bulk.
(Secretary's note: the manufacturers have since agreed to a P.O. proposal that CP 203 be discharged).
8th July 1964
1009.1 Telephone No.712 (SCP's 177 & 201) (Last Minute 1003.1)
The P.M.G. had approved the luxury telephone and it was expected that an official announcement would soon be made. Mr.
Harding (S Branch) said that it had been agreed to purchase 10,000 Telephones No.
712: 1000 would be required at an early date so that the Administration could carry out
market research, and to enable the E.D. to carry out proving trials. If modifications were found to be necessary, it was hoped that
they could be incorporated in the balance of 9000. In reply to Mr. Harding (S Branch), Mr.
Towner (S.T.C.) said that it was hoped to provide handsets, in August or September for
acoustic tests at Dollis Hill.
It was agreed to discharge SCP 177.
6th October 1964
SCP 201 Addendum 4 Luxury Telephone No. 712
Mr. Harding reported that six handsets had been received. and. had been passed to Dollis Hill for
transmission performance tests. He was a little concerned about the tone caller.
Under normal circumstances it was possible to operate up to six bells in series, but
it was only possible to operate one tone caller in series with one bell. A
mixture of tone callers and bells would be acceptable provided, signalling
limits were act exceeded. Mr. Towner was looking into the point raised by Mr.
Harding and he pointed out that there was a possible alternative by wiring in
The P.O. had as yet come to no conclusion regarding a name for the new telephone,
Trimphone and Deltaphone were two names which had been mooted by the P.O. and S.T.C. respectively.
14th July 1965
SCP 201 Addendum 4 Telephone No. 712
Mr. Harding reported that over 100 sets had been received, and he understood that others were at present being
factory tested. Mr. Towner said that models of the flow design of tone caller would be sent to the P.O. in about a month’s time. Some
production difficulties had been encountered, but they were now being resolved, and he expected more progress to be made when the factory resumed working after the holiday. The Chairman commented on the
effect upon estimating demand due to the three months delay in commencing the field trial.
13th October 1965
SCP 201 Addendum 4 Telephone No. 712
Mr. Harding reported. that the N.W. (London) Area field trial, which had been delayed, was expected to be
started within the next two weeks. Seventy instruments had recently been
installed in Headquarters offices and preliminary reports indicated that the lack of directional effect of the tone caller was likely to cause embarrassment in offices where two or more of the instruments were located. No solution to this problem was immediately foreseeable.
Six models of the new design of tone caller had been received by the P.O. initial test results were encouraging.
14th April 1966
SCP 201 Addendum Telephone No. 712
Mr. Seymour understood that public reaction to the instrument had been more favourable than anticipated, and an order for 30,000 of the Mark II version
was therefore being arranged. Internal modifications to the final version were in hand with S.T.C. Mr. Towner reported that the balance of the first 9,000 of the first order for 10,000 had recently commenced.
Mr. Williams mentioned that the Trimphone would appear in a forthcoming edition of the BBC TV program “Tomorrow’s World”.
13th July 1967
SCP 201 Addendum 4 Telephone Nos. 712 and 722
It was noted that the Mark IV version would be known as the Telephone No. 722.
12th October 1966
SCP 201 Addendum 4 Telephone Nos. 712 and 722
It was noted that Mr. M. C. C. Taylor had replaced. Mr. F. J. Harvey as
P.O.L.O. The Chairman confirmed that there was now no Mark III version of this development.
15th March 1967
SCP 201 Addendum 4 Telephone Nos. 712 and 722
The final model of the 722 telephone set had been received by the P.O. and S.T.C. had already been advised verbally
that approval had been given. This decision would be confirmed in writing.
Mr. Towner stated that S.T.C had been considering the possibility of a smaller dial for the Trimphone. The
considerations were mainly associated with export aspects and the object was to reduce costs and to save space in order to accommodate a ringer. It would still be possible to have a beta light on a smaller dial. Samples had been sent to the P.O. and Mr. Towner hoped that the P.O. would agree to
discuss the possibility further. Mr. Harding replied that P.O. would not wish to consider the development of further models of the Trimphone unless they offered considerable advantages over the Mark IV version. Although
attracted to the possibility of reducing costs and the choice of using a magneto bell or a
tone caller, Mr. Harding said that it must be borne in mind that the tone caller was an attraction from the sales point of view, and for this reason the P.O. would be reluctant to replace it.
The Chairman remarked that as the design life of a luxury telephone was regarded as being seven years he thought that it would be more appropriate to consider new ideas
such as the one proposed by S.T.C. in relation to a successor to the Trimphone. The P.O. would therefore bear
S.T.C.’s offer in mind. for future developments.
13th September 1967
SCP 201 Addendum 4 Telephone Nos. 712 and 722
It was noted that the second paragraph of the Liaison Officers’ report should be amended to
read: “Draft Drawings of the Transmission of Circuit Board have been circulated to the Manufacturers for
Mr. Harding reported that the performance of recent samples of
Handset No. 10 had been
noticed to be 2 dB down on the performance of Handset No. 8.
This appeared to be a function of the receiver rather than the Handset. The problem was being
pursued further by the P.O.