The Evolution of British Post Office Telephones
Taken from an article in the Post Office Telecommunications Journal Aug-Sept 1953

With later additions by R. Freshwater 2007
Click on any picture for a bigger view or more information

This article traces the development of the telephone instrument within the British Post Office alone and, as the history of the telephone is truly an international one, this is far from being the complete story of its evolution. Also, only the “main stream” of development within the Post Office is dealt with, so that many special purpose telephones have received no mention.

When Graham Bell invented the telephone the function of the Post Office was to provide postal and telegraph services and consequently the tendency was for it to be regarded as an auxiliary telegraph instrument. After a short period of complete indifference to the invention telephones were offered by the Post Office as alternative instruments on A.B.C. telegraph lines.
The type of telephone used for this service in 1878 is illustrated in Fig. 1. It was brought into circuit in place of the A.B.C. instrument by means of a switch and while it was recognised as being quicker and simpler to use than the telegraph instrument it was not considered to be as reliable. This telephone was a combination of a Bell receiver, very similar to Bell’s original invention, and a Crossley transmitter, a British invention which was one of a number of Carbon Pencil transmitters based on Professor Hughes’s theory of microphonic contact. It was used with a local battery and induction coil, which were contained within the telephone. The decorated top of the telephone was a guard for a large pinewood diaphragm on the rear of which the carbon pencils were fixed.

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Fig. 1 Fig. 2

Telephones of this type were soon superseded by Gower-Bell telephones like the one shown in Fig. 2. The Gower transmitter was another variation of the Carbon Pencil theme only with more pencils than the Crossley. The receiver worked on the Bell principle, but was considerably larger and heavier than the conventional receiver, so much larger and heavier in fact that it could not be held to the ear in the usual manner but was listened to through speaking tubes. The change from the conventional receiver of the earlier telephone to speaking tubes sounds a strange step today but in those days speaking tubes were quite familiar and their use was thought justified by the increased power of the new receiver. Much later in the history of the telephone the arrangement of the magnets in this receiver was repeated, on a much smaller scale, in the design of auxiliary “watch” receivers.

The Gower-Bell telephone remained the Post Office choice for many years, and was continuously developed. By 1891 it had become known as the Universal Telephone, so-called because it could be adapted for use under practically any conditions likely to be met with in Post Office service. It retained the Gower Transmitter but the receiver and speaking tubes had given place to a pair of Bell receivers. The calling signal was a trembler bell which was rung by a battery at the subscriber’s premises under the control of a relay operated by a signalling current received from the exchange. A feature of this system of working which has a modern sound was that when an operator rang the subscriber’s bell she could hear the interruption in current caused by the trembler bell contact, a forerunner of ringing tone.  While the Post Office was providing the telephone as an alternative telegraph service private companies sprang up throughout the country for the purpose of providing telephone exchange service proper. The companies had the right of use of the vital telephone patents, Bell’s receiver patent, the Carbon Microphone patent of Edison, and later the patent for Blake’s transmitter, and their eventual choice of instrument was one using Bell’s receiver with a Blake transmitter, as is illustrated in Fig. 3.

In the 1880s important legal decisions were made which confirmed that the Postmaster General’s monopoly in providing telegraphic communication extended also to telephonic communication and placed the destiny of the telephone in Britain in the hands of the Post Office. The Post Office straightway announced its willingness to provide telephone exchange services and in pursuance of this policy converted a number of A.B.C. Telegraph switching centres to Telephone Exchange working, using Gower-Bell telephones, but it was in no position to fill the void that would have been created had the private systems been closed down. Consequently licences were issued to the private companies and their activities continued until 1912, although several local authorities continued for some years to operate their own telephone systems under licence and one (Hull) still does so.

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Fig. 3 Fig. 4

Patent difficulty
The Post Office’s efforts to compete with the private companies were hampered by the restricted choice of instruments imposed by the possession by its rivals of the telephone patents which did not expire until 1890-91, and the small scale of the Post Office telephone activities up to this time can be judged by its total of 4,691 working exchange lines.
The year 1889 was a landmark in the history of telephony, for Deckert, an Austrian, patented his Granular Carbon transmitter. It was by no means the first transmitter of the type. The original invention had been made by the Rev. Hennings in England as early as 1878, but the earlier granule transmitters, while being powerful compared with other types, had suffered from “packing” of the granules and many inventors had wrestled with this problem before Deckert found a solution to it. A good carbon granule transmitter is 1,000 times as powerful as the Bell receiver, which had been used as a transmitter in the very early days of telephony, and without this great increase in power long-distance telephony before the age of electronic amplifiers would have been impossible.

The Deckert transmitter was adopted as their standard by both the Post Office and the private companies, now amalgamated to form the National Telephone Company. It was a mixed blessing for the Company, as its increased power caused serious overhearing between lines on earth return systems, which they used to economise in line plant, and it had to be confined to metallic circuits only. The Post Office had always been worried about the possible effects of induction and had installed metallic circuits from the beginning so that this difficulty was not met with.

Another innovation of this time was the Handset. Ericsson in Sweden had thought of combining the transmitter and receiver in a handset in 1884 but the transmitters of the day did not take kindly to being moved about and the receivers were bulky so that the idea was not much used. With the coming of the Carbon Granule transmitter, and of improved magnetic materials for receivers the idea became practicable; the movement of the transmitter was actually advantageous, as it mitigated “packing”. Both the Post Office and the Company put into service telephones using the Ericsson handset; a Company telephone of 1900 fitted with one is shown in Fig. 4. The handset is fitted with a carbon granule transmitter similar to the Deckert but of Ericsson design. Handset telephones of this type were very popular with users and they remained in service for some purposes until very recently.

Post Office activity in the telephone field increased greatly with the turn of the century. The Telegraph Act of 1899 restricted the Company’s activity to those areas where it had already established exchange services, so that the onus of providing service elsewhere, mainly rural areas where the numbers to be served did not make the system economic, fell upon the Post Office. In addition, the Post Office set out to provide a telephone service within London alongside the Company’s. The time was very propitious for large scale expansion as the Common Battery system had just been invented in America and had proved to be much superior to the older systems. The Post Office was able to introduce telephones for the C.B. system, based on American designs, which were destined to remain unchanged in principle for very many years.  At this time the Post Office started to number its telephones in the series that is still being used.

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Fig. 5 Fig. 6

Telephones No. 1 and No.2 are illustrated in Figs. 5 and 6. They were both designed for C.B. working, being electrically identical, and illustrate a Post Office policy, still followed, of giving the subscriber a choice between a table telephone and a wall telephone. The wall telephone was a complete installation in itself but the table telephone had to be used with a bell-set, the “Candlestick” shape (used for its economy in desk space) having no room for the bell, capacitor and induction coil.
Electrically the most important feature of these telephones was the transmitter. This was a carbon granule transmitter invented by A. C. White, an American, and known as the “Solid Back” transmitter. It was a most efficient transmitter for its day and remained as the standard Post Office transmitter for C.B. use until displaced by the modern inset transmitter in 1930.

Handset advantages
It will be noticed that neither of the telephones in Figs. 5 and 6 uses a handset. There were two main reasons for this. First, the handset has to be made with the distance between mouthpiece and earpiece large enough to cater for the unusual head so that, for the average user, the distance between the lips and the transmitter is greater than that obtainable with separate receivers and transmitters and there is some loss of transmitting efficiency. With the increasing number of trunk calls which, as the trunks were not amplified, demanded the utmost transmission efficiency, this could not be tolerated. Secondly, carbon granule transmitters of the time varied considerably in resistance with changes in position and might even become disconnected when the plane of diaphragm was horizontal. On the C.B. system the transmitter forms part of the signalling circuit and high resistance or disconnections in it give false clearing signals to the exchange. Owing to these shortcomings of the handset, both the Post Office and the Company standardised on fixed or hinged transmitters for C.B. use.

While using the C.B. systems for large exchanges the Post Office evolved new systems for small rural exchanges. These were known as Central Battery Signalling systems and while they gave the signalling advantages of the C.B. system the transmitter was fed with current from a battery which was part of the telephone instead of from the main battery at the telephone exchange. The advantage of this was that it allowed the use of small primary batteries at the telephone exchange instead of larger secondary cell installations, the charging of which presented problems before the electricity supply mains were as widespread as now.

For the C.B.S. systems, telephones No. 3 and No. 4 were designed. They were the local battery counterparts of telephones No. 1 and No. 2, which they resembled physically, but were fitted with a special transmitter for use with local batteries. This transmitter was made in the form of an easily replaced capsule known as a transmitter inset No. 3. Transmitters made in this way are not repairable and when faulty are scrapped. They are cheaper to make initially and the ease of replacement is a valuable maintenance feature. Some “Solid Back” transmitters were also made in inset form and in modern times the use of inset transmitters is universal.

When taking over the Company’s system in 1912, the Post Office inherited, besides some C.B. exchanges, a large number of magneto exchanges and their associated instruments. An essential part of a magneto telephone is a hand generator by which the subscriber calls and rings off. A telephone which incorporated this bulky component in an ingenious way is illustrated in Fig. 7. This was an Ericsson telephone, nicknamed the “Skeleton”, which was absorbed into the Post Office series as telephone No. 16. The decorative legs are actually the magnets of the generator.

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Fig. 7 Fig. 8

Oldest in use
The Post Office had introduced a magneto instrument, telephone No. 11, into service in 1902. This was very similar to the C.B.S. wall telephone, telephone No. 3, with the addition of a generator, and it is illustrated in Fig. 8. It bears the distinction of being the oldest Post Office telephone still in current use, being the standard wall instrument for those places where the magneto system lingers on.

The first major change in the main series of Post Office telephones was caused by the introduction of automatic working which, while the automatic system was fundamentally C.B., required the addition of a dial. Between 1914 and 1924 several telephone dials and circuits were tried and eventually standard telephones were introduced. The new telephones were coded Nos. 150 and 121 and are illustrated in Figs. 9 and 10. Apart from the provision of dials they differed little from their predecessors of the C.B. system and in 1929 these were superseded by the automatic instruments, the dial being replaced by a dummy for C.B. working. This has the advantages that the number of different telephones is reduced, and that when a C.B. system is converted to automatic working it is necessary only to replace the dummy by a dial instead of changing the instrument. These two telephones are still the Post Office standard telephones.

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Fig. 9 Fig. 10

The first 30 years of the 20th century had been a period of extraordinary stability in telephone design, for the transmitter and receiver being used in 1930 were the same as those introduced at the beginning of the C.B. system. Things now began to stir however and the next Post Office telephone was revolutionary in many ways.

The new shape, which was first seen in 1929, was that of telephone No. 162, illustrated in Fig. 11. The Post Office, in co-operation with a manufacturer (an innovation of its own), designed and developed it specifically for its own services. It involved the return of the Handset, made possible by the design of a new more sensitive transmitter and a new receiver, and used a circuit designed to reduce sidetone. (Sidetone is the hearing in the receiver of sounds picked up by the transmitter of the same telephone. It has a deleterious effect on receiving efficiency for the background noise picked up by the transmitter at the receiving end tends to mask the wanted sounds from the distant end. It also has an adverse effect on the transmitting efficiency of a telephone, for its presence in the receiver causes a talker to lower his voice subconsciously. Reducing the loudness of the sidetone results in the speaking level being kept up with consequent increase in transmission.
This is similar to the often observed effect which causes some deaf people to talk more loudly than people with normal hearing.)

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Fig. 11 Fig. 12

Emphasis on intelligibility
The transmission elements of the new instrument were designed with emphasis on intelligibility rather than on sheer loudness of the transmitted sounds, which had been the criterion of efficiency in the past.  The new transmitter is of the carbon granule type, made up in inset form, but it differs from earlier ones in that it uses the immersed electrode principle to minimise changes in resistance caused by changes of position, and even when it is held with the diaphragm horizontal the signalling path through it is maintained.

Physically the telephone was a table-set made up largely from plastic mouldings. Like the candlestick it does not contain all the essentials of the subscriber’s installation and has to be used with a bell set. For the new telephone an entirely new bell set was designed the cover of which is also a plastic moulding. While suitable for independent fixing to a wall the bell set can also be attached to the base of the telephone to form a combined set, the bell set cover and the telephone case being shaped to form a pleasing assembly as shown in Fig. 12. Besides its use as a table model the new telephone, when mounted on a specially designed moulded plastic bracket, is also suitable for fixing to a wall so that a wall telephone equivalent of the new design is not necessary. The use of plastics for the telephone enabled it to be made satisfactorily in different colours, for a plastic is coloured red right through, and eventually telephones in Ivory, Jade Green and Chinese Red were made as alternatives to the usual black.

Further improvements
At the same time as this handset telephone was introduced to service a change was made to the telephones Nos. 150 and 121 so that the new inset transmitter could replace the old “Solid Back” and the telephone illustrated in Fig. 9 actually shows this change. The efficiency of these telephones was thus improved but they still suffer from the ill effects of sidetone. A Local Battery equivalent of the telephone No. 162 was also introduced under the title telephone No. 196.

Slight changes have been made to these handset telephones since they were first designed. The circuit has been changed to a more efficient anti-sidetone one and a new and more efficient receiver with an improved frequency response has been fitted. The only externally apparent change is the fitting of a sliding drawer in the base of the telephone which is used for keeping lists of dialling codes made necessary by the growth of the automatic system.  These were introduced in the No. 232 type.

The No. 162 and 232 type of telephone was followed in 1938 by a telephone the shape of which originated in Sweden. It is a combined set, a desirable feature for installation purposes, and is neater than a combined set made up from the No. 162 type of telephone and its bell set, but is not so suitable when the telephone and bell are wanted in different places, as an extra bell then has to be used. The handset and electrical circuit are identical with later versions of the previous telephone so that there is no improvement in transmission. A valuable feature of the design is that up to three push button keys can be fitted to it and these can be used for auxiliary purposes in extension working. The telephone illustrated in Fig. 13 uses them to control an extension bell, and the key facility also makes the telephone very suitable for some kinds of shared service working.

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Fig. 13 Fig. 14

Telephones of this shape are too long to be used on brackets as wall telephones, but hitherto this has not mattered as telephones of the earlier shape met the need. Now, however, the widespread use of shared service has created a demand for a wall telephone to which keys may be fitted and a new telephone is being designed for this purpose. A prototype of this telephone of the near future is illustrated in Fig. 14 in which one of the calling keys may be seen.

The standard of transmission provided by present day instruments is generally accepted as striking the right balance between economy in instruments and line plant on the one hand, and effective transmission of speech on the other. Any improvements to transmitters, receivers and circuits will probably be used to allow increased resistance of line between the telephone and the exchange (the performance of telephones falls as line length increases) so giving economies in line plant and possibly numbers of exchanges. The transmitting power and receiving sensitivity of telephones are ultimately limited by overhearing on the lines to which they are connected and it may be necessary to include a regulating device in any telephones much more efficient than the present ones to prevent the limits being exceeded when they are used on short lines.

In recent years the contribution of the higher frequencies of speech towards intelligibility has been fully recognised and the frequency response of transmitters and receivers has been improved. The limit of worth-while improvement in this direction has almost been reached, however, for it has been found that little increase in intelligibility is achieved by transmitting speech frequencies above 3,500 cycles per second so that there is no economic justification for making telephones which transmit and receive higher frequencies. It has always to be remembered that a new telephone for general use will have to work alongside five or six million older telephones for perhaps 20 years, so that no violent break with the past, such as a big change in the balance between transmitting and receiving sensitivities, is possible.


The late 1950's onwards
During this decade the Post Office worked with numerous manufacturers to produce a modern, stylish phone with much improved transmission qualities.  This phone started life as the No. 700, but was new circuitry, in a No. 300 type case with a modern handset.  It just didn't look right, and with the advent of of modern plastics the phone changed completely and in 1959 two variants were produced: one with conventional circuit wiring and the other with a printed circuit board.  This phone was the No. 706, had provision for 2 buttons a new rocking armature receiver and the No. 16 transmitter, which was an improvement on the transmitter produced in 1929.  About a year later a wall phone, the No. 711, which was designed along similar lines went into production.  A four button phone known as the No. 710 was also produced and this brought the number of standard phones down to three, a considerable cost saving in stock for such a large organisation.

As well as the No. 706 phone the Post Office was asking manufacturers to present a new design bed side phone.  This phone was to be modern and could vary in style and technology, if the manufacturers could produce to a price specified by the PO.

1960's are here
The bed side phone was field trialed around 1965 and was called the No. 712.  It had a tone caller, an illuminated dial and was sleek and very, very modern.  This phone was eventually modified to get round technical problems and became the No. 722 - the Trimphone. It also attracted a premium rental charge.

In the mean time field trials were taking place with keyphones, where the dial was replaced by buttons.  These phones had restrictive use as they were costly to produce or had a signalling system that was not compatible with most telephone exchanges.  It would be a while before they were produced in quantity.

Miniaturisation of electrical components was taking place and the No. 706 was replaced by the No. 746 in 1969, a cost reduced phone with a smaller printed circuit board.  This phone had the same circuitry as the No. 706 but was produced with an integral handle in the casing.  A wall phone and a four button phone were also produced and these where the No. 741 and No. 740.

Towards Liberalisation
Although the phones were to a good quality the Post Office was always being pushed to expand the range and in 1974 the No. 776 - The Compact Phone - was made available.  This phone was once again electrically similar to the No. 746, but had a modern handset, that would be seen again on the later Ambassador and Statesman telephones.  It also had an external bell, something that had not been seen since the No. 232.

The No. 776 could be supplied with a wall mounting bracket that would accept the bell and in 1978 a special edition was produced in a colour called Balmoral Blue for the Queens Silver Jubilee.

Towards the mid 1970's miniaturisation and semiconductor technology was producing small discrete components and the 700 style phones were produced with press buttons.  To start with the Loop Dial version had a battery, that was charged from the exchange line, but this was superseded by a self contained model with no battery.  Muti-frequency 4 phones (MF4) were also produced and these were generally fitted to the new ranges of modern electronic telephone systems installed on customers premises.

But still the public wanted more choice and as the Post Office Telecommunications was privatised to become British Telecommunications, a new range of phones were made available on rental and for outright sale.   These these new phones, known as Special Range Telephones (SRT's) were normally named by Manufacturer and Model name.  Originally fitted with a Plug No. 420, to allow easy removal, they were later supplied with plugs to fit the new style sockets.

Outright sale was a new concept in the UK Public Network and changes were made to UK wiring practices to allow for a phone to be easily removed, thus opening up the market to open competition on telephone instrument sales.

The Ambassador Telephone was then introduced, which look square and chunky.  It was a self contained keyphone and it was thought that it may supersede the 700 range of phones.  It was not cheap to produce, but it could be wired to suit various extension plans.  A cheaper version was introduced but this was short lived as the Statesman came on stream and beat the Ambassador for price.  It was also becoming obvious that the extension plans had out lived their usefulness and that standard telephones could be introduced with just a switch for earth or time break recall, which would suit most telephone systems, so the Ambassador was dropped.

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Last revised: December 19, 2010