The one-thousand-line private automatic telephone exchange at Woolwich, 1914

It is necessary, at the outset, to record with due precision that the particular installation here under consideration is the one-thousand-line private automatic telephone exchange erected by Siemens Brothers and Company Limited at their Woolwich works in the year 1914. This equipment, which employed a development of the Strowger step-by-step principle as modified by Siemens and Halske of Germany and subsequently adapted by the British firm, is of first importance to the student of early internal automatic telephony in the United Kingdom, being the first private automatic branch exchange, or P.A.B.X., to be placed in service in Britain and one of the first two British automatic exchanges of any kind to provide ringing tone to the calling party. The companion installation of one hundred and fifty lines, completed the previous year at King’s College Hospital, Denmark Hill, shares that distinction in respect of tone provision, but the Woolwich board, by reason of its greater capacity and its experimental interconnection with the public network, occupies a position of particular significance in the documentary record.

The underlying switching method originated with the patents of Almon B. Strowger of Kansas City, granted in the United States between 1889 and 1892, and first demonstrated in commercial form at La Porte, Indiana, on the third of November 1892. The system employed two-motion selectors in which a wiper assembly was first lifted vertically by a stepping magnet responding to dial impulses and thereafter rotated horizontally across a bank of contacts, commonly arranged in ten or twenty levels of ten contacts each. Linefinders or, in later British practice, uniselectors performed the initial hunting function. In the United Kingdom the first public application of this principle occurred at Epsom, Surrey, on the eighteenth of May 1912, the equipment being supplied by the Automatic Telephone Manufacturing Company of Liverpool and having an initial capacity of five hundred lines with an ultimate designed capacity of one thousand five hundred lines. A second Strowger installation, known as the Official Switch, was brought into service at Post Office headquarters, St Martin’s-le-Grand, on the thirteenth of July 1912, equipped initially for nine hundred lines with the same ultimate capacity of one thousand five hundred. Both of those exchanges employed Keith plunger-type line switches and vertical-pattern group and final selectors of American pattern.

Siemens Brothers, whose Woolwich establishment had been founded in 1863 for the manufacture of submarine telegraph cables and which by 1914 employed some two thousand persons, had, prior to the outbreak of war, maintained close technical relations with Siemens and Halske. The German firm had acquired the German patent rights in the Strowger designs and had introduced a number of modifications. These included the rotary preselector, later standardised by the British Standards Institution under the designation uniselector, which served as a linefinder and permitted a reduction in the quantity of two-motion apparatus; a central motor-driven interrupter as the source of stepping pulses in place of self-driving mechanisms on each switch; two-motion selectors controlled entirely by relays and lacking the side-switch of the original American design; and the provision of both dial tone, indicating that the exchange equipment had been seized and that dialling might commence, and free tone, known in German as Freizeichen, indicating that the called line was idle. The Siemens Brothers installations of 1913 and 1914 were the first in Britain to furnish ringing tone. They also employed German pattern handset telephones rather than the pedestal or candlestick instruments supplied by Automatic Telephone Manufacturing for the Epsom and Official Switch exchanges.

The telephone-equipment factory at Woolwich in which the one-thousand-line board was assembled had itself been constructed in 1911–1912 as a five-storey building, employing the Kahn system of reinforced concrete, immediately to the south of the earlier cable works of 1910–1911. The building contained laboratory and experimental rooms for a newly recruited staff of telephone engineers. Siemens Brothers took out its first British telephone-exchange related patent in 1913. The Electrical Review of the twenty-seventh of February 1914 recorded that the Post Office had, as a rather special favour, permitted the Woolwich automatic exchange to be coupled to the public exchange system for experimental purposes. This arrangement necessitated the addition of a private manual exchange through which Post Office calls were passed to the local stations, the automatic apparatus being cut out of circuit when a line was through to the Post Office. The same principle of hybrid working was later provided on certain Siemens and Halske private automatic exchanges supplied in this country by Automatic Internal Telephones Limited.

The outbreak of war in August 1914 interrupted contact with the German design department and, following the Trading with the Enemy Act, the British state expropriated the controlling German interest in Siemens Brothers in 1916. Orders for telephone exchanges virtually ceased for a period, the works being directed instead toward field-telephone cable and related military requirements. Nevertheless the firm later supplied a Siemens-system public exchange of one thousand eight hundred and sixty lines, ultimate capacity four thousand, at Grimsby, opened on the fourteenth of September 1918, and further public exchanges at Stockport in 1919 and Southampton in 1923. The Woolwich private exchange of 1914 therefore stands as an isolated but technically complete example of the pre-war Siemens adaptation of Strowger practice applied to internal communication on an industrial scale.

Subsequent private automatic equipment produced by Siemens Brothers continued the same general principles. By December 1931 the company had introduced three new private automatic equipments of ten, twenty-five and fifty lines respectively. In July 1932 the Engineering Supplement to the Siemens Magazine, number 86, described a larger type designated P.A.X. No. 21, having a capacity of one hundred lines. That equipment employed relays and two-motion switches together with one uniselector acting as a call-distributor. The operating voltage range was from twenty volts to thirty volts. Finder-connectors could be fitted to provide for a maximum of ten simultaneous calls. The rack was of floor type, mounted on a chassis provided with rollers. A back-plate fixed to the wall carried a distribution frame consisting of two sets of terminal blocks. The cover enclosing the equipment was of metal-faced plywood; only the front was removed when attention was required, the sides and top remaining attached to the rack, while the rear halves of the sides were arranged to swing outwards on hinges to permit the opening of the gates on which the line relays and common equipments were mounted. The overall dimensions of the P.A.X. No. 21 were five feet five inches high by two feet eight inches wide by one foot eleven inches deep. It was stated at the time to possess the smallest dimensions of any equipment of one hundred lines capacity then on the market. Relays were provided with twin contacts throughout.

These later compact units illustrate the continuity of design from the large open-rack installation of 1914. The early Siemens exchanges used open-type single-sided racks, a practice that the company also applied to its first public exchange at Grimsby and that subsequently became standard for British electromechanical equipment. Numbering on private systems frequently departed from Post Office conventions; extensions might commence with the digits one, nine or zero, and mixed one-, two- or three-digit numbering was employed. Special facilities commonly offered on Siemens and competing Automatic Telephone Manufacturing private exchanges included conference circuits, code-call systems whereby an official away from the instrument could be summoned by lights or buzzers, watchman’s supervisory service, executive right-of-way or break-in, fire and emergency alarm circuits, and tie-lines linking two or more private exchanges. On Siemens Brothers systems the zero level was normally taken to second selectors, codes such as double-zero being used for tie-lines.

It should be recorded that alternative automatic systems were also under trial in the same period. The Lorimer system, of Canadian origin, employing a rotary switch with a one-hundred-point cylindrical bank, electromagnetic clutches and continuously rotating shafts, was installed as a public exchange at Hereford in August 1914 with an initial capacity of five hundred lines and an ultimate of nine hundred. The Western Electric rotary system was placed in service at Darlington on the tenth of October 1914 with eight hundred lines, ultimate two thousand two hundred and sixty. The all-relay system of G. A. Betulander and N. G. Palmgren, exploited in Britain by the Relay Automatic Telephone Company (originally the Betulander Automatic Telephone Company, established by Marconi’s Wireless Telegraph Company Limited in 1913), was considered by the Post Office to be better suited to small private automatic branch exchanges; the first such installation for the Post Office was brought into service at Debenhams, Wigmore Street, London, on the eighth of December 1923. The Relay Automatic Telephone Company also supplied private exchanges from 1922, including a fifty-line board at the Liverpool Courier and a seventy-line board at North Eastern Marine Engineering. The Post Office, after comparative trials that included the Strowger, Lorimer, rotary, Siemens and relay systems, standardised on a development of Strowger step-by-step working for the public network in 1922.

The Woolwich one-thousand-line exchange of 1914 therefore occupies a defined place within this sequence of experimental and commercial installations. Its capacity of one thousand lines placed it among the larger private systems of the period; contemporary Automatic Telephone Manufacturing private installations of 1913–1919 ranged from twenty-five lines at Polmaise to five hundred lines at Rosyth. The Official Switch of 1912, although of comparable ultimate capacity, served Post Office internal traffic rather than a manufacturing works. The Siemens equipment introduced tone signalling and handset instruments to British automatic practice two years before the first London director exchange, Holborn, was cut over on the twelfth of November 1927. The director system itself, a register-translator development of Strowger working adopted for London and later for Manchester (1930), Birmingham (1931), Glasgow (1937), Liverpool (1941) and Edinburgh (1950), lies outside the scope of purely internal exchanges but illustrates the subsequent public-network application of the same two-motion selectors.

Preservation of knowledge concerning the 1914 Woolwich board is rendered more difficult by the subsequent demolition of the five-storey telephone-equipment factory in which it was constructed. The adjacent cable factory of 1910–1911 at 37 Bowater Road survives and is listed at Grade II, but the exchange apparatus itself has long since been recovered. Surviving documentation consists principally of the contemporary notice in the Electrical Review, the later accounts in the Post Office Electrical Engineers’ Journal, and the descriptions of successor private automatic exchanges such as the P.A.X. No. 21 of 1932. The student must therefore reconstruct the arrangement from the known characteristics of Siemens practice of the period: uniselector preselection, relay-controlled two-motion selectors without side switches, central pulse generation, open single-sided racks, operating voltages in the region of twenty to thirty volts on later compact units, and the hybrid manual-automatic working required for public-network access. These details, together with the exact dates of the companion King’s College Hospital exchange of 1913, the Epsom public exchange of 18 May 1912, the Official Switch of 13 July 1912, the Grimsby public exchange of 14 September 1918, and the first Relay Automatic Telephone Company Post Office P.A.B.X. of 8 December 1923, constitute the essential chronological and technical framework within which the Woolwich one-thousand-line installation of 1914 must be understood. Further research into original circuit diagrams, rack layouts and traffic records, should they come to light, would permit a still more complete account of this early and fully documented example of British private automatic telephony.