![Uniselector Stepper detail](https://upload.wikimedia.org/wikipedia/commons/8/8d/Uniselector_Stepper_detail.jpg)
The Strowger switch is the first commercially successful electromechanical
stepping switch
In electrical control engineering, a stepping switch or stepping relay, also known as a uniselector, is an electromechanical device that switches an input signal path to one of several possible output paths, directed by a train of electrical puls ...
telephone exchange system. It was developed by the
Strowger Automatic Telephone Exchange Company founded in 1891 by
Almon Brown Strowger. Because of its operational characteristics, it is also known as a step-by-step (SXS) switch.
History
Strowger, an
undertaker
A funeral director, also known as an undertaker (British English) or mortician (American English), is a professional involved in the business of funeral rites. These tasks often entail the embalming and burial or cremation of the dead, as ...
, was motivated to invent an automatic telephone exchange after having difficulties with his telephone service. He became convinced that the
manual telephone exchange operators were deliberately interfering with his calls, leading to loss of business. According to the local
Bell Telephone Company manager Herman Ritterhoff, Strowger swore to "get even" with the telephone operators and "put every last one of them out of a job." Ritterhoff claimed that the real cause of Strowger's difficulties was a metal sign hung on his wall over his telephone, causing an intermittent short circuit when blown by the wind.
Strowger conceived his invention in 1888, and was awarded a patent for an
automatic telephone exchange
telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...
in 1891. The initial model was made from a round collar box and some straight pins.
While Almon Strowger devised the initial concept, he was not alone in his endeavors and sought the assistance of his brother Arnold, nephew William, and others with a knowledge of electricity and financing to realize the concept. The
Strowger Automatic Telephone Exchange Company was founded in 1891.
In the original design patent, four keys were added near the telephone, one each for thousands, hundreds, tens and units, with each key having an additional wire connection to the central exchange. Each key had to be tapped the correct number of times to step the switch and make the desired connection. To connect to number 1256, the user would press the first key once, the second key twice, the third key five times and the final key six times.
The company installed and opened the first commercial exchange in his then-home town of
La Porte, Indiana on November 3, 1892.
The exchange had around 75 subscribers. The installation followed the original patented design, with four keys and four additional line wires connected to the exchange, but not all of the keys were used.
Early advertising called the new invention the "girl-less, cuss-less, out-of-order-less, wait-less telephone".
In 1896 the company patented the finger-wheel dial as an improvement to the existing four-key design.
The Strowger Automatic Telephone Exchange Company became the
Automatic Electric Company
Automatic Electric Company (A.E. Co.) was an American telephone equipment supplier primarily for independent telephone companies in North America, but also had a worldwide presence. With its line of automatic telephone exchanges, it was also a lo ...
, which Strowger was involved in founding, although Strowger himself seems not to have been involved in further developments. The Strowger patents were exclusively licensed to the Automatic Electric Company. Strowger sold his patents in 1896 for US$1,800 and sold his share in Automatic Electric in 1898 for US$10,000. His patents subsequently sold for US$2.5 million in 1916. Company engineers continued development of the Strowger designs and submitted several patents in the names of its employees.
The Strowger system was widely used until the development of the more reliable
crossbar switch, an electromechanical switch with a matrix of vertical and horizontal bars and simpler motions.
Patent details
Strowger's patent specifies dialing equipment at the customer location and the switching equipment at the central office.
The telegraph keys or telephone dial creates trains of on-off current
pulses
In medicine, a pulse represents the tactile arterial palpation of the cardiac cycle (heartbeat) by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed near the surface of the body, such as at the nec ...
corresponding to the digits 1-9, and 0 (which sends 10 pulses). This equipment originally consisted of two telegraph keys engaged by
knife switch
A knife switch is a type of switch used to control the flow of electricity in a circuit. It is composed of a hinge which allows a metal lever, or knife, to be lifted from or inserted into a slot or jaw. The hinge and jaw are both fixed to an ...
es, and evolved into the
rotary dial
A rotary dial is a component of a telephone or a telephone switchboard that implements a signaling technology in telecommunications known as pulse dialing. It is used when initiating a telephone call to transmit the destination telephone number ...
telephone.
The central office switching equipment has a two-motion
stepping switch
In electrical control engineering, a stepping switch or stepping relay, also known as a uniselector, is an electromechanical device that switches an input signal path to one of several possible output paths, directed by a train of electrical puls ...
. A contact arm is moved up to select one of ten rows of contacts, and then rotated clockwise to select one of ten contacts in that row, a total of 100 choices. The stepping motion is controlled by the current pulses coming from the originating customer's telegraph keys, and later from the rotary dial.
Two-motion mechanism
The Strowger switch has three banks of contacts. Toward the upper end of each shaft are two
ratchet
Ratchet may refer to:
Devices
* Ratchet (device), a mechanical device that allows movement in only one direction
* Ratchet, metonomic name for a socket wrench incorporating a ratcheting device
* Ratchet (instrument), a music instrument and a ...
s. The upper one has ten grooves, and raises the shaft. The lower one has long vertical teeth (on the other side, hidden).
The Strowger switch uses two telegraph-type keys on a telephone set for dialing. Each key requires a separate wire to the exchange. The keys are tapped to step the switch in two stages. The first set of incoming pulses raises the armature of an electromagnet to move a shaft which selects the desired level of contacts, by engaging a pawl with the upper ratchet. Another pawl, pivoting on the frame, holds the shaft at that height as it rotates. The second set of pulses, from the second key, operates another electromagnet. Its pawl engages the (hidden) vertical teeth in the lower ratchet to rotate the shaft to the required position. It is kept there against spring tension by a pawl pivoted on the frame. When the switch returns to its home position, typically when a call is complete, a release magnet disengages the pawls that hold the shaft in position. An interlock ensures that the spring on the shaft rotates it to angular home position before it drops to its home position by gravity.
Development
![Télégraphie, Systeme Strowger](https://upload.wikimedia.org/wikipedia/commons/2/24/T%C3%A9l%C3%A9graphie%2C_Systeme_Strowger.jpg)
The commercial version of the Strowger switch, as developed by the Strowger Automatic Telephone Exchange Company, used a
rotary dial
A rotary dial is a component of a telephone or a telephone switchboard that implements a signaling technology in telecommunications known as pulse dialing. It is used when initiating a telephone call to transmit the destination telephone number ...
for signalling to the exchange. The original final selector (connector) switch which connected to 100 customers was supplemented by preceding group selector stages, as the "cascading" enabled connection to many more customers, and to customers at other exchanges. Another requirement for commercial systems was a circuit to detect a busy connection (line) and return a
busy signal
Reanno Devon Gordon (born 24 January 1979), better known by his stage name Busy Signal, is a Jamaican dancehall reggae artist.
Biography
Reanno Devon Gordon p/k/a Busy Signal was born in Saint Ann Parish,Johnson, Carolyn (2006)A Very Busy Chi ...
to the calling subscriber.
Instead of dedicating an expensive first-stage selector switch to each customer as in the first exchange, the customer was given access to the first-stage switch of a telephone network, often by a
line-finder which searches "backward" for the calling line; so requiring only a few relays (in most cases two, a Line, and a Cut-off relay), for the equipment required for each customer line.
Later Strowger (SXS) exchanges often use a subscriber
uniselector
In electrical control engineering, a stepping switch or stepping relay, also known as a uniselector, is an electromechanical device that switches an input signal path to one of several possible output paths, directed by a train of electrical pulse ...
as part of the line equipment individual to each line, which searches "forward" for a first selector. This is more economical for higher calling-rate domestic or business customers, and has the advantage that access to additional switches can readily be added if the traffic increases (the number of linefinders serving a group is limited by the wiring multiple installed). Hence exchanges with subscriber uniselectors were usually used at British exchanges with a high proportion of business customers, e.g.
director
Director may refer to:
Literature
* ''Director'' (magazine), a British magazine
* ''The Director'' (novel), a 1971 novel by Henry Denker
* ''The Director'' (play), a 2000 play by Nancy Hasty
Music
* Director (band), an Irish rock band
* ''D ...
exchanges, or in New Zealand where the provision of local free calling meant that residential customers had a relatively high calling rate.
The fundamental modularity of the system combined with its step-by-step (hence the alternative name) selection process and an almost unlimited potential for expansion gives the Strowger system its technical advantage. Previous systems had all been designed for a fixed number of subscribers to be switched directly to each other in a mesh arrangement. This became quadratically more complex as each new customer was added, as each new customer needed a switch to connect to every other customer. In modern terminology, the previous systems were not "
scalable".
British deployment
From 1912, the British
General Post Office, which also operated the British telephone system, installed several automatic telephone exchanges from several vendors in trials at Darlington on 10 October 1914 and Dudley on 9 September 1916 (
rotary system
The rotary machine switching system, or most commonly known as the rotary system, was a type of automatic telephone exchange manufactured and used primarily in Europe from the 1910s. The system was developed and tested by AT&T's American engineerin ...
), Fleetwood (relay exchange from Sweden), Grimsby (Siemens), Hereford (Lorimer) and Leeds (Strowger).
Events in Telecommunications History - 1927
BT Archives The GPO selected the Strowger switches for small and medium-sized cities and towns. The selection of switching systems for London
London is the capital and List of urban areas in the United Kingdom, largest city of England and the United Kingdom, with a population of just under 9 million. It stands on the River Thames in south-east England at the head of a estuary dow ...
and other large cities was not decided until the 1920s, when the Director telephone system
The director telephone system was a development of the Strowger or step-by-step (SXS) switching system used in London and five other large cities in the UK from the 1920s to the 1980s.
A large proportion (c. 70% to 80%) of telephone traffic in ...
was adopted. The Director systems used SXS switches for destination routing and number translation facilities similar to the register used in common-control exchanges. Using similar equipment as in the rest of the network was deemed beneficial and the equipment could be manufactured in Britain.
References
Further reading
* Kempster, Blanchard Miller, ''American Telephone Practice'', McGraw, 1905, pp. 692ff
full text
External links
AT&T Archives: 1951 Training Video Regarding the Step-by-Step Switch
Prof Mark Csele, Niagara College, Ontario
Telephone Systems Training Course: Central Office Equipment
1954, Western Electric Hawthorne Works
{{Telecommunications
Telephone exchange equipment