Strowger switch
Encyclopedia
The Strowger switch, also known as Step-by-Step or SXS, is an early electromechanical telephone switching system invented by Almon Brown Strowger. It is a specialized version of a stepping switch
.
, was motivated to invent an automatic telephone exchange after having difficulties with the local telephone operator
s, where the wife of a competitor was one of them. He was said to be convinced that she, as one of the manual telephone exchange operators was sending calls "to the undertaker" to her husband, who ran a competing undertaker business.
He first conceived his invention in 1888, and patented the automatic telephone exchange in 1891. It is reported that he initially constructed a model of his invention from a round collar box and some straight pins.
s. The upper one has ten grooves, and raises the shaft. The lower one has long vertical teeth (on the other side, hidden).
The first set of incoming pulses goes to an electromagnet; its armature raises the shaft, to select 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 next set of pulses goes to a second 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 home (typically, when a call is complete), a release magnet disengages the pawls that hold the shaft in position; a simple interlock ensures that the spring on the shaft rotates it to home (angular) position before it drops (by gravity) to home.
This system was widely used until the advent of the crossbar switch
— an electromechanical switch with a matrix of vertical and horizontal bars and simpler motions, that worked more reliably.
exchange. Another enhancement was the inclusion of circuits to detect busy connections and return a busy signal
to the calling subscriber.
It is this fundamental modularity of the system combined with its step-by-step (hence the alternative name) selection process and an almost unlimited potential for expansion that 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 simply not "scalable
".
While Almon Strowger may have devised the 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 money to realize his concepts. With this help the Strowger Automatic Telephone Exchange Company was formed and it installed and opened the first commercial exchange in his then-home town of La Porte, Indiana on November 3, 1892, with about 75 subscribers and capacity for 99. He married Susan A. (1846–1921) from Massachusetts
in 1897 as his second wife. The patents were exclusively licensed to Automatic Electric Company, another company Strowger helped found. Strowger sold his patents in 1896 for $1,800 and sold his share in Automatic Electric for $10,000 in 1898. His patents subsequently sold for $2.5 million in 1916.
The company's engineers continued development of Strowger's designs and submitted several patents in the names of its employees. It also underwent several name changes. Strowger himself seems to have not taken part in this further development. He subsequently moved to St. Petersburg, Florida
and appears to have returned to being an undertaker, as H. P. Bussey Funeral Home records report an unidentified body being moved "for Mr. Strowger" in December 1899. The same funeral home subsequently buried Strowger himself. Strowger was a man of some wealth at his death and was reported as owning at least a city block of property.
Stepping switch
In electrical controls, a stepping switch, also known as a stepping relay, is an electromechanical device which allows an input connection to be connected to one of a number of possible output connections, under the control of a series of electrical pulses. It can step on one axis , or on two axes...
.
History
According to legend, Almon Strowger, an undertakerFuneral director
A funeral director , also known as a mortician or undertaker, is a professional involved in the business of funeral rites. These tasks often entail the embalming and burial or cremation of the dead, as well as the planning and arrangement of the actual funeral ceremony...
, was motivated to invent an automatic telephone exchange after having difficulties with the local telephone operator
Telephone operator
A telephone operator is either* a person who provides assistance to a telephone caller, usually in the placing of operator assisted telephone calls such as calls from a pay phone, collect calls , calls which are billed to a credit card, station-to-station and person-to-person calls, and certain...
s, where the wife of a competitor was one of them. He was said to be convinced that she, as one of the manual telephone exchange operators was sending calls "to the undertaker" to her husband, who ran a competing undertaker business.
He first conceived his invention in 1888, and patented the automatic telephone exchange in 1891. It is reported that he initially constructed a model of his invention from a round collar box and some straight pins.
Two-motion mechanism
These particular switches have three banks of contacts; what appear to be continuous arcs of metal might be shields; individual contacts are hidden. Toward the upper end of each shaft are two copper-colored ratchetRatchet (device)
A ratchet is a device that allows continuous linear or rotary motion in only one direction while preventing motion in the opposite direction. Because most socket wrenches today use ratcheting handles, the term "ratchet" alone is often used to refer to a ratcheting wrench, and the terms "ratchet"...
s. The upper one has ten grooves, and raises the shaft. The lower one has long vertical teeth (on the other side, hidden).
The first set of incoming pulses goes to an electromagnet; its armature raises the shaft, to select 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 next set of pulses goes to a second 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 home (typically, when a call is complete), a release magnet disengages the pawls that hold the shaft in position; a simple interlock ensures that the spring on the shaft rotates it to home (angular) position before it drops (by gravity) to home.
Details of the patent
consists of:- A device for use by the customer — this creates trains of on-off current pulsesPulse (signal processing)In signal processing, the term pulse has the following meanings:#A rapid, transient change in the amplitude of a signal from a baseline value to a higher or lower value, followed by a rapid return to the baseline value....
corresponding to the digits 1-9, and 0 (which sent 10 pulses). This equipment originally consisted of two telegraph keys engaged by knife switches, and evolved into the rotary dialRotary dialThe rotary dial is a device mounted on or in a telephone or switchboard that is designed to send electrical pulses, known as pulse dialing, corresponding to the number dialed. The early form of the rotary dial used lugs on a finger plate instead of holes. Almon Brown Strowger filed the first patent...
telephone. - A two motion stepping switchStepping switchIn electrical controls, a stepping switch, also known as a stepping relay, is an electromechanical device which allows an input connection to be connected to one of a number of possible output connections, under the control of a series of electrical pulses. It can step on one axis , or on two axes...
at telephone exchange. A contact arm could be moved up and down in order to select one of ten rows of contacts, and then rotated to select one of ten contacts in that row, a total of 100 choices. The stepping motion was controlled by the current pulses coming from the originating customer's telegraph keys, and later from the rotary dial.
This system was widely used until the advent of the crossbar switch
Crossbar switch
In electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....
— an electromechanical switch with a matrix of vertical and horizontal bars and simpler motions, that worked more reliably.
Further development
Later systems cascaded Strowger switch stages, enabling connection among many more subscribers. Rather than dedicating a single, expensive, first-stage switch to each subscriber, provision was made to connect the subscriber to any available first-stage switch. This was done either by the use of linefinders in the case of low-calling-rate domestic subscribers, or by the use of subscribers' uniselectors in the case of either commercial subscribers or, later, for any subscriber connected to a DirectorDirector telephone system
The Director System was a system which made it possible to call subscribers at other telephone exchanges without operator intervention in large multi-exchange cities, and to have a mixture of automatic and manual exchanges within these cities...
exchange. Another enhancement was the inclusion of circuits to detect busy connections and return a busy signal
Busy signal
A busy signal in telephony is an audible or visual signal to the calling party that indicates failure to complete the requested connection of that particular telephone call....
to the calling subscriber.
It is this fundamental modularity of the system combined with its step-by-step (hence the alternative name) selection process and an almost unlimited potential for expansion that 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 simply not "scalable
Scalability
In electronics scalability is the ability of a system, network, or process, to handle growing amount of work in a graceful manner or its ability to be enlarged to accommodate that growth...
".
While Almon Strowger may have devised the 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 money to realize his concepts. With this help the Strowger Automatic Telephone Exchange Company was formed and it installed and opened the first commercial exchange in his then-home town of La Porte, Indiana on November 3, 1892, with about 75 subscribers and capacity for 99. He married Susan A. (1846–1921) from Massachusetts
Massachusetts
The Commonwealth of Massachusetts is a state in the New England region of the northeastern United States of America. It is bordered by Rhode Island and Connecticut to the south, New York to the west, and Vermont and New Hampshire to the north; at its east lies the Atlantic Ocean. As of the 2010...
in 1897 as his second wife. The patents were exclusively licensed to Automatic Electric Company, another company Strowger helped found. Strowger sold his patents in 1896 for $1,800 and sold his share in Automatic Electric for $10,000 in 1898. His patents subsequently sold for $2.5 million in 1916.
The company's engineers continued development of Strowger's designs and submitted several patents in the names of its employees. It also underwent several name changes. Strowger himself seems to have not taken part in this further development. He subsequently moved to St. Petersburg, Florida
St. Petersburg, Florida
St. Petersburg is a city in Pinellas County, Florida, United States. It is known as a vacation destination for both American and foreign tourists. As of 2008, the population estimate by the U.S. Census Bureau is 245,314, making St...
and appears to have returned to being an undertaker, as H. P. Bussey Funeral Home records report an unidentified body being moved "for Mr. Strowger" in December 1899. The same funeral home subsequently buried Strowger himself. Strowger was a man of some wealth at his death and was reported as owning at least a city block of property.
See also
- Director telephone systemDirector telephone systemThe Director System was a system which made it possible to call subscribers at other telephone exchanges without operator intervention in large multi-exchange cities, and to have a mixture of automatic and manual exchanges within these cities...