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The Strowger switch was the first example of an electromechanical stepping switch telephone exchange system. It was invented by Almon Brown Strowger, and first patented in 1891. Because of its operational characteristics it is also known as a step-by-step (SXS) switch.
According to some accounts, Strowger, an undertaker, was motivated to invent an automatic telephone exchange after having difficulties with the local telephone operators, one of whom was the wife of a competitor. 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.
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. The Strowger Automatic Telephone Exchange Company was founded in 1891.
The company 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. It used two telegraph type keys on the telephone, which had to be tapped the correct number of times to step the switch, but the use of separate keys with separate conductors to the exchange was not practical for a commercial system. Early advertising called the new invention the "girl-less, cuss-less, out-of-order-less, wait-less telephone". 
The Strowger Automatic Telephone Exchange Company became the Automatic Electric Company, 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.
Strowger's patent (US 447918 ) specifies dialing equipment at the customer location and the switching equipment at the central office.
The customer device creates trains of on-off current pulses 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 dial telephone.
The central office switching equipment had a two-motion stepping switch. A contact arm could be moved up and down 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.
The Strowger switch had 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 ratchets. 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 used two telegraph type keys on a telephone set for dialing. Each key required a separate wire to the exchange. The keys were 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 of the Strowger system
The commercial version of the Strowger switch, as developed by the Strowger Automatic Telephone Exchange Company, used a rotary dial 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 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 searched "backward" for the calling line; so requiring only a few relays for the equipment required for each customer line.
Later Strowger (SXS) exchanges often used a subscriber uniselector as part of the line equipment individual to each line, which searched "forward" for a first selector. This was more economical for higher calling-rate domestic or business customers, and had the advantage that access to additional switches could readily be added if the traffic increased (the number of linefinders serving a group was 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 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 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 not "scalable".
The sound of a step-by-step call reaching a busy circuit.
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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 (rotary system), Fleetwood (relay exchange from Sweden), Grimsby (Siemens), Hereford (Lorimer) and Leeds (Strowger). The BPO selected the Strowger switches for small and medium cities and towns. However, the selection of switching systems for London and other large cities was not decided until the 1920s, when the Director telephone system 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.
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- Kempster, Blanchard Miller, American Telephone Practice, McGraw, 1905, pp. 692ff. full text
- AT&T Archives: 1951 Training Video Regarding the Step-by-Step Switch
- Telephone Switches Prof Mark Csele, Niagara College, Ontario
- Telephone Systems Training Course: Central Office Equipment 1954, Western Electric Hawthorne Works