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Uniselector. Note drive gear on the right-hand side; this type has no electromagnet for stepping.

Bank of two-motion switches

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.

History[link]

According to legend, Almon Strowger, an undertaker, was motivated to invent an automatic telephone exchange after having difficulties with the local telephone operators, 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 the initial model was made from a round collar box and some straight pins.

Two-motion mechanism[link]

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 ratchets. 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. An 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[link]

US 447918  consists of:

1. A device for use by the customer — this 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.
2. A two motion stepping switch 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 — an electromechanical switch with a matrix of vertical and horizontal bars and simpler motions, that worked more reliably.

British Strowger exchange, BPO 2000-type equipment

Further development[link]

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 Director 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".

	Strowger switch in use The sound of a step-by-step call reaching a busy circuit.
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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.

References[link]

• Kempster, Blanchard Miller, American Telephone Practice, McGraw, 1905, pp. 692ff. full text

See also[link]

Wikimedia Commons has media related to: Strowger switch

• Director telephone system
• AT&T Archives: 1951 Training Video Regarding the Step-by-Step Switch

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