Electromechanical switch

Strowger switch

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.


According to legend, Almon Strowger was motivated to invent an automatic telephone exchange after having difficulties with the local telephone operators. He was said to be convinced that the local manual telephone exchange operators were sending calls to 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.

Details of the patent

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.

Further development

Later systems cascaded Strowger-style switches, enabling connection among more customers. They also added a line finder selector, to connect a subscriber to any available switch rather than dedicating a single switch to each customer, reducing the number of switches needed. Another enhancement was the inclusion of circuits to detect busy connections and send the subscriber's call to a busy signal.

It is 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 geometrically 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 come up with the idea, he was not alone in his endeavours and sought the assistance of his brother Arnold, nephew William and others with a knowledge of electricity and money to realise 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. Strowger sold his patents in 1896 for $1,800 and sold his share in the Automatic Electric Company 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.


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

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