Several electromechanical switching system were developed around 1880-1890 to eliminate the limitations of manual exchanges and to establish automatic exchanges to improve the speed and carry more leads (subscribers). Among those electromechanical automatic switches, strowger's step by step switching system was the most pupular and widely used and even now in some part of the world, it is in use. The history of this switching system is described briefly in the following paragraph.
The first automatic electromechanical switch was developed by connolly and Mcig the in 1879. But Almon B. Strowger, an undertaker in Kensas city, USA was the first to put it to effective use. Strowger developed a electromechanical system with electromagnets and pawls. Along with his nephew Walters, Strowger produced a working model in 1888 (US patent No. 447918, 10/6/1891). In this system, a moving wiper (with contacts in the end) moved upto and around a bank of many other contacts, making a connection with any one of them. Together with Joseph B. Harris and Moses A. Meyer, strowger formed his company "Strowger Automatic Telephone Exchange'' in october 1891.
The reasons for survival of this system even in some part of the world are its (a) high system availability (b) comprehensibility and (c) cheapness and simplicity.
There are two types of basic elements which performs most of the functions of the strowger switching system. They are (a) Uniselectors and (b) Two motion selectors.
Uniselectors. A uniselector is a one which has a single rotary switch with a bank of contacts. Depending upon the number of switching contacts, uniselectors are identified as 10 outlet or 24 outlet uniselectors. A single 10 outlet or 24 outlet uniselector can be used as a switching element for 10 or 24 subscribers. Several uniselectors can be graded together so that multiple incoming circuits can be connected to multiple outgoing circuits. Fig. 4.5 shows the simple arrangement of uniselectors.
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