History[ edit ] 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. The initial model was made from a round collar box and some straight pins. 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".
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This system uses selectors for switching. The selectors used in Strowger exchange are mainly of two types Uniselector Both the selectors belong to the same types of switches called rotary switches. The uniselector consists of moving contacts called wipers. These are used to make electrical connections with any one of several contacts, called bank contacts, in an arc around it. The arc in most cases consists of ten steps.
The wiper assembly is divided into three sets of wipers so that the switch has to turn through only one third of a full circle when operated. These wipers are operated by an electromagnet, called driving magnet, with the help of a ratchet and pawl mechanism. When current flows through the windings of the driving magnet, it is energised and attracts the armature; the pawl slips over one tooth of the ratchet wheel. The ratchet is prevented from movement in the reverse direction by a detent.
When the current stops through the windings of the driving magnet, it is de-energised, and the armature comes back to its rest position. During the reverse movement of the armature and hence that of the pawl, the ratchet wheel stop ahead in the clockwise direction by one tooth and the wipers move to the next contact. The uniselector rotates as many steps as the electromagnet is energised and de-energised. The schematic diagram of the uniselector is shown in the figure.
In the vertical direction the wipers move upward to the desired level and make no connections with the bank contacts. While in the horizontal direction the wipers make connection with the bank contacts. The two-motion selector has 10 levels; each having 10 contacts, thus a total of contacts are accessible.
Two Motion Selector Each contact represents the terminals of one switch of the higher stage or of one telephone line in the case of final selector. The dialing pulses cause the wiper assembly to step up or down to the desired level. If we take the example of a final selector, where up to lines can be connected, the vertical and horizontal stepping of the selector are controlled by the digits dialed by the subscriber.
When the first digit is dialed, the dialing pulses energise and de-encrgises the vertical magnet. The vertical magnet with the help of ratchet and pawl mechanism step up the wiper assembly, corresponding to the digit dialed. This is called vertical stepping. When the second digit is dialed the dialing pulses are now diverted to another magnet, called horizontal magnet, with the help of a relay.
These pulses energise and de-energise the horizontal magnet, which with the help of ratchet and pawl causes the wiper assembly to rotate to the proper contact, corresponding to the second digit dialed.
This is called horizontal stepping. Thus the wiper assembly makes connection with the required number dialed by the subscriber. After completion of the call the wiper assembly comes back to the home position. For this purpose the rotary magnet is operated by the current and thus the wiper assembly moves through the remaining contacts of the level.
A spring forces the wiper assembly to drop vertically and then to return horizontally to its normal position. Thus the two-motion selector does not require an additional magnet for its "homing process".
TSSN - Strowger Switching System
Next Page In this chapter, we will discuss how the Strowger Switching system works. The first ever automatic telephone switching was developed by Almon B Strowger. As the operator at the Manual telephone exchange was the wife of his competitor and was diverting all the business, Strowger thought of developing a switching system, which does not require an operator. This led to the invention of the automatic switching system developed by Strowger. The Strowger Switching system is also called the step-by-step switching system as the connections are established in a step-by-step manner.