Strowger switch

Each key had to be tapped the correct number of times to step the switch and make the desired connection.

[4] The company installed and opened the first commercial exchange in his then-home town of La Porte, Indiana on November 3, 1892.

Early advertising called the new invention the "girl-less, cuss-less, out-of-order-less, wait-less telephone".

Company engineers continued development of the Strowger designs and submitted several patents in the names of its employees.

This equipment originally consisted of two telegraph keys engaged by knife switches, and evolved into the rotary dial telephone.

The stepping motion is controlled by the current pulses coming from the originating customer's telegraph keys, and later from the rotary dial.

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.

Its pawl engages the (hidden) vertical teeth in the lower ratchet to rotate the shaft to the required position.

This is more economical for higher calling-rate domestic or business customers, and has the advantage that access to additional switches can readily be added if the traffic increases (the number of linefinders serving a group is limited by the wiring multiple installed).

Previous systems had all been designed for a fixed number of subscribers to be switched directly to each other in a mesh arrangement.

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 and Dudley on 9 September 1916 (rotary system), Fleetwood (relay exchange from Sweden), Grimsby (Siemens), Hereford (Lorimer) and Leeds (Strowger).

The Director systems used SXS switches for destination routing and number translation facilities similar to the register used in common-control exchanges.