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This section reviews the solo rotary stepper switch. This type is distinguished from the ganged, motor driven, rotary switch used in the "7A Rotary Automatic Machine Switching System" and successors.  Check out the 7A switch for more on motor driven rotary switches. The solo rotary is a cousin of the motorized version. It is a standalone device (solo) and not part of a larger rotary array. Its linage is from the Lorimar brothers and Strowger's ideas and was created by Western Electric circa 1910. It exists in many versions and it was rather simple to construct compared to the many complex switches reviewed on this website. 

The solo rotary action is driven by an internal electromagnet. Each time the magnet is energized the armature and pawl advances (steps) the wipers one step in a circular fashion. The arm is called a wiper because its attached terminal "wipes" the stationary contacts as it touches them. The wipers' connecting wires are not shown in this diagram. 

simplified rotary selector switch diagram

Rotary steppers, versatile in nature, are employed in both small and large exchanges.  For example, one may find a stepper as a line finder, connector, dialed digit counter, trunk hunter and more. Some early small business exchanges used only rotary steppers as their primary switch. As an example, Western Electric built 35 and 70-line units circa 1922. For a picture see [Aitken].  See far below for example of 9-line private intercom using a single rotary stepper and some control relays circa 1960.

The four pictures below are versions of rotary steppers. 

four different rotary switches telephone systems

For context, the example on the upper left showcases the motor-driven 7A Rotary Line Finder type, which isn't a standalone device, unlike the other three presented.

Rotary stepping switches are classed either as unidirectional or bidirectional. Unidirectional rotary stepping switches step in only one direction. When all stationary bank contacts have been traversed, the mechanism is in its reset or "home" position. Such a rotary stepping switch is often stepped to its home position after each use. The "two-way" switch (Clare-1) has a step electromagnet and a fast return electromagnet (top magnet). 

Western Electric, the Automatic Electric Company (AEC, founded 1901) and C.P. Clare Company were the primary US manufacturers of the sole stepper switch type (1910s-1970's).  

The following is an explainer video tutorial on the Rotary Stepper switch. 

 

The provided short video demonstrates an AEC unidirectional stepper at work. The arc spans 180 degrees. It features two arms: one active and the other reserved for the subsequent cycle. It's an ingenious design. 

The Siemens' High Speed Motor Uniselector is notable for its excellent engineering and use in rotary exchanges in Europe. 

Siemens rotary stepper switch uniselctor type
Westren electric 207c KTU device image with rotary stepper switch

Western Electric 207C KTU - 9 line private intercom (1-digit) with rotary stepper switch

To more fully appreciate the nature of these switches view three, 3D videos of electro-mechanical steppers and a Western Electric relay. Check them out here. 

Relay Tester

 

The "Relay Tester" appliance, crafted in 2021, simplifies the testing of individual relays and rotary steppers. It comprises:
 

  • Square wave pulse generator (PG) with variable frequency (1 to > 200 Hz) and variable pulse widths. This is useful for testing external relays for operational speed and operate/release times.  For example, the PG can emulate a dial with 10 pulses per second rate and a 66% on and 33% off pulse ratio. 

  • Repeater relay (Western Electric U-type, operates manually or from PG source) for driving heavy external loads.

  • A WE Wire Spring relay for use as needed.  

  • ​Current meter to measure external switched device power consumption.

  • 24 and 48 VDC power available for external devices. 

References

Aitken, William Automatic Telephone Systems, Vol 3, 1924

To see the 207C intercom in action view a short video by Phil McCarter.

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