net to rotate the wipers the other way. A third variety "winds" a spring as the wiper steps progressively, and a ratchet holds the wipers from returning to home position. When the circuit is no longer needed, another electromagnet releases the holding pawl; the spring then returns the wipers to their home position.
Stepping switches were quite noisy in operation (especially when self-stepping), because their mechanisms accelerated and stopped quickly to minimize operating time. One could compare their sound to that of some snap-action mechanisms. Nevertheless, they were engineered for long life, given periodic maintenance; they were quite reliable.
Single-axis stepping switches are sometimes known as uniselectors.
Two axis stepping switch
Slightly more complicated was the two axis stepping switch, (also called Strowger switch or two motion selector in Britain). Typically, a single compact group of wipers could connect to one of 100 (or 200) different fixed contacts, in ten levels. When the switch was idle, the wipers were disengaged from the fixed contacts. The wipers moved up and down on a vertical shaft, and rotated into the contact bank to make a connection. A spring, internal to the vertical shaft, returned the wipers to their home position at the bottom.
This type had two stepping coils with pawls and ratchets, one to raise the wipers to the desired banks of contacts, and one to rotate the wipers into the banks. These were commonly used in telephone switching with ten banks of ten contacts. The coils were typically driven by the electrical pulses derived from a rotary telephone dial. On a two-motion selector, as a digit was dialed, the wipers would step up the banks, then automatically rotate (self-step) into the selected bank until they found an "unused" outlet to the next switch stage. The last two digits dialed would operate the connector switch (final selector in Britain). The second to last digit would cause the wipers to move up and the last digit would cause them to rotate into the bank to the called customer's line outlet. If the line was idle then ringing voltage would be applied to the called line and ringing tone was sent to the calling line.
Another variant of the two-axis switch was the Stromberg-Carlson X-Y Switch which was quite common in telephone exchanges in the western USA. It was a flat mechanism, and the moving contacts moved both sidewise, as well as to and fro. It was quite reliable, and could be maintained by people with minimal training.
Applications of stepper switches
The sound of a step-by-step call reaching a busy circuit.
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As well as the decoding of pulses from telephones, stepping switches could be used for a variety of purposes, depending on how they were wired. By connecting several in series with the highest output of one going to the stepping contact of the next, a counter could be constructed. Or by feeding the stepping contact with an endless pulse train via a relay, and controlling the relay from the switch's own output, it can be made to automatically hunt for the first unpowered line (or powered, depending on whether the relay is normally open or normally closed). They could also be used as a demultiplexer, so that two input lines could control a number of output devices. One input line steps the switch until the correct device is selected, and the other then powers that device. Many other applications are possible.
Such switches were used in a series of Japanese cypher machines during World War 2: CORAL, JADE, PURPLE (the names were American). Some of the equipment used to break the Enigma machine code also used many such switches, which some observers called the "Machine Gun" for the loud noise.