The Western Electric 60 AP Selector is a sort of all-mechanical intelligent relay, one which will only open when it receives a particular coded sequence of pulses. Used as the heart of many railroad and industrial phone switching systems in the early part of the 20th century, the core of the device is a rotating circular wheel with 17 holes around its edge. The wheel rotates a fixed number of degrees every time a pulse passes through the coil armature and is restrained by springs such that when power is removed from the coils, the wheel will snap back to its original position. A restraining arm lines up with the wheel along the holes in it's edge; pins placed in these holes that line up with the restraining arm when power is removed will hold the wheel in place, preventing the springs from snapping it back to home position. By placing pins into the various positions along the wheel a series of numbers can be encoded into the device, a matching set of pulses will cause the wheel to rotate through its full range and close a pair of contacts.
It should be noted that a limitation of this device is that it can only be encoded with a series of numbers that totals up to 17, the number of pulses needed to rotate the wheel to its final position.
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