Editing Strowger switch (section)

=== Two-motion mechanism ===
The Strowger switch has three banks of contacts. Toward the upper end of each shaft are two [[Ratchet (device)|ratchet]]s. The upper one has ten grooves, and raises the shaft. The lower one has long vertical teeth (on the other side, hidden).

The original Strowger switch used two telegraph-type keys on a telephone set for dialing. The keys are tapped to step the switch in two stages, a vertical (V) and a rotating (R) stage, each requiring a separate electrical circuit between exchange and telephone set. The first set of incoming pulses (V) 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. Another pawl, pivoting on the frame, holds the shaft at that height as it rotates (R). The second set of pulses, from the second key, operates another electromagnet. Its pawl engages the (hidden) vertical teeth in the lower ratchet to rotate the shaft to the required position. It is kept there against spring tension by a pawl pivoted on the frame. When the switch returns to its home position, typically when a call is complete, a release magnet disengages the pawls that hold the shaft in position. An interlock ensures that the spring on the shaft rotates it to angular home position before it drops to its home position by gravity.