1. A rotary switch comprising a housing that is open at one end and has an end wall with an opening at the other end, a base plate secured to said housing and closing off said one open end thereof, said base plate having a circular array of stationary switch contacts exposed to the interior of said housing and terminal means for said stationary switch contacts accessible for circuit connections outside of said housing, said base plate also having a circular track of first teeth exposed to the interior of said housing, a shaft rotatably mounted in said opening in said end wall, said shaft having an axial extension within said housing, a detent plate mounted on said axial extension, means coupling said detent plate to said axial extension so that said detent plate will rotate with said axial extension but is slidable as a whole longitudinally of said axial extension, a circularly extending array of second teeth carried by said detent plate so as to rotate therewith, said second teeth facing said first teeth, a compression spring surrounding said axial extension and urging said detent plate longitudinally of said axial extension toward said base plate so as to hold said second teeth in engagement with said first teeth, and a wiper unit connected to said detent plate, said wiper including a first electrically conductive member disposed so as to engage each stationary switch contact in turn as said shaft is turned, said base plate having another stationary contact exposed to the interior of said housing with terminal means accessible for a circuit connection outside of said housing, said wiper unit including a second electrically conductive contact member conductively connected to said first contact member and engaging said another stationary contact, said first and second teeth and said compression spring cooperating in the absence of an applied turning force to lock said detent plate and shaft against rotation, said first and second teeth and Said compression spring also cooperating when said shaft is rotated by means of an applied turning force to produce reciprocal sliding motion of said detent plate axially of said axial extension so that as said first contact member moves from one switch contact to another it also reciprocates first away from and then back to said array of switch contacts.

8. A rotary switch comprising a housing, a circular array of stationary switch contacts in said housing, terminal means for said stationary switch contacts accessible for circuit connections outside of said housing, a stationary circular track of first teeth within said housing, a shaft rotatably mounted in one end of said housing, said shaft having an axial extension within said housing, a detent plate mounted on said axial extension, means coupling said detent plate to said axial extension so that said detent plate will rotate with said axial extension but is slidable axially of said axial extension, an array of second teeth carried by said detent plate so as to rotate therewith, a compression spring surrounding said axial extension and urging said detent plate in a direction to hold said second teeth in engagement with said first teeth, a wiper connected to said detent plate, said wiper including a contact member disposed so as to engage each stationary switch contact in turn as said shaft is turned, another stationary contact with terminal means accessible for a circuit connection outside of said housing, and means providing an electrical connection between said wiper and said another stationary contact, said first and second teeth and said compression spring cooperating in the absence of an applied turning force to lock said detent plate and shaft against rotation, said first and second teeth and said compression spring also cooperating when said shaft is rotated by means of an applied turning force to produce reciprocal sliding motion of said detent plate axially of said axial extension so that as said contact member moves from one switch contact to another it also reciprocates first away from and then back to said array of switch contacts.