regulator

n. 1. One who, or that which, regulates. [ 1913 Webster ]

2. (Mach.) A contrivance for regulating and controlling motion, as: (a) The lever or index in a watch, which controls the effective length of the hairspring, and thus regulates the vibrations of the balance. (b) The governor of a steam engine. (c) A valve for controlling the admission of steam to the steam chest, in a locomotive. [ 1913 Webster ]

3. A clock, or other timepiece, used as a standard of correct time. See Astronomical clock (a), under Clock. [ 1913 Webster ]

4. A member of a volunteer committee which, in default of the lawful authority, undertakes to preserve order and prevent crimes; also, sometimes, one of a band organized for the comission of violent crimes. [ U.S. ] [ 1913 Webster ]

A few stood neutral, or declared in favor of the Regulators. Bancroft. [ 1913 Webster ]