Detector \De*tect"or\, n. Specifically:
   (a) An indicator showing the depth of the water in a boiler.
   (b) (Elec.) A galvanometer, usually portable, for indicating the direction of a current.
   (c) (Elec.) Any of various devices for detecting the presence of electric waves.

Source: Webster's Revised Unabridged Dictionary (1913)
 

Detector \De*tect"or\, n. [L., a revealer.]

One who, or that which, detects; a detecter. --Shak.

A deathbed's detector of the heart. --Young.

Bank-note detector, a publication containing a description of genuine and counterfeit bank notes, designed to enable persons to discriminate between them.

Detector lock. See under Lock.

Source: Webster's Revised Unabridged Dictionary (1913)