A device capable of registering a specific substance or physical phenomenon, and that optionally sounds an alarm or triggers a warning.
An indicator showing the depth of the water in a boiler.
A galvanometer, usually portable, for indicating the direction of a current.
Any specialized cell or structure that responds to sensory stimuli.
A protein on a cell wall that binds with specific molecules so that they can be absorbed into the cell in order to control certain functions.