The simplest form of thermal mass flowmeter is the hot-wire anemometer, used to measure air
speed. This flowmeter consists of a metal wire through which an electric current is passed to heat
it up. An electric circuit monitors the resistance of this wire (which is directly proportional to wire
temperature because most metals have a definite temperature coefficient of resistance). If air speed
past the wire increases, more heat will be drawn away from the wire and cause its temperature to
drop. The circuit senses this temperature change and compensates by increasing current through
the wire to bring its temperature back up to setpoint. The amount of current sent through the wire
becomes a representation of mass air flow rate past the wire