Hot-Wire Anemometer:

The hot-wire anemometer, principally used in gas flow measurement, consists of an electrically heated, fine platinum wire that is immersed into the flow.  As the fluid velocity raise, the rate of heat flow from the heated wire to the flow stream increases.  Therefore, a cooling effect on the wire electrode occurs and will causing its electrical resistance to change. Inside a constant-current anemometer and the fluid velocity is determined from a measurement of the resulting modified within wire resistance.  In a constant-resistance anemometer, the fluid velocity is determined from the current required to maintain a constant wire temperature and therefore, the resistance constant.