The wire is maintained at a constant temperature higher than that of the ambient, by passing a current through it. The flow past the probe cools it and momentarily reduces its temperature. The change in temperature results in a change in resistance of the wire . This is sensed by a feedback control circuit which passes more current through the wire to restore it to its original resistance and temperature. The excess voltage required to maintain the wire temperature constant, when exposed to a flow, is a measure of the flow velocity itself. This unique relationship between and must be obtained by calibration, i.e. must be determined in a controlled experiment where is precisely known. Typical calibration curves for hot-wire are shown in Figure 2.11 in which and denote the wire temperature and the local fluid temperature respectively. In a calibration experiment, it is common to keep and individually constant to generate each of the curves