n a Pirani gauge (see Figure 2), two filaments (platinum alloy in the best gauges), act as resistances in two arms of a Wheatstone bridge. The reference filament is immersed in a fixed-gas pressure, while the measurement filament is exposed to the system gas.
A current through the bridge heats both filaments. Gas molecules hit the heated filaments and conduct away some of the heat. If the gas pressure (or composition) around the measurement filament is not identical to that around the reference filament, the bridge is unbalanced and the degree of unbalance is a measure of the pressure. In reality, modern Pirani gauges electronically adjust the unbalance and use the current needed to bring about balance as a measure of the pressure. This improves the linearity of measurement.