Thermocouple vacuum gauge

In this type of gauge, a wire filament is heated by running current through it. As molecules interact with the filament, heat is transferred to them, which changes the filament temperature. The temperature is measured by a thermocouple attached to the filament. As the pressure drops, the thermocouple gets hotter and its output increases. The readout is accomplished on a millivolt meter calibrated to indicate pressure.

The exhibit is the thermocouple gauge and the gauge head from the MS902 double-focusing mass spectrometer manufactured by AEI Scientific Apparatus Ltd. in 1969. The gauge contains two crossed wires soldered together, which serve as a heater and a thermocouple. There were 5 pieces of these gauges in the MS 902 - in the rough side of the source and analyzer vacuum system and as in the fine side of inlet systems.  These gauges provided a convenient means of measuring pressures from atmosphere to about 0.01 torr.

Wikipedia: Thermocouple as vacuum gauge 
Wikipedia: Pressure measurement