The Agilent 531 and 536 TC gauges are designed to work with gauge controllers with an internal voltmeter and current source.
Thermocouple vacuum gauges work by measuring the potential difference between two conductors (TC wires) with different temperature coefficients.
The two conductors are placed in contact with each other at one end, and the other end is connected to a voltage source.
The filament provides the heat with 165mA current source.
When the two conductors are at different temperatures, a potential difference develops between them.
This potential difference can be used to measure the vacuum pressure.