Abstract: A thermal conductivity vacuum gage has a vacuum-sensing probe and a separate operating unit for indicating the vacuum sensed by the probe and supplying electrical power for the vacuum sensing by the probe. An electrically-conducting cable connects the probe to the operating unit to form measuring and power supply circuits therebetween. The portion of the measuring circuit and the portion of the power supply circuit in the operating unit are identical or common, i.e. the same circuit portion, so that variations in the operation of this common circuit portion affect circuits the same in contrast to prior gages having separate measuring and power supply circuits, whereby variations in the operation of one could affect the other.