For pressure measurements in the UHV range.The idea of the ion gauge is to ionize the rest gas and to measure the pressure via the ion concentration. The operating principle is illustrated below.

A hot filament emits electrons which are accelerated to go inside inside a cylindrical wire cage. In the cage the electrons hit rest gas molecules and ionise them. The ions are collected at the wire in the middle of the cage and the current from that wire is a measure for the pressure. The ion current is proportional to the pressure of the rest gas. A problem is that the sensitivity of the gauge depends on the chemical identity of the rest gas. The ion gauge can be used in for pressures between 10-4 and 10-11 mbar. The upper limit is given by the need to operate the filament in vacuum, so that it is not destroyed. The lower limit is given by an unwanted effect: when an electron hits the cage it can produce a photon which then can cause the emission of an electron from the centre wire, giving rise to a current contribution in the opposite direction from the ion current. While this effect sounds extremely unlikely it becomes the dominant contribution to the current at very low pressures. When the ion gauge is exposed to a flux of electrons from some other source, it can also start to show an artificially low pressure for the same reason.