A d.c. voltage of 2.3 kV is applied between the anode and cathode. Free electrons
are accelerated from the cathode toward the anode and collide with gas
molecules causing ionisation. The resulting electrons and positive ions are also
accelerated in the electrostatic field and cause further ionisation. The mean path
length of each ion is extended as they follow a complex spiral path in the
magnetic field. The net ion current flowing between the cathode and anode is
detected by the control unit and is a function of the pressure in the gauge head.