Cold-cathode ionization gauges

Abstract

The practicability of using the cold-cathode glow discharge to measure gas pressure in the region below 10⁻³ mbar was first realized by Penning¹, who used a strong magnetic field to increase the electron path length from cathode to anode, so raising the ionization to a measurable level. In 1937 he described a practical gauge in which a discharge took place between an anode (made in the form of a loop), and two zirconium discs connected together electrically (one on each side of the ring) acting as cathode. The geometrical arrangement is shown in Figure 4.1. A permanent magnet was placed with its field parallel to the axis of symmetry. With a field strength of about 0.04 tesla and a constant anode-cathode voltage, the discharge current was found to be proportional to pressure in the range 10⁻⁵ to 10⁻³ mbar. At constant pressure, the current increased with increasing voltage; at 2000 V (the recommended operating value) the current was approximately 1 mA at a nitrogen pressure of 10⁻³ mbar.