Page "Pressure measurement" Paragraph 116

Likewise, cold-cathode gauges may be reluctant to start at very low pressures, in that the near-absence of a gas makes it difficult to establish an electrode current-in particular in Penning gauges, which use an axially symmetric magnetic field to create path lengths for ions that are of the order of metres.

In ambient air, suitable ion-pairs are ubiquitously formed by cosmic radiation ; in a Penning gauge, design features are used to ease the set-up of a discharge path.

For example, the electrode of a Penning gauge is usually finely tapered to facilitate the field emission of electrons.