7.9 Penning (Cold Cathode)
Ionization Gauge
Range 10−3 Torr—10−13 Torr
Electrons are emitted from a cold cathode
(room temperature) when we apply a large
enough voltage. If we accelerate the electrons
with the same field as was used to extract
them, we can have energetic electrons
capable of ionizing the gas in the system. We
increase the mean free path of the electrons
by applying a magnetic field. This increases
the rate of ionization per electron, and we
obtain an ion current that is large enough to
measure even without amplification.