Ionisation gauges cold cathode

Fig. 15. Cold-cathode ionisation gauge, where A = anode, C = cathode, M = horseshoe magnet, and = microammeter (15).

There are two principal types of ionisation gauge, viz. the hot cathode type in which electrons are emitted by a heated filament, and the cold cathode type in which electrons are released from the cathode by the impact of ions. In both cases the vacuum is measured in terms of the ion current. The electrons are accelerated by a potential difference (usually about 2000 V) across the ionisation tube (see Fig. 6.22). Positively charged ions are formed by the electrons striking gas molecules. The number of positive ions produced is a function of the gas density (i.e. the pressure) and the electron current ie which is normally held constant. The ions are collected at a negatively charged electrode and the resulting ion current it is a direct measure of the gas pressure. The hot cathode version is the most sensitive of the two and can be used to measure vacua down to about 10 10 torr ( 10 8 N/m2). 

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