= Thermionic emission
Release of <Electrons> from a heated material; one source of <Electrons> in an <X-ray tube>.
Heating a <metal> gives some <Electrons> enough <energy> to escape its surface. A heated <diode cathode> can therefore supply an <electric current> across a <vacuum> gap to a collecting <diode anode>. This makes <temperature> a control for the supply of <Electrons>. Conversely, measuring that supply at several <temperatures> lets us estimate the surface <work function>: the <energy> barrier measured from the <Fermi level> to the nearby <vacuum> level. A <Richardson plot> turns the strong <exponential function> dependence into a slope.
Three questions must be kept separate: how many <Electrons> cross the surface, how many traverse the gap, and how many the collecting <diode anode> records. The <Richardson-Dushman law> answers the first under specified surface assumptions. <Space charge> can obstruct the second, while incomplete collection can alter the third.
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