Unless a voltage difference exists across the plates, no external work
can be done. In the thermocouple the voltage difference was caused by
the different electrical properties of the _p_ and _n_ semiconductors.
Both the emitter and collector in the thermionic converter are good
metallic conductors rather than semiconductors, so a different tack must
be taken.
The key is the use of an emitter and a collector with different work
functions. If it takes 4.5 electron volts to force an electron from a
tungsten surface and if it regains only 3.5 electron volts when it
condenses on a collector with a lower work function, then a voltage drop
of 1 volt exists between the emitter and collector.
To summarize, then, the thermionic emission of electrons creates the
potentiality of current flow. The difference in work functions makes the
thermionic converter a power producer.
There is an interesting comparison that helps describe this phenomenon.
Consider the emitter to be the ocean surface and the collector a
mountain lake. The atmospheric heat engine vaporizes ocean water and
carries it to the cooler mountain elevations, where it condenses as rain
which collects in lakes. The lake water as it runs back toward sea level
then can be made to drive a hydroelectric plant with the gravitational
energy it has gained in the transit. The thermionic converter is similar
in behavior: hot emitter (corresponding to the sun-heated ocean); cooler
collector (lake); electron gas (water); different electrical voltages
(gravity). Without gravity the river would not flow, and the production
of electricity would be impossible.