The work function of a surface is the energy that an electron needs
to escape into space. For a metal, this energy varies for
different arrangements of atoms on crystal faces.
Once an electron is free, it can be accelerated by an electric field
toward the anode of a vacuum tube.
John Ambrose Fleming created the first thermionic vacuum tube, the Fleming valve.
Guthrie was a mentor to Fleming
but Fleming was more inspired by the Edison effect
embodied in vacuum tubes.
See also in The book of science:
Readings on wikipedia: