Crossword Clue:

an electron tube with a photoemissive cathode

AnswerCrossword Clue
ELECTRONTUBAInstrumental device with an anode and a cathode?
GRIDWire mesh in an electron tube.
ANODEPositive terminal of an electron tube
DIODEElectron tube with two elements
PITOTPhysicist with an eponymous tube