What condition is required to induce thermionic emission within the cathode filament?

• A. low current
• B. high current
• C. direct current
• D. 2 phase current

Answer: (B)

Thermionic emission happens when the cathode filament is heated enough that electrons gain enough thermal energy to overcome the metal’s work function and escape the surface. The heating comes from passing current through the filament, so the key condition is a high enough current to raise the filament to a high temperature. When the current is large, the filament gets hot and many electrons are emitted. If the current is too low, the filament stays cool and emission is minimal or nonexistent. The exact type or phase of the current (direct, alternating, or other) doesn’t determine emission; it's the temperature achieved by the heating power that matters, which is why a high current is the correct condition.