Question

In a cathode ray tube, why is the cathode coated with thorium and carbon?

Answer 1

A tungsten filament on which a mixture of thorium and carbon are coated is known as thoriated tungsten. The advantage in the emitter is that heat is produced by passing electric current through tungsten filament whereas thermions are emitted from thorium. The work function of thorium is $2.6eV$ and corresponding to a temperature of about $2000K$. If the temperature is increased beyond $2000K$, the rate of thermionic emission also increases.