How you can construct Cathode?

The substance used for Electron Emission is known as Emitter or Cathode.

The Cathode is heated in an evacuated space to emit Electrons.
As Cathode is sealed in Vacuum,therefore,the most convenient way to heat it is Electrically.

The heating processes may be done by two methods.The Thermionic Cathode are divided into two types,

01)Directly Heated Cathode
02)Indirectly Heated Cathode

01)Directly Heated Cathode:-

In this type,the Cathode consists of Oxide-Coated Nickel ribbon,which is known as "Filament".The heating Current is Directly passed through this Nickel ribbon which Emits the Electrons.

The Directly Heated Cathode is more efficient in converting heating power into Thermionic Emission.Therefore,it is generally used in Power Tubes that needs large amount of Emission and in small tubes operated from Batteries,where efficiency and quick heating are important.

The principal limitation of this type of Cathode is that any variation in heater Voltage affects the Electron Emission and this produce "Hum" in the Circuit.

02)Indirectly Heated Cathode:-

In this type,the Cathode consists of a thin metal sleeve coated with Barium and Strontium Oxides.A Filament or heater is enclosed within the sleeve and insulated from it.There is no Electric Connection between the heater and the Cathode.

The heating Current is passed through the Heater and the Cathode is heated indirectly through heat transfer from the heater element.

Advantages of Indirectly Heated Cathode:-

01)As Cathode is completely separated from the heating circuit,therefore,it can be readily connected to any desired potential as needed,independent of the heater potential.

02)Because of relatively large mass of Cylindrical Cathode,it takes time to heat or cool and as such does not introduce "Hum" due to heater Voltage Fluctutions.

03)a.c. can be used in the heater Circuit to simplify the Power requirement.

What is mean by Filament.

It is the thin wire,denotes the element through which the Cathode heating Current flows.In case of Directly Heated type,Cathode is itself the Filament.Whereas,heater is the Filament in Indirectly Heated Type.

You may also interested in following................
Which are the Important Methods for obtaining Electron Emission?
What are the different types of materials used for Semiconductors.
What is mean by Electron Emission?