CONDUCTION OF ELECTRICITY THROUGH GAS

When voltage is applied across an anode and a cothode that are enclosed in a vacuum which contains a gas and the gas is subjected to a reduced or low pressure, the gas conduct electricity.

CONDITIONS UNDER WHICH GASES CONDUCT ELECTRICITY:
Gases conduct electricity under low pressure and high voltage.

EXPLANATION OF OPERATIONS:
When the pressure inside the tube is reduced to a very low pressure ( say -0.01 mm Hg)  and a high voltage ( say 1000 volt)  is connected across the cathode and the anode of the tube, the gas in  tube breaks into ions ( I.e ionized ). The positive ions move toward the cathode while the negative ions and free electrons move toward the anode. At the cathode, the positive ions knock off electrons from the cathode. These electrons produced at the cathode are called cathode rays.

CATHODE RAYS:
Cathode rays are stream of electrons.
PRODUCTION OF CATHODE RAYS:
Cathode rays are produced when the pressure inside a cathode ray  tube is reduced to a very low pressure ( say -0.01 mm Hg)  and a high voltage ( say 1000 volt)  is connected across the cathode and the anode of the tube, the gas in the  tube breaks into ions ( I.e ionized ). The positive ions move toward the cathode while the negative ions and free electrons move toward the anode. At the cathode, the positive ions knock off electrons from the cathode. These electrons produced at the cathode are called cathode rays.

Properties of cathode rays:
• Cathode rays are stream of moving particles.
• Cathode rays are negatively charged particles.
• Cathode rays are attracted toward the anode in electric field.
• Cathode rays make glass and other materials to glow or fluores.
• Cathode rays are deflected toward the North pole in a magnetic field.
• Cathode rays e gas to ionized.
• Cathode rays turn wheel paddle placed in their parts.
• Cathode rays pocess energy.
• Cathode rays produce heart energy.
• Cathode rays have high penetrating power.
• Cathode rays affect photographic plates.
• Cathode rays can penetrate through some metals.

APPLICATION OF GAS DISCHARGE TUBE:
• Fluorescent lamps and neon signs:
The  pprinciple of gas discharge tube is used fluorescent tubes for lighting and display of signs.
Fluorescent tubes contains mercury vapour or sodium vapour which glow at low pressure when cathode rays pass through the vapour. The cathode rays produced inside the tube cause emission of ultraviolet rays when they bombard the vaporised atoms of mercury. The inside of the fluorescent lamp is coated with phosphor so that it gives off visible light when the ultraviolet rays strike it. Fluorescent tubes are also used as display signs. Neon gas is commonly use because it gives bright orange  light. Fluorescent amps are more efficient than filament lamps.

Production of neon sign:
Neon sign is produced when an evacuated discharge tube is filled with neon gas at low pressure of 5 mm of Hg.  The gas discharge between the electrodes and give its characteristic colour. The colour that the discharge tube produce depend n the nature of the gas used inside the tube and the nature of the coating inside the tube.

THE HOT CATHODE:
Hot cathode is another way of producing free electrons apart from what happened in a discharge  tube when electrons  are produced from a cold cathode.
Whenever a metal is heated to a sufficient high temperature, electrons are emitted from the surface of the metal.

Thermionic emission:
Thermionic emission is the process whereby electrons are emitted from metal surface.
 Diagram to explain thermionic emission:

Explanation of the operations:
When the cathode or hot filament is heated to a high temperature by a voltage source across its terminal, the filament emits electrons by thermionic emission. extra energy is given to the free electrons at the metal surface. The extra energy enable the electrons to brake away from the metal surface and exist as free elections. The electrons accelerate to the positively biased anode and therefore produce current which flow through the galvanometer.

Thermionic Devices:
These are devices that operate on the principle of thermionic emission. They devices :
• Diode valve
• X- ray tube
• Cathode ray tube

Explanation:
• Diode valve:
Diode value is a simple application of thermionic emission. It consist of an anode, filament made of tungsten wire and a cathode which surrounds the filament.
Diagram of valve:

Operations of diode valve:
When the filament is heated to a high temperature by a voltage source across its terminal, the filament emits electrons by thermionic emission. The electrons accelerate to the positively biased anode and therefore produce current which flow at the anode. The current can be detected by a galvanometer.

Diode valve characteristics:
The diode value characteristics is used to show and explain the action of  diode valve.
Diode valve characteristic circuit:

Operations of the circuit:
 When the filament is heated to a high temperature by a voltage source across its terminal, the filament emits electrons by thermionic emission. The electrons accelerate to the positively biased anode and therefore produce anode current which flow at the anode. The current can be detected by a galvanometer. A voltmeter is connected across the anode and the cathode to know the the anode voltage Va that produced the anode current Is. Different anode voltage Va are applied to obtain different anode current Ia. Each of the anode voltages Va are plotted its corresponding anode current  Ia to obtain the diode valve characteristics.

Diode valve characteristics curve:



Explanation of the curve:
The diode characteristics curve shows the behaviour of diode valve. The cure shows that diode current Ia is not directly proportional to diode voltage Va. Initially, as the anode voltage Va is increased, the diode current Ia increases until it reaches its maximum value. But with further increase in anode voltage , the anode current remained constant or decreases. This point of maximum value of Ia is called saturation point. The anode current this point is called saturation current. For this reason, diode valve does not obey ohms  and it is called a non ohmic conductor or substance. In diode valve, current flow only in one direction.

Similarities between thermionic emission and photoelectric emission:
• Both of them are surface phenomenon.
• Electrons are released in both cases.
• Energy is released in both cases.
• Energy transformation take place in both cases.
• Increase in temperate is involved in both cases.

Differences between thermionic emission and photoelectric emission
Thermionic emission Photoelectric emission
Electrons are released in the presence of heat in a vacuum.
Electrons are released at a particular temperature depending on the nature of the cathode. Electrons are released in the presence of sun light.
Electrons are released at threshold frequency depending on the nature if the metal surface.

Cathode ray oscilloscope:
Cathode ray oscilloscope is the application of thermionic emission. It is a vacuum tube that contains  electron gun at one end and  a fluorescent screen at the other end. Between these two are two pairs of reflectors that are used to control the electron unto the screan.
Diagram of cathode rays oscilloscope:


Explanation of operation of cathode rays oscilloscope:
The heated filament heats the cathode and cause it to produces stream of electrons by thermionic emission, which travel to the screen. The positive biased anodes accelerate the electron unto the screen. The X-plates are used  to move the electron beam  left and right while the y- plates move the electron beam up and down. The grid is used to regulate the amount of electrons that gravel to the screen and cause the screen to. Cooling fin is attached to cool the temperature of the tube.