To determine the ionization potential of mercury used inside a Thyratron Tube

To determine the ionization potential of mercury used inside a Thyratron Tube.

Apparatus Required

1. Thyratron Tube
2. Ammeter
3. Variable Power Supply
4. Voltmeter
5. Connecting Wires


Ionization potential is defined as the minimum amount of energy required in (ev) electron - volts to just remove an electron from an atom. Here the ionization potential of mercury can be determined by filling the vapours of mercury in a diode or triode tube. The hot cathode has filled triode is known as Thyratron.

In our experiment when a positive potential is applied to the plate and it is increased slowly, plate current also increases slowly. But when plate potential increases beyond a particular value, plate current increases more rapidly. This is because the electrons arriving at the anode gain enough energy to knock out electrons from the atoms of the gas close to anode. These electrons are also attracted by plate and hence causing increase in plate and current. he positive ions so produced neutralize the space charge which further helps in increasing the kinetic energy of thermal electrons. This value of plate potential at which plate current shows large increase is known as ionization potential of the gas (vapours). The circuit connections are shown in the figure given below:

circuit diagram of thyratron tube


1. (i). Make connections as shown in the circuit diagram figure above and switch on power supply.

2. Vary plate voltage, Vg = 1 ? 20 V and take reading of plate current, Ip, for each one.

3. Draw graph between Vg & Ip .

graph of thyratron tube experiment

4. Draw a straight line AB between last few points. This line intersects with X-axis at A. OA gives the value of observed ionization potential of mercury.

Observation Table

observation table thyratron tube


The ionization potential of mercury is ............ Volts.


  • A gas filled tetrode must be used.
  • The connections should be proper before switching on the power supply.

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