In the periodic chart, the mercury atomic number is 80 and it is symbol Hg. To determine the ionisation potential of mercury a mercury diode valve is used.

Mercury Ionisation Potential Introduction :

In this experiment, a simple principle is used to determine the ionisation potential of mercury. . A filament which is put under the cathode plate glows and provide the thermal heat to the Cathode.

As a result, the cathode plate starts to eject the electrons from their surface. These electrons will approach the anode plate if we connect it with the positive terminal of the battery. Initially, when the potential difference between the cathode and anode is small very few electrons reach up to the plate. And you observe very small current.

Working of the ionisation Tube: Physics Lab

But slowly when you increase the potential difference in between the anode and cathode plate some more electrons attract towards the anode plate. The current increases every time because a number of electrons attract towards the anode plate.

You can also conclude that when you are increasing the potential between the plates the kinetic energy of emitted electrons is also increasing. At a certain amount of kinetic energy of the emitted electrons, it reaches that level where they are capable to knockout the electron from mercury atoms.

Here, the ejected electrons from the mercury atom are known as secondary electrons. Now secondary electrons are also contributed in the current along with the primary electrons.

When a sufficient voltage is applied all the electrons emitted from the cathode reach at the plate along with the secondary electrons. This is the reason that the current suddenly increases at a certain applied voltage. That value of the applied voltage known as the ionisation potential of mercury.

What is space charge and how to neutralize that?

When we increase the potential electrons get accelerated and ionize the mercury atoms. As a result, we observe the remaining positive ions near the anode plate. This ion charges block the current. So further when we increase the potential it moves towards the cathode plate and neutralizes the space charge.

The electrons released from the ionisation of mercury reach up to the plate, in addition to the electrons emitted from the cathode and as a result current increase suddenly. This is the point of ionisation potential.

Here, in this post, I have discussed the primary and secondary electrons. How space charge creates? and how one neutralize that? Why the current suddenly increases? and How to define the ionization potential of the mercury atom?

Plot Graph

To find the ionisation potential of mercury, you will need to plot the current and voltage. Take potential at the x-axis and current on the y-axis. Check that point where current suddenly increases. For it please use slop at both the lines. Wherever they cut each other you join that point on the x-axis. In this way, you will get the ionization value. Compare it with the standard result and find the experimental error.

For mercury, ionisation potential is 10.39 V

For the % error you take the ration [(standard value-observed value) / standard value] x 100 and check. You can observe yourself that what are the possible causes of this % error.


One important point I want to share with you that don’t run longer the high potential into the mercury diode valve, otherwise, it can be burn. Check your circuit diagram and try to understand it with the black box, a box of apparatus. Most of the time in Indian Universities they provide a box and block diagram on that.

AIM: To find the ionization potential of mercury using a gas-filled diode.

APPARATUS: A gas-filled (mercury vapor) diode, a D.C. power supply, a voltmeter, a millimeter, and connecting wires.


The atom or molecule’s term ionisation energy is the energy needed to remove electrons out of an atom or molecule. Large atoms require low energy for ionisation, however, small elements require high energy for ionisation. This amount was simply referred to as the potential for ionisation, i.e. the energy required to remove electrons from an atom and expressed in volt (V). ⠀

The ionisation potential of mercury: Mercury ionisation potential can be measured by adding mercury vapour in an evacuated tube fitted with a cathode and anode at a low pressure of 10 mm to 50 mm of the mercury. The gas diode cathode can be heated directly or indirectly.

diode filled with gas is called phantom (or thyratron). The mark in the pipe indicates the gas or vapour presence.

When you connect the thyratron with the battery as per the circuit diagram. Then first you connect filament with it, so it starts to glow. Then after you provide the positive potential to the plate respective to the cathode.

Now the electrons that emit from the cathode starts to move towards the plate. As a result, you observe the current. These electrons emit from the cathode that is in Vaccum tube (this thyratron bulb) and moves inside the diode gas valve towards the plate. Remember, this flow of charges in space. When it reaches at the plate it complete the circuit and you observe the current.

This electronic current depends on these factors:

  • The number of electrons emitted per unit area from the cathode.
  • The temperature of the cathode that you are providing by the filament.
  • The effect of the space charge region, i.e., the negatively charged region containing the electron cloud due to the accumulation of electrons emitted by the cathode.

You may have seen when you increase the plate potential current also increases. Do you know why? The answer to it you have to understand with proper basic concept. See, the temperature is fixed, the number of electrons are emitting from the cathode. But not all electrons are reaching on the plate, so number is less. But when you increase the potential number of electrons increases, as a result, current increases.

When the potential is increased beyond a critical value, where all the electrons attracted toward the plate than that time do you know what happens? See, when potential increases the attraction will increase and simultaneously, it will enhance the kinetic energy of the emitted electrons. Now, these high-speed electrons strike on the mercury atom. That results, ionisation of the mercury atoms.

The electrons which emitted from the cathode are known as primary electrons while electron that ejects from the mercury atom are known as secondary electrons.

This value of plate potential at which the electron ejects from the mercury atoms is called the ionization potential. At this point, you observe the sudden change into the current.

OBSERVATIONS: As an example, you can see here;

Sr. No. Plate voltage in volts Plate current in mA
1 1 0.2
2 2 0.5
3 3 1.0
4 4 1.5
5 5 2.0
6 6 2.5
7 7 3.0
8 8 7.0
9 9 11.0

Draw The Graph:

To draw the graph first do scaling that what value you are taking for 10 small division on X-axis and Y-axis. This number may vary, you can consider 1 small division or 1 box like that. Right after making it clear that on X-axis you are taking potential and current on Y-axis.

Now draw the points on the graph. You will see the non-linear behaviour of the V-I plot. Now how to determine the value of ionisation potential? for it, you take two slopes and observe the intersection points. This is the result.

Now compare it with the standard result of mercury and show percentage error. Explain in your notebook that what you learned from the experiment and how it is significant?