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

Mercury Ionization Potential Introduction :

In this experiment, a simple principle is used to determine the ionization 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 Ionization 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 ionization 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 ionization 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 ionization 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 ionization 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, ionization 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.