I saw this number on Wikipedia and other videos, but the calculation part is no where to be seen. Can someone explain how this value is calculated?

  • $\begingroup$ Why would you assume that there is a calculation part? $\endgroup$
    – user137289
    Commented Nov 5, 2020 at 9:45
  • $\begingroup$ Then from where did this value arise from? $\endgroup$ Commented Nov 5, 2020 at 10:37
  • $\begingroup$ @IhsanAhmedK What is the other way to determine physical quantities? $\endgroup$
    – user137289
    Commented Nov 5, 2020 at 11:47
  • $\begingroup$ @Pieter That is what I want to know. I know the discharge depends on pressure, temperature and the dielectric constant of the medium, in this case, air. But I have no clue how this value is reached. $\endgroup$ Commented Nov 5, 2020 at 17:01
  • 1
    $\begingroup$ @IhsanAhmedK pieter is trying to say experiments $\endgroup$ Commented Nov 6, 2020 at 12:06

1 Answer 1


With proper measuring instruments, it can be done as follows.

First the setup......

We are going to need two parallel metallic plates (just like a parallel plate capacitor). Then we need to connect a high voltage DC supply across these two plates. The magnitude of voltage should be adjustable and the maximum magnitude of voltage across the plates should be up to a million volts. Next we need to be able to measure the distance between the plates. For getting the required value quickly, we should use smaller distances; from 1mm to 5mm.

Now the procedure.....

First adjust the gap between the plates to a predetermined value say 2 mm. Now start increasing the voltage applied across the parallel plates still there is a spark jumping from positive to negative plate. Note down the magnitude of the voltage at which the spark occurs. Try this at least two to three times to get an accurate reading of of voltage (V) and distance between the plates (d). To ensure accuracy, repeat the same procedure for different distance between the two plates.

For each set of readings, use the formula E = V/d to calculate the electric field exactly at or before the occurrence of spark.

The event of occurrence of spark exhibits dielectric breakdown and the electric field at which this breakdown occurs is the maximum magnitude of electric field that the medium(in this case air) can carry without losing its dielectric property that is its dielectric strength.

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    $\begingroup$ You may find that the result relates to the ionization energy of water molecules. I have heard that dry nitrogen makes a pretty good insulator. $\endgroup$
    – R.W. Bird
    Commented Nov 6, 2020 at 14:30
  • $\begingroup$ Nitrogen or any other inert gases are good insulators. To prevent early dielectric breakdown, temperature should be lower and pressure, higher. $\endgroup$
    – Pramod
    Commented Nov 9, 2020 at 3:52

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