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I was doing solid states in chemistry and in the section "electrical properties" there were given the range of electrical conductivity of conductors, insulators and semi-conductors.

My question is

If current do not flow throw insulators then how we can measure its electrical conductivity.

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    $\begingroup$ Current will flow, albeit in small amounts under high voltage gradients. Also, there is a difference between resistivity (material property) and resistance (realized device property) which you can use to your advantage in measurements to increase measured current and decrease the required voltage drop (e.g. make a large cross-section, thin film resistor). $\endgroup$
    – Jon Custer
    Commented Sep 22, 2015 at 18:30
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    $\begingroup$ If there was no current, the conductivity would be zero. Since people do measure conductivities, I think it's safe to assume a little bit of current does flow. $\endgroup$
    – Javier
    Commented Sep 22, 2015 at 18:31
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    $\begingroup$ To put it another way, why do you think that NO current flows in an insulator? Here dictionary.reference.com/browse/insulator is a definition. "Negligible" is not zero. $\endgroup$
    – WhatRoughBeast
    Commented Sep 22, 2015 at 19:26
  • $\begingroup$ Could this be by chance ac conductivity? $\endgroup$
    – LLlAMnYP
    Commented Sep 22, 2015 at 19:51
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    $\begingroup$ Basically one applies a voltage to the insulator and measures the current, just like with conductors, however, in practice it is very difficult to do this without being swamped by surface conduction. The proper setup requires very clean surfaces and guard electrodes that absorb all surface currents. $\endgroup$
    – CuriousOne
    Commented Sep 22, 2015 at 21:02

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Conductors, insulators, and semi-conductors all have physical definitions relating to how their valence electrons behave. But from the simpler electrical point of view, the difference between conductors and insulators is simply their conductance/resistance. Insulators conduct very poorly, conductors conduct well. You can make a conductor (say, a metal wire) conduct poorly (by making it very thin or by heating it). It's more difficult to make an insulator conduct better.

Semiconductors are a very different beast. Their conductance depends on many more factors than mentioned here and are out of the scope of the question.

But to make a long story short, nearly everything conducts, but how much it conducts can vary by many orders of magnitude.

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  • $\begingroup$ You should change your last sentence to "everything conducts" before I start asking "Can you name a substance that doesn't?". $\endgroup$
    – CuriousOne
    Commented Sep 22, 2015 at 21:03
  • $\begingroup$ Actually there are "superinsulators" out there which do not conduct at all [en.wikipedia.org/wiki/Superinsulator](superinsulator) $\endgroup$
    – Dr Xorile
    Commented Sep 22, 2015 at 22:26
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    $\begingroup$ See I knew there would be an exception if I said "everything conducts"! $\endgroup$
    – Dave
    Commented Sep 22, 2015 at 22:30

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