How the electrical conductivity of insulators is measured?

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.

• 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). Commented Sep 22, 2015 at 18:30
• 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. Commented Sep 22, 2015 at 18:31
• 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. Commented Sep 22, 2015 at 19:26
• Could this be by chance ac conductivity? Commented Sep 22, 2015 at 19:51
• 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. Commented Sep 22, 2015 at 21:02