How good insulator?

Answer 1

There are several types of insulators: thermal, electric,, sound insulators and other. I assume you are talking about electrical insulators.

The electrical resistance per unit area of a wire and unit length is called specific resistance or resistivity r and it is obtained by the wire resistance R as

r=R A / L

where A is the wire area and L is the wire length. It is the resistivity that qualify a material as a good electrical insulator. Perhaps the best insulator among common materials is fused quartz whose resistivity is about 1016 Ohm m (that is one with 16 zeros after it !!). Also glass is a very good insulator, pure glass (that is pure amorphous SiO2 without any metal addition) has a resistivity of about 1014 Ohm m.

Just for an additional reference, gold that is a very good electrical conductor has a resistivity of 2.35 10-8 Ohm m, that is 0,00000000235 !)

It is to be observed that, due to the mechanism underlying electrical and thermal conductivity, solid materials that are good electrical insulators are generally also good thermal insulators (but few exceptions do exist).