Conductors and insulators

As is well known, to conduct an electric current is the same as to transmit electrons. All substances have some ability to transmit electrons but they differ greatly in the ease with which electrons pass through them. For instance, a copper wire conducts electricity readily, glass seems to conduct so little current that it is hardly measurable. Substances through which currents easily pass are known to be conductors. Those substances that strongly resist the flow of current are termed insulators. An insulator is also called a dielectric. It has very few free charges that are able to move under the influence of the electric field.

There is, however, no sharp distinction between conductors and insulators (dielectrics). Under ordinary conditions there is no perfect conductor and no perfect non-conductor. There is a continuous gradation from good conductors to good dielectrics.

For instance, paper though a poor conductor is by no means a perfect insulator. Indeed, all substances conduct a little electricity – even such materials as porcelain, rubber, paper, and glass, which are considered as good insulators. However, the insulators have so few electrons that can move about freely that, in practice, they allow only a negligible current to flow through them.

Almost all metals are good conductors of electricity but silver is believed to be the best conductor of all. Copper comes next, it is followed by aluminium. Copper is our most commonly used conductor. In addition to its high conductivity, copper is abundant, easily mined and processed.

Some liquids also conduct electric currents. They even prove to be good conductors of electricity. Water with salt conducts electricity well. On the other hand, we know distilled water to have high resistivity.

Most gases conduct current under proper conditions of pressure and temperature. However, they are not as good electrical conductors as metals.

As a rule, most of the non-metals are found to transmit only a negligible current; that is why they are considered as insulators. The most common materials used to resist electric flow are: glass, rubber, porcelain, paper, oil, cotton, and silk. Non-ionized clean air is also considered to be a good insulator. It is often used for this purpose in electrical apparatus.

It would be quite wrong to think that conducting materials are the only materials to be used for power transmission. We need both conductors and insulators. Indeed, we cannot do without the copper wire which conducts the electric current, that is, acts as a conductor. However, we have to use an insulator to prevent electrical loss.

In the transmission of power, we generally employ the best conductor available in order that as little power as possible might be lost in heating the transmission line. For other purposes, however, the poorer conductors are often used because they happen to possess other desirable properties. Thus, tungsten is usually the metal to be chosen for the filaments of electric lamps.