MATERIALS

If we connect a battery across a body, there is a movement of free electrons towards the positive end. This movement of electrons is an electric current. All materials can be classified into three group according to readily they permit an electric current to flow. There are: conductors, insulators, and semiconductors.

In the first category are substances, which provide an easy path for an electric current. All metals are conductors, however some metals do not so conduct well. Manganin, for example, is a poor conductor. Copper is a good conductor, therefore it is widely used for cables. A non-metals, which conducts well, is carbon. Salt water is an example of a liquid conductor.

A material, which does not easily release electrons, is called an insulator. Rubber, nylon, porcelain, and air are all conductors. There are no perfect insulators. All insulators will allow some flow of electrons, however this can bi ignored because the flow they permit is so small.

Semiconductors are midway between conductors and insulators. Under certain conditions they allow a current to flow easily but under others they behave as insulators. Germanium and silicon are semiconductors. Mixtures of certain metallic oxides also act as semiconductors. These are known as thermistors. The resisance of thermistors falls rapidly as their temperature rise. They are therefore used in temperature-sensing devices.