Copyright © 2007 Dorling Kindersley
A conductor is a material that allows electric charge to move through it as an electric current. Usually the charge is carried by electrons, and the conductor is a metal. Metals make good conductors because the outer electrons of their atoms are loosely attached, and the electrons can drift through the metal when a voltage is applied. Some materials have all their electrons firmly fixed in place, so they do not conduct electricity well. A material like this is called an INSULATOR.
Usually, the free electrons in a conductor whizz around in all directions. When a voltage is applied, however, they move more towards the positive terminal than in any other direction.
Some materials, called superconductors, have no resistance at all to the flow of current. Electrons move through them in a more organized way than in ordinary conductors. They are good for jobs like building huge electromagnets for medical scanners, but there is a problem. They only work if they are kept very, very cold. The highest temperature that even the most advanced superconductor can take is –135°C (–211°F).
Ohm discovered the law governing current flowing through conductors. He found that doubling the voltage between the ends of a wire doubled the current through it, while doubling the wire’s length halved the current. The wire had a resistance proportional to its length, and the current was the voltage divided by this resistance. Ohm’s law appeared in 1827.
Insulators conduct electricity poorly or not at all. Their electrons are bound tightly and will only move if an extremely high voltage is applied. Insulators are essential in electrical engineering, stopping current flowing where it should not. Most common materials, except metals, are insulators, but not all are suitable for electrical engineering. The earliest practical insulators were air, pottery, glass, and rubber. All are still used, but most insulators today are plastics.
Some insulators have to work under extreme conditions. These electricity supply insulators have to withstand a voltage of 440,000 V (440 kV, or kilovolts) and stop current flowing from the power cables to earth even in the middle of a rainstorm. They also have to take the weight of the cables. Plastics are not good enough for a job like this, but a much more ancient material – pottery – takes the strain with ease.