Between these three groups there are no hard and fast dividing lines. However, engineers arbitrarily list various substances in the various groups, depending on the ease with which they conduct electricity.
Under the first group the following substances are considered to fall: Silver, copper, aluminum, zinc, brass, platinum, iron, nickel, tin, lead, in fact nearly all metals.
The substances classed as resistors are: German silver, carbon, graphite, nichrome, and various metallic alloys.
The insulators are: Slate, porcelain, bakelite, dry wood, glass, silk, cotton, rubber, mica, dry air, in fact, most non-metallic materials.
But don’t forget, however, that no material is a perfect conductor and no material is a perfect insulator. Even the best conductors known offer some resistance to current flow. On the other hand, even such good insulators as glass, mica and porcelain break down, if called upon to handle too high an e.m.f. However, below this point, which is known as the break-down point, insulators are for all practical purposes, non-conductors.
RESISTANCE IN CONDUCTORS
An electrical conductor may well be compared to a water pipe. When water is flowing through the pipe, its flow will be hindered to some extent by friction, that is, the inner walls of the pipe offer some frictional resistance to the flow of water through it. If these walls are rough, the friction will be considerable and the resistance to water flow will be high. If these walls are smooth, the resistance will be less, but some will be there all the same and we can’t eliminate it.
A large pipe will offer less resistance to water flow than a small pipe--and if too high a pressure is exerted on a small pipe, it will burst. The longer the pipe, the greater will be the loss in pressure through the pipe.
So too, an electrical conductor offers some resistance to current flow and this resistance is greater in a small wire, less in a larger wire, and varies with the length of the wire.
What happens to the energy that is lost because of resistance in a water pipe? It may seem peculiar, but the energy that is lost is converted into heat energy and dissipated. The amount of heat generated in a water system is so small that it is inappreciable, but in an electrical circuit, the heat generated by a
