An experiment to compare the electric conducting powers of various substances was described on page 365. There it was seen that different materials could be classified as good or poor conductors and insulators. As far as current electricity is concerned, we usually think in terms of the ability of a substance to resist the flow of electricity through it. A good conductor is therefore said to have a low resistance and a poor conductor a high resistance. With regard to insulation, materials which are good insulators for static electricity are also good insulators for current electricity.
We shall see later (page 457) that the resistance of a wire depends on its dimensions and the material from which it is made. For a wire of given dimensions, silver offers least resistance to the current. Silver is too expensive for normal use, and therefore copper, which comes next on the list, is used for electric cables and for connecting wires in electric circuits.
When a high resistance is required, e.g., to reduce the current in a circuit, special alloys are used. The most common of these are constantan (otherwise called contra or eureka), manganin and nichrome. Constantan is used for general purposes, while manganin is used for making highquality standard resistors. Nichrome is the alloy from which the elements of electric fires are made, since it resists oxidation when red hot. Constantan and manganin wires respectively have a resistance of about twenty-five times, and nichrome about sixty times, that of a copper wire of the same dimensions.