Conductors

Nearly all substances as they occur in nature conduct electricity - i.e., if the substance is joined to a source of electrical energy, a magnetic field is created around it. Roughly, three groups of conductors may be formed, but of very varying degree: 1st, good conductors, pure metals, and alloys of metals; 2d, at a long interval, solutions of electrolytes - i.e., solutions capable of being decomposed by the passage of an electric current through them; and 3d, very bad conductors, such as India rubber, ebonite, shellac, sulphur, glass, slate, marble, stoneware, mica, dry wood and paper, animal fibers (silk, wool, furs), petroleum oil, paraffin wax, ozokerit, pitch, bitumen, etc. Usually, in practical work, the first class is spoken of as conductors, and the third class as insulators.

Resistance

The resistance of a conductor is

(a) Directly proportional to its length; (6) Inversely proportional to its cross-sectional area; (c) Directly proportional to its specific resistance; (d) and usually increases with its temperature.

Specific Resistance

The specific resistance of a substance is usually stated as the resistance between the faces of a cube of the substance, 1 centimeter in length and 1 square centimeter in cross-sectional area.

The law of resistance may be stated thus, neglecting the effect of temperature:

R = pl / s; where

R = the resistance in ohms; l =the length of conductor; s = the cross-sectional area of the conductor; p = the specific resistance of the material.