Every material offers a certain amount of opposition to the flow of electricity through it. Resistance may be defined in a general way as that property of a body which opposes the flow of electricity. A conductor is a substance of such small resistance that it readily allows electricity to flow through it. The best conductors Eire carbon and metals, notably silver, copper and aluminium.
In conductors electrons are able to pass readily from atom to atom. The outer electrons of the atoms of a good conductor, such as copper, are not rigidly held by their attracting nuclei, so that there is, at any Instant, a number of outer electrons, detached from their parent atoms and existing as free electrons, can move about in the spaces between the atoms, passing from the orbital system of one atom to that of another, and so on.
Although sliver is better conductor than copper, its use as a conductor is very limited because of its cost. In a few instances it is used where a delicate and highly conducting material is necessary. Copper, because of its high conductivity and moderate cost, is used more extensively as a conductor than any other material.
It has many advantages such as ductility, high tensile strength, is not corroded by the atmosphere and is readily soldered. In P.V.C. wires, C.T.S. wires, paper Insulated armoured cables, trolley lines for tram cars, trolley-bus, electric locomotives, for armature windings of different machines and appliances, copper conductors are used.
ADVERTISEMENTS:
Aluminium has sixty one per cent of the conductivity of copper. It is softer than copper and its tensile strength being much less, sometimes steel conductor or conductors are reinforced with aluminium conductors. At present aluminium conductor is extensively used for P.V.C. wires, tropodur cables, distribution lines, overhead lines for street light connections etc.
When steel conductor Is reinforced with aluminium conductor, it is known as A.C.S.R. conductor (Aluminium Conductor Steel Reinforced). This type of conductor is generally used as a conductor for high voltage transmission lines where its lightness and large diameter are an advantage.
Iron and steel have about nine times the resistance of copper for the same cross-section and length. These materials are most commonly used as resistors in connection with rheostats and as earth connecting leads.
Lead wire is used as fuse wire. In most instances this wire is made of lead-tin alloy. Conductors becomes worse conductors when they Eire heated, i.e., the resistance they offer to the flow of electricity becomes greater.
ADVERTISEMENTS:
The size of a conductor depends on the following factors:
(i) It depends on the amount of current flowing through a circuit. The greater the flow of current, the larger is the size of the conductor.
(ii) It depends on the voltage drop in the line. If the voltage drop is unusually high, the intensity of illumination obtained from, a lamp decreases and the speed of an electric motor slows down. To reduce the voltage drop in the line conductors, therefore, the resistance of the conductors has to be decreased. Thus, the size of a conductor does not depend on the amount of circuit current alone. Its size should be such as to keep the voltage drop in it within reasonable limit.
(iii) In case of overhead lines the size of a bare conductor is also determined by its tensile strength. According to Indian Electricity Rule No. 74(1) the breaking strength of all conductors of any overhead line shall not be less than 317.51 kilograms.
ADVERTISEMENTS:
(iv) Configuration and arrangement of a circuit also determine the size of its conductors. For example, if a branch line is not tapped off from a main line through a fuse cut-out or circuit breaker, the size of the conductor in the branch line should be the same as that of the conductor in the main line.