Ordinary telegraph wire is about 1⁄6th of an inch thick; a wire twice as thick would conduct four times as well, having four times the area of cross section; hence an equal length of it would have only 1⁄4th the resistance.
3. The resistance of a conducting wire of given length and thickness depends upon the material of which it is made—that is, upon the specific resistance of the material.
Conductivity.—This is the inverse of resistance. The term expresses the capability of a substance to conduct the electric current.
If the symbol Y represent the conductivity of a substance, and I the current then:
I/Y = its resistance;
and if R represent the resistance of a substance, then
I/R = its conductivity.
Good conductors of heat are also good conductors of electricity.
Specific Conductivity.—The figure which indicates the relation between one substance and another as to their capacity to conduct electricity is called specific or relative conductivity. Taking the specific conductivity of silver as 100, that of pure copper is 96.
The specific resistance of a substance is the reverse of its relative conductivity. The specific resistance of a metal is generally expressed in millionths of an ohm as the resistance of a centimeter cube of that metal between opposite sides.