266. Resistance of Conductors.—With an ammeter one may study the change produced in the amount of current flowing in a wire when a change is made in the wire conducting the current. For example, if one measures with an ammeter the current flowing from a dry cell through a long and then through a short piece of fine copper wire, it will be seen that less current flows when the long piece is used. That is, the long wire seems to hinder or to resist the passing of the current more than the short piece. In other words, the long wire is said to have more resistance.
The resistance of a conducting body is affected by several conditions.
(a) It is directly proportional to the length of the conductor, one hundred feet of wire having twice the resistance of fifty feet.
(b) It is inversely proportional to the square of the diameter; a wire 0.1 inch in diameter has four times the resistance of a wire 0.2 inch in diameter.
(c) It differs with different substances, iron having about six times as much as copper.
(d) It varies with the temperature, metals having greater resistance at a higher temperature.
Since silver is the best conductor known, the resistances of other substances are compared with it as a standard.
The ratio of the resistance of a wire of any substance as compared to the resistance of a silver wire of exactly the same diameter and length is called its relative resistance.
Purified substances arranged in order of increasing resistance for the same length and sectional area (Ayrton-Mather) are given on p. 294.