The ohmic resistance of a conductor depends upon four factors:
(a) Material. Most metals are conductors of electricity. Silver and
copper have comparatively low resistance, allowing current to flow much more freely
than do insulators or nonconductors such as rubber, glass, and plastics, which have
high resistance. Copper is used almost exclusively in electric wiring as it offers less
resistance to the flow of electric current than any other common material.
(b) Length. For a given material, the resistance of a conductor is
directly proportional to its length. For example, 10 feet of wire has 10 times as much
resistance as 1 foot of the same wire.
(c) Cross-section. The greater the cross-section of a conductor, the
smaller will be its resistance per foot of length. For example, a wire 1 millimeter (mm) in
diameter has four times as much resistance as a wire of the same length and material,
but 2 mm in diameter, because the latter has four times as much cross-sectional area.
The resistance of a conductor is inversely proportional to its cross-sectional area (or to
the square of its radius).
(d) Temperature. For most conductors, the resistance increases with
an increase in the conductor's temperature. The hotter the conductor becomes, the
greater will be its resistance.
Section III. ELEMENTARY ELECTRIC CIRCUITS
Every circuit has three essential requirements: (1) a source of potential
difference (voltage), (2) a current-carrying wire (conductor), and (3) a resistive device to
regulate the rate of current flow (resistance). Figure 2-2 shows some of the common
diagrammatic symbols used in electrical drawings.
a. Series. A series circuit is one in which all the resistances are connected end
to end so that the same current flows through each part of the circuit, using but one
path. In the simple series circuit shown in figure 2-3, all of the current flowing from the
battery must flow through the switch, the resistor, and the lamp, then back to the
b. Parallel. Parallel circuits provide more than one path in which the current can
flow. In figure 2-4, the current from the battery divides to follow two paths.
Recombination of the current occurs when one portion of the current flows through the
resistor and the other through the lamp. The amount of current flowing in each path
(branch) depends on the resistance in that branch of the circuit.
c. Closed. A closed circuit results when a complete path is available through
which the electrons can flow.