In the single-wire electric starting system of an automobile, current
from the storage battery reaches the starting motor through the
starting switch and a single heavy cable, and returns through the
frame and other metal parts of the car itself, or vice versa. This
is another instance of a ground-return circuit.
Both the primary and secondary sides of the ignition system
of an automobile are also grounded circuits. In contrast with

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ELECTRICAL EQUIPMENT



this, the circuit may be composed of copper cables directly con-
necting both poles of the battery and switch with the starting
motor. The highly insulated cable employed for both ignition
and starting systems is expensive and the use of a single wire greatly
simplifies the connections, considerations which account for the
general use of this type of circuit. A circuit is said to be open when
there is a break in it which prevents the current from flowing, as



starting Motor




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Fig. 1. Typical Starting-Lighting Wiring Diagrams, (a) Series Circuit of
Starting Motor; (b) Multiple Circuit of Lamps



when the switch is opened, or when a connection or the wire itself
is broken.
Series Circuit The connections between a storage battery,
switch, and starting motor, comprise the simplest form of circuit,
in which the motor is said to be in series with the battery, and'
the cells of the battery are in series with one another. This is
termed a series circuit and a break in it at any point opens the
entire circuit. The starting motor, Fig. 1 (a), requires the entire
output of the storage battery for its operation.
