Lesson 4/Learning Event 2
Construction
Most lamps are provided with a single contact for each filament within the lamp, the current
through each filament being completed to the shell of the lamp base. A double-filament lamp with
single-contact construction has two contacts on the lamp base, each being connected with one of
the filaments. The return from both filaments is to the lamp base shell, which is grounded
through the lamp socket. Thus, there are two separate circuits with two contacts on the base, each
of which might properly be termed a single contact, for a grounded circuit.
Because the voltage used in automotive electrical systems is low and the current required as a result
is high, the filaments are much shorter and stronger than those used for standard house lighting
lamps. A short and thick, rather than a long and thin, filament can withstand rough treatment, and
this is desirable in the case of a lamp subjected to the vibrations of a motor vehicle. A short
filament also provides a concentrated light source that will give a better focus.
Size
Lamps range in size from the small one-half candlepower instrument panel lamps to the large 50 or
more candlepower driving lamps.
Current Requirements
The two-candlepower lamp consumes 0.21 ampere at 12 volts. The four-candlepower lamp
consumes 0.22 ampere at 12 volts. A lamp with two filaments, one of 32 candlepower and the
other of 21 candlepower, will draw 1.3 and 1.8 amperes. One reason for the rapid discharge of
storage batteries in winter is the increased number of hours that lamps are used. There is a direct
relation between the total current use and the number of lamps used. All storage batteries are rated
by ampere-hours; that is, the number of hours a battery can be used at a certain discharge
amperage before it becomes depleted. For instance, two headlight lamps burning at 4 amperes
each, a total of 8 amperes, would discharge a storage battery rated at 80 ampere-hours in
approximately 10 hours, unless the generator charged the battery.
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