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Cars are equipped with fuel gauges which are operated along with the
vehicle's electrical system. There are two types: the thermostatic
type and the balancing coil type. The thermostatic type is made of a
standing unit, located in the fuel tank, and the gauge itself
(registering unit), which is located on the instrument panel. The
fuel gauge used in some cars and trucks is of the electrically
operated balanced coil type. These have a dash unit and a tank unit.
The dash unit has two coils, spaced about 90 degrees apart, with an
armature and integral pointer at the intersections of the coil axis.
The dial has a scale in fractions between "Empty" and "Full". The
tank unit has a housing, which encloses a rheostat, and a sliding
brush which contacts the rheostat. The brush is actuated by the float
arm. The movement of the float arm is controlled by the height of the
fuel in the supply tank. The height of the fuel (called variations in
resistance) changes the value of the dash unit coil so that the
pointer indicates the amount of fuel available. A calibrated friction
brake is included in the tank unit to prevent the wave motions of the
fuel from fluctuating the pointer on the dash unit. Current from the
battery passes through the limiting coil to the common connection
between the two coils, which is the lower terminal on the dash unit.
The current is then offered two paths, one through the operating coil
of the dash unit and the other over the wire to the tank unit. When
the tank is low or empty, the sliding brush cuts out all resistance
in the tank unit. Most of the current will pass through the tank unit
circuit because of the low resistance and only a small portion
through the operating coil to the dash unit. As a result, this coil
is not magnetized enough to move the dash unit pointer, which is then
held at the "Empty" position by the limiting coil.
If the tank is partly full or full, the float rises on the surface of
the fuel and moves the sliding brush over the rheostat, putting
resistance in the tank unit circuit. More current will then pass
through the operating coil to give a magnetic pull on the pointer,
which overcomes some of the pull of the limiting coil. When the tank
is full, the tank unit circuit contains the maximum resistance to the
flow of the current. The operating coil will then receive its maximum
current and exert pull of the pointer to give a "Full" reading. As
the tank empties, the operating coil loses some of its magnetic pull
and the limiting coil will still have about the same pull so that the
pointer is pulled toward the lower reading. Variations in battery
voltage will not cause an error in the gauge reading because its
operation only depends on the difference in magnetic effect between
the two coils.
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