magnetic field is constant, and the amount of speed cup
the magnetic type is operated by a permanent magnet,
deflection is at all times proportional to the speed at
which is driven by a flexible shaft. Around this
which the magnet is being revolved.
revolving permanent magnet is a stationary field plate.
(Some instruments have a revolving field plate.) A
TOTAL ODOMETER.--The total odometer (fig.
movable speed cup is located between the magnet and
6-85, view A) is driven through a series of gears
field plate, with the indicating pointer attached to the
originating at a spiral gear out on the magnet shaft. This
end of the speed cup staff.
gear, known as the first gear, drives an intermediate
As the magnet revolves within the speed cup, it sets
second gear and a third gear, which is connected to a
up a rotating magnetic field, which exerts a pull, or
fourth gear at the odometer. The fourth gear turns the
magnetic drag, on the speed cup, making it revolve in
odometer through a series of star pinion gears inside the
the same direction. Movement of the speed cup is
odometer dials, or wheels. The total odometer usually
retarded and held steady by a hairspring attached to the
has five figure wheels, or dials, and is so constructed
speed cup staff. The speed cup comes to rest at a point
and geared that as any one wheel finishes a complete
where the magnetic drag is balanced by the retarding
revolution, it turns the next figure wheel to the left
force created by the hairspring. An additional function
one-tenth of a revolution. Most models record to 99,999
of the hairspring is to pull the pointer back to zero when
miles, and then automatically zero themselves.
the vehicle or engine stops.
TRIP ODOMETER.--The trip odometer (fig.
There is no mechanical connection between the
6-85, view B) is also driven by the third gear, through
revolving magnet and the speed cup. As the speed of the
the trip odometer drive gear and another gear at the trip
magnet increases due to vehicle acceleration or (as in
odometer. The trip odometer usually has four wheels,
the case of a tachometer) to an increase in engine speed,
and is so constructed that as any one finishes a complete
the magnetic drag on the speed cup also increases and
revolution, it turns the next wheel to the left one-tenth
pulls the speed cup farther around, thus registering a
of a revolution. The wheel on the extreme right registers
faster speed by the pointer and face dial. The magnet's
in tenths of a mile. Most models record to 999.9 miles,
INTERMEDIATE GEAR
TOTAL ODOMETER
THIRD
GEAR
TRIP
ODOMETER
FIRST
GEAR
RESET
SHAFT
MAGNET
SHAFT
SECOND
GEAR
B
A
ASf06085
Figure 6-85.--(A) Total odometer; (B) Trip odometer.
6-69
