
TM 9-1829A, April 1944
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2021-07-07
b.
Operation:
Even though the internal parts of the various make magnetic-type instruments vary in
construction and appearance, they all incorporate the same basic components (as described below) and
operate on the same principles.
(1)
Speed indication (fig. 5):
The speed-indicating portion of a speedometer or tachometer of the magnetic-
type operates on the magnetic principle and includes a revolving permanent magnet (driven by the
flexible shaft). Around this revolving permanent magnet is a stationary field plate (some Auto-Lite
instruments have a revolving field plate). Between the magnet and field plate is a movable speed cup,
with the indicating pointer attached to the end of the speed cup staff. The magnet revolves within the
speed cup. The revolving magnet sets up a rotating magnetic field which exerts a pull or magnetic drag
on the speed cup, making it revolve in the same direction. The movement of the speed cup is retarded
and held steady by a hair spring attached to the speed cup staff. The speed cup comes to rest at a point
where the magnetic drag is just balanced by the retarding force created by the hair spring. An additional
function of the hair spring is to pull the pointer back to zero when the vehicle or engine stops. There is
no mechanical connection between the revolving magnet and the speed cup. As the speed of the magnet
increases due to vehicle acceleration or (as in the case of a tachometer) increase in the engine speed, the
magnetic drag on the speed cup also increases and pulls the speed cup further around, thus registering a
faster speed by the pointer and face dial. The magnetic field is constant, and the amount of speed cup
deflection is at all times proportionate to the speed at which the magnet is being revolved.
Figure 5: Phantom view of basic components of speed-indicating portion of speedometer