(i) Changes in atmospheric pressure affect measurement accuracy
through corresponding changes in pressure or force, air friction, anti
oxygen content. Because of air friction, a pendulum clock would run faster
at a high altitude than at sea level.
(j) Vibration generally is not desired in measuring equipment. It
causes microphonic noises in vacuum tubes.
In very high-magnification
instruments, which are read visually, vibration causes a blurring or
oscillation of the image with a reduction in readable resolution. Sometimes
a better reading can be obtained when vibration is present. Altimeters are
designed to be read while subjected to normal cockpit vibration.
When
vibration is not present, the mechanism of the altimeter lags behind changes
in pressure because of friction in the bearings and gears.
(k) Stray AC magnetic fields can induce undesired voltages into
lead wires, chokes, and transformers.
DC magnetic fields can deflect a
stream of electrons in a vacuum.
An unshielded cathode tube placed
vertically will have its spot deflected several millimeters by the earth's
magnetic field.
External DC magnetic fields can vary the accuracy of a
D'Arsonval-type voltmeter. The meter could read over or under, depending on
the polarity of the field. Some meters of this type are calibrated to be
used only when mounted on steel panels and when used out of this panel they
read high.
AC magnetic fields should be kept away from the permanent
magnets of meter movements, ferrite isolators, etc, to avoid damage by
partially demagnetizing them.
(l) AC electrostatic fields can be radiated from and can be picked
up by unshielded wiring. If the signal being measured is quite small and is
followed by considerable magnification, these stray pickups can create
objectionable noise levels or signal distortion. Static electricity, or DC
electrical fields, also exert an attraction or repulsion. This causes error
in the position of the meter needle when a slight static charge is on the
face of meter.
(m) For example, in the measurement of air pressure with a
mercurial barometer, the force of gravity is a pertinent factor and has to
be compensated for. The strength of the earth's gravitational field varies
considerably with altitude and latitude and to a lesser extent with the
nature of the earth's crust locally. This changes the effective weight of
the mercury in the barometer.
(n) In general, if a particular property is being measured, large
random amounts of the same property in the immediate environment will
interfere with the measurement.
(3) Instrument errors.
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