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Electric
Meters : instruments for measuring and indicating magnitudes of
electrical values, such as current, charge, potential, and power, and the
electrical characteristics of circuits, such as resistance, capacitance,
and inductance. The information is usually presented in terms of defined,
standard electrical units, which principally are the ohm, volt, ampere,
coulomb, henry, farad, watt, and joule . Given that all forms of matter exhibit one or more electrical
characteristics, electrical measurements may be taken from an essentially
unlimited number of sources.
II. Basic
Meter Mechanisms
By
their nature, electrical values cannot be measured by direct observation.
Therefore, some property of electricity must be used to produce a physical
force that can be observed and measured. For example, in a galvanometer,
which was the earliest type of meter devised, the force existing between a
magnetic field and a pivoted, current-carrying coil within the field
causes an observable deflection of the coil. Because the deflection is
proportional to the current, a calibrated scale is employed to measure the
electrical current. Electromagnetic action between currents, force action
between electric charges, and the heating effect caused by conductor
resistance are some other examples of indirect methods used for electric
meter analog readouts.
III.
Meter Calibration
To
ensure uniformity and accuracy, electric meters are calibrated according
to the accepted standards of measurement for the given electrical unit
(such as volt, ampere, ohm, and watt) as established in the United States,
for example, by the National Institute of Standards and Technology.
IV.
Primary Standards and Absolute Measurements
The
primary standards for the ohm and ampere are based on internationally
accepted definitions of these units in terms of mass, conductor dimension,
and time. The measurement techniques that employ these basic units are
accurate and reproducible. Absolute ampere measurements, for example,
involve the use of a weighing-balance scheme that measures the force
between a set of fixed coils and a moving coil. Absolute measurements of
current and potential difference are of importance primarily in laboratory
work, and for most purposes relative measurements are sufficient. The
meters described in the following paragraphs are all intended for relative
measurements.
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