(^446) • 2019 eeworldonline.com | designworldonline.com
TEST & MEASUREMENT HANDBOOK
How to measure current
and energy use accurately
The direct measurement of current is difficult. It’s
usually impractical to count individual electrons, so the
two primary techniques for measuring current actually
measure “side-effects” of current. The first technique uses
the fact that moving charged particles create a magnetic
field (Ampère’s Law). The second technique uses the fact
that charged particles moving through resistance create
a voltage (Ohm’s Law). Both these techniques can be
derived from Maxwell’s equations.
The fact that current creates a magnetic field was first
discovered by Ørsted in 1820 using a compass. This
technique was later refined into the modern galvanometer.
Most modern galvanometers have a permanent magnet
and a pivoting coil of wire. In a typical D’Arsonval/Weston
type galvanometer, current flowing through the coil pushes
towards or away from the permanent magnet. The coil
magnetic field is counteracted by the permanent magnet
which forces the coil to twist, moving the pointer. If you
have ever seen analog multimeters or vintage stereo
equipment, you have likely seen a galvanometer.
General-purpose test equipment can have trouble measuring small
currents, particularly if they only happen briefly. New specialized
instruments go a long way toward solving this problem.
MATT LIBERTY | JETPERCH LLC
Jetperch — Test and Measurement HB 06-19 copy.indd 44 6/7/19 2:05 PM
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