TE S T & ME ASUREMENT HANDBOOK
How to measure current and energy use accurately 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.
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.
44
Jetperch — Test and Measurement HB 06-19 copy.indd 44
6 • 2019
MATT LIBERTY | JETPERCH LLC
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.
eeworldonline.com | designworldonline.com
6/7/19 2:05 PM
