Page 64 - Teach Yourself Electricity and Electronics

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CHAPTER

Measuring devices

NOW THAT YOU’RE FAMILIAR WITH THE PRIMARY UNITS COMMON IN ELECTRIC-
ITY and electronics, let’s look at the instruments that are employed to measure these
quantities.
Many measuring devices work because electric and magnetic fields produce forces
proportional to the intensity of the field. By using a tension spring against which the
electric or magnetic force can pull or push, a movable needle can be constructed. The
needle can then be placed in front of a calibrated scale, allowing a direct reading of the
quantity to be measured. These meters work by means of electromagnetic deflection
or electrostatic deflection.
Sometimes, electric current is measured by the extent of heat it produces in a re-
sistance. Such meters work by thermal heating principles.
Some meters work by means of small motors whose speed depends on the mea-
sured quantity. The rotation rate, or the number of rotations in a given time, can be
measured or counted. These are forms of rate meters.
Still other kinds of meters actually count electronic pulses, sometimes in thou-
sands, millions or billions. These are electronic counters. There are also various other
metering methods.

Electromagnetic deflection

Early experimenters with electricity and magnetism noticed that an electric current
produces a magnetic field. This discovery was probably an accident, but it was an ac-
cident that, given the curiosity of the scientist, was bound to happen. When a mag-
netic compass is placed near a wire carrying a direct electric current, the compass
doesn’t point toward magnetic north. The needle is displaced. The extent of the er-
ror depends on how close the compass is brought to the wire, and also on how much
current the wire is carrying.
Scientific experimenters are like children. They like to play around with things. Most
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