Nace CP 1, Course Manual

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Basic Electricity 1:22
CP 1 – Cathodic Protection Tester Course Manual
©
NACE International, 2000
02/01/05
Analog Meters
Basic Meter Movement
The basic movement of an analog meter is a stationary permanent
magnet/moving coil system called a
D’Arsonval movement
. This is
shown in Figure 1.15. The needle provides a continuous
representation of the measurement, called an
analog
of the
measurement value.
The D’Arsonval meter responds to a current flow through the
moving coil. Electrical current is converted into a mechanical force,
as indicated in the following steps:
1.
Current from the external circuit flows through the coil and
creates a magnetic field.
2.
The coil rotates due to the reaction of its magnetic field with the
magnetic field of the permanent magnet.
3.
As the coil rotates, it works against a mechanical spring, causing
tension in the spring.
4.
The pointer comes to rest along the calibrated scale based on the
balance in forces between the moving magnetic coil and the
spring tension.
5.
The reading on the scale represents the current flow through the
coil from the external circuit. Note that the energy needed to
operate the meter comes from the circuit itself.