AC electromechanical meter movements come in
two basic arrangements: those based on DC movement designs,
and those engineered specifically for AC use. The basic
permanent-magnet meter movement explored in the DC meter
chapter of this book will not work correctly if directly
connected to alternating current, because the direction of
needle movement will change with each half-cycle of the AC.
Permanent-magnet meter movements, like permanent-magnet
motors, are devices whose motion depends on the polarity of
the applied voltage (or, you can think of it in terms of the
direction of the current).
In order to use a DC-style meter movement such as the
D'Arsonval design, the alternating current must be
"rectified" into DC. This is most easily accomplished
through the use of devices called "diodes." We saw diodes
used in an example circuit demonstrating the creation of
harmonic frequencies from a distorted (or rectified) sine
wave. Without going into elaborate detail over how and why
diodes work as they do, just remember that they each act
like a one-way valve for electrons to flow: acting as a
conductor for one polarity and an insulator for another.
Arranged in a bridge, four diodes will serve to steer AC
through the meter movement in a constant direction