Permanent-Magnet Instrument

an instrument for direct measurement of the strength of electric current, voltage, or the quantity of electricity in DC circuits. The movable part of a permanent-magnet instrument is displaced by the interaction between the magnetic field of the permanent magnet and the conductor carrying the current.

The most widespread type of permanent-magnet instrument has a movable frame located in the field of a permanent magnet. When a current flows through the windings of the frame, forces are generated that produce torque. Current is supplied to the frame through springs, or tension members, which generate a counteracting mechanical torque. The action of both torsional moments causes deflection of the frame through an angle proportional to the strength of the current in the frame. To avoid overheating of the winding and tension members, only moderate currents, ranging from several microamperes to several dozen milliamperes, may be passed directly through the frame. Extension of the measurement range for current and voltage is achieved by connecting shunts and series resistors to the frame; they may be external or built-in.

Permanent-magnet instruments exist in which the permanent magnet is located in the interior of a moving coil, as well as with moving magnets mounted on an axle inside a moving coil. Permanent-magnet quotient meters are also used. Permanent-magnet instruments with moving magnets are simpler, smaller, and lighter, but they are less precise and sensitive than instruments with moving frames. Readings are shown by pointers or luminous indicators: a ray of light from a source is directed at a mirror mounted on the moving part of the instrument and is reflected from the mirror, generating a bright spot with a dark line in the center on the scale of the instrument.

The distinguishing features of permanent-magnet instruments are uniform scales, good damping, high precision and sensitivity, and low current consumption. Such instruments are sensitive to overloads and mechanical shock and impact but are relatively insensitive to the effect of external magnetic fields and ambient temperatures.