Basic principle operation of Moving-iron Instruments: We have mentioned earlier that the instruments are classified according to the principles of operation. Furthermore, each class may be subdivided according to the nature of the movable system and method by which the operating torque is produced. Specifically, the electromagnetic instruments are sub-classes as (i) moving-iron instruments (ii) electro-dynamic or dynamometer instruments, (iii) induction instruments. In this section, we will discuss briefly the basic principle of moving-iron instruments that are generally used to measure alternating voltages and currents. In moving –iron instruments the movable system consists of one or more pieces of specially-shaped soft iron, which are so pivoted as to be acted upon by the magnetic field produced by the current in coil. There are two general types of moving-iron instruments namely (i) Repulsion (or double iron) type (ii) Attraction (or single-iron) type. The brief description of different components of a moving-iron instrument is given below.

• Moving element: a small piece of soft iron in the form of a vane or rod
• Coil: to produce the magnetic field due to current flowing through it and also to magnetize the iron pieces.
• In repulsion type, a fixed vane or rod is also used and magnetized with the same polarity.
• Control torque is provided by spring or weight (gravity)
• Damping torque is normally pneumatic, the damping device consisting of an air chamber and a moving vane attached to the instrument spindle.
• Deflecting torque produces a movement on an aluminum pointer over a graduated scale.