There are two types of moving iron instrument which are discussed below briefly
Figure (a) below demonstrates an attraction type of moving-iron instrument is When current flows in the solenoid, a pivoted soft-iron disc is attracted towards the solenoid & themovement causes a pointer to have some movement across a scale.
In this type of moving-iron instrument, shown below in Figure (b), 2 pieces of iron are placed inside the solenoid, one of which being fixed, & the other joined to the spindle carrying the pointer.
When current is made to pass through the solenoid, the 2 pieces of iron are magnetized in the similar direction and hence repel each other. The pointer thus moves across the scale. The force moving the pointer is, in each type, proportional to I 2. Because of this the direction of current does not matter & the moving-iron instrument can be used on d.c. or a.c. The scale, however, is non-linear.