Fig. 8 shows the sectional front and an end elevation of the attracted-iron type instrument. It consists of a solenoid (or coil) C and oval shaped soft-iron disc D in such a way that it can move in or out of the solenoid. To this iron a pointer P is attached so that it may deflect along with the moving iron over a graduated scale. The soft-iron disc is made of sheet metal specially shaped to give a scale as nearly uniform as possible.
When the current to be measured (or a definite fraction of the current to be measured or proportional to the voltage to be measured) is passed through the solenoid, a magnetic field is set up inside the solenoid, which in turn magnetises the iron. Thus the soft-iron disc is attracted into the solenoid/coil, causing the spindle and the pointer to rotate. Damping is provided by vane V attached to the spindle and moving in an air chamber, and control is by hair spring.
Fig. 8. Attraction-type moving-iron instrument.