To this iron a pointer is attached so that it may deflect along with the moving iron over a graduated scale. The iron is made of sheet metal specially shaped to give a scale as nearly uniform as possible. The moving iron is drawn in to the field of solenoid when current flows through it. The movement of iron is always from weaker magnetic field outside the coil into the stronger magnetic field inside the coil regardless the direction of flow of currant in the coil.
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 magnetises the iron. Thus the iron is attracted in to the coil, causing the spindle and the pointer to rotate. Deflecting Torque Exact calculation of deflecting torque is even more difficult than in the case of the repulsion type, and design of the shape of the iron is largely by the trial.
Let,
I = Initial current in ampere in coil
L = the inductance of coil in Henry
θ = Angle of deflection in radian Controlling Torque The controlling torque in these instruments is provided by spring coil. But gravity control can be used for panel type of instruments which are vertically mounted. Damping Torque The damping torque is provided by air friction usually be a vane moving in a sector shaped chamber. Shape of Scale Such instrument has a scale cramped at the lower end and greatly expanded at the upper end, as the torque is quite low which the moving iron is just entering the solenoid and increases rapidly as the iron is drawn further in.
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