Let's talk about repulsion type moving iron instrument
Let us take an electromagnet. Now two rectangular pieces of iron strip are placed side by side near to the electromagnet
as shown here. Now due to magnetic induction, both of the iron pieces achieve similar magnetic properties
in them. North pole and south pole are induced at similar ends of the iron pieces
If this pole of that iron piece becomes north pole, then this end of second iron piece also
becomes north pole. Similarly, this end of both iron pieces becomes south pole and due to the similarity, both
of them will repel each other. The force of repulsion depends upon the strength of electromagnet because this electromagnet
causes induction in the iron pieces. If now polarity of electromagnet alters, means the direction of current in the coil alters
south poles will be induced in this end of both iron pieces
Although in this case polarity alters, but still there is repulsion force acting on them
because of nearby similar poles. Hence it can be concluded that the direction of the force between the iron pieces are not
changed due to change in direction of current in the coil. Now if we make this iron piece fixed in respect to the position of the coil and we make this
iron piece free to move, then movement of the moving iron piece can indicate the quantity
of the current in the coil. If we can calibrate this movement properly in ampere scale
Now if we attach one end of moving iron piece with a hinge and fix a long pointer on with
this piece, the movement of the iron piece becomes rotational. If a circular scale is attached below the tip of pointer, then the tip moves along the
circular scale to give reading of the current flowing through the coil
This movement of pointer tip on the scale depends upon quantity of the current through
the coil, not on the direction of the current. Hence this measuring system can be utilized to measure both AC and DC
The measuring instruments using this technique to measure electrical quantities are called
repulsion type moving iron instruments. Repulsion type moving iron instruments are mainly used for the measurement of alternating
currents and voltages, though it can also be used for DC measurements
Now we will talk about actual forms of repulsion type moving iron instrument
In actual construction, a hollow circular coil is used and one of the iron pieces is
attached inside the hollow coil. The other moving iron piece or moving vane is attached to the shaft
Controlling spring and pointer is also connected with the shaft. As both vanes are placed in the same magnetic field, they are magnetized with the same polarity
Hence the defecting torque is produced due to the force of repulsion between two iron
pieces or vanes. If these two iron pieces develop pole strength of M1 and M2 respectively, then instantaneous
deflecting torque F is proportional to M1 into M2 which again is proportional to h square
where h is the field strength. If the permeability of iron is assumed constant, then h proportional to I where I is the coil current
The instantaneous deflecting torque F is being proportional I square. Average deflecting torque T suffix D proportional to mean of I square over a cycle
Since the instrument is spring controlled, T suffix C is proportional to theta where theta
is the angle of deflection of the moving piece. In the steady position of deflection, T suffix D equals to T suffix C. So, theta proportional
mean of I square over a cycle. Thus, the deflection is proportional to the square of the coil current
As the angular displacement of the moving system of the meter is proportional to square
of the current to be measured, it is proved that the instrument can be used for both AC
and DC. I hope you understood this also. Thank you