IOCL Junior Operator Solved Questions & Answers || IOCL Junior Operator General Science Questions and Answers

• In a moving iron instrument, the pointer’s deflection is proportional to the square of the current (I²).
• This indicates that if you double the current, the deflection will increase by four times.
• The deflection of the pointer is influenced by the magnetic field created by the coil, which is directly related to the current flowing through it.

Moving Iron (MI) instruments can be used for both AC and DC measurements:

• AC Measurement: When used with AC, the AC current constantly changes direction (positive and negative cycles). The moving iron element is attracted or repelled by the magnetic field. Crucially, the force acting on the iron is proportional to the square of the current (I²). Because squaring any number (positive or negative) results in a positive value, the force is always in one direction, regardless of the AC current’s instantaneous polarity. It allow the instrument to measure the RMS (Root Mean Square) value of the AC signal.
• DC Measurement: When used with DC, the current is constant, producing a steady magnetic field. The iron piece deflects proportionally to the square of this constant current, giving a stable reading.

The force acting on the moving iron piece in a Moving Iron (MI) instrument is independent of the direction of the current.

• MI instruments work because the iron is magnetized by the current flowing through a coil.
• The force on the iron is proportional to the square of the current.
• Squaring a number always results in a positive value, regardless of whether the original number was positive or negative. Therefore, the direction of the current doesn’t matter; the force will always be in the same direction.
• The force does depend on the magnitude of the current, the number of turns in the coil (which affects the magnetic field strength), and the shape of the iron piece (which influences how it interacts with the magnetic field).

• MI instruments have a non-uniform scale because the needle’s movement is based on the square of the current.
• This square-law relationship results in a non-linear scale.

• The disadvantage of moving iron instruments is that they are affected by stray magnetic fields.
• External magnetic fields can disrupt the internal magnetic field created by the current-carrying coil in an MI instrument. This disruption may result in incorrect readings.

• A moving coil meter can measure resistance by Comparing the unknown resistance with a known resistance.
• This is achieved using a meter bridge, which is based on the Wheatstone bridge principle.
• The meter bridge balances the ratio between known and unknown resistances to determine the unknown value.

• When the test leads of an ohmmeter are directly shorted together, the meter movement will have a maximum amount of current through it.
• With zero resistance between the leads, the meter should register full-scale deflection.
• This is because ohmmeters are designed so that full deflection (to the right of the scale) represents zero resistance.

• The principle of resistance measurement with a moving coil meter is based on Ohm’s Law
• Ohmmeters use a voltage source and measure the resulting current through an unknown resistance.
• By applying Ohm’s Law (V = IR), the meter calculates and displays the resistance value.

• To adapt a moving coil meter for voltage measurement by adding a resistance in series with the meter.
• A series resistor, also known as a multiplier resistor, is added to limit the current flowing through the moving coil meter when it’s connected across a voltage source.
• This allows the meter to measure voltage without being damaged by excessive current.
• The higher the resistance, the higher the voltage that can be measured

• To measure AC voltage with a moving coil meter, you need a rectifier.
• Moving coil meters are inherently DC devices.
• To measure AC voltage, the AC signal must first be converted to DC. A rectifier does this by allowing current to flow in only one direction, effectively changing the alternating current into a direct current.
• The moving coil meter can then measure the average value of this rectified current, with the scale calibrated to display the corresponding RMS AC voltage