Galvanometer works on the principle when a current carrying conductor is kept in a magnetic field then it experience a force .it is a electromechanical instrument which use to detect electric charge following through conductor.
Construction of Moving Coil Galvanometer
In moving coil galvanometer the coil is suspended between the pole of a strong horse shoe magnet.The pole piece is made cylindrical and a soft iron cylindrical core is placed within the coil without touching the inner surface of pole piece cylindrical.it makes the magnetic field radial.In such a field plane of the coil is always parallel to the magnetic field .When the current passes through the coil then a torque will be experienced by the suspended coil.The figure given below show the diagram of Galvanometer.
Sensitivity Of Moving Coil Instrument
Sensitivity of any type of instrument is defined as the change in output when there is a minimum change occurs in any parameter of the instrument.A high sensitivity Instrument gives more accurate result.A moving coil galvanometer is said to be sensitive if it gives high deflection for a very change in instrument current.Let there is a small (i.e dI) change in instrument current and correspond to this change there is a small deflection (i.e dα) in pointer thesensitivity of the instrument is given as
there are four types of sensitivity defines for moving coil galvanometer and i shall discuss each one by one
Current Sensitivity
Current sensitivity of moving coil galvanometer is defined as the ratio of change in deflection when there is a change in current which is flowing through the coil.
In simple it is called deflection produced in galvanometer per unit change in current flowing through it .
Sensitivity = dα/dI
or
Sensitivity = NBA/C
where
N=Number of turns of suspended coil
B= Radial Magnetic field density in tesla
A= area of suspended coil
C= Ttorisonal Constant
Voltage Sensitivity
Voltage Sensitivity is defined as change in deflection when there is a change in applied voltage across the meter.In other wordsDeflection produced in the galvanometer per unit voltage applied to it.
it is given as
Voltage Sensitivity = α/V
or
Voltage Sensitivity = NBA/RC
N=Number of turns of suspended coil
B= Radial Magnetic field density in tesla
A= area of suspended coil
C= Ttorisonal Constant
R=Resistance of Galvanometer
Megaohm Sensitivity
Resistance required in circuit of moving coil galvanometer in mega ohm to cause a deflection of of one scale division on impressing 1 Volt is called mega ohm sensitivity.
Ballistic Sensitivity
Ballistic sensitivity is defined as the first maximum deflection in mm caused on scale placed at a distance of 1 meter from the mirror by a unit charge suddenly passed in a galvanometer.
Ammeter
Ammeter is a device which is used to measure the electric current and it is always connected in series with the circuit which current is to be measured.The reading of the ammeter is always smaller than the actual current of the circuit.To get more accurate result by ammeter,it is required to minimize the meter resistance.
Conversion of Galvanometer Into Ammeter
By inserting a resistance of proper value a moving coil galvanometer can be converted into either a ammeter or Voltmeter.To Convert a Galvanometer into ammeter its resistance need to be lowered so that maximum current can pass through it and gives exact reading.The connected low resistance is called shunt.
Votlmeter
Voltmeter is a device which measures the potential difference between two point and it is always connected in parallel with the circuit element across which potential difference is to be measured.its reading is always less than the true value.
As greater the resistance of voltmeter ,more accurate will be its reading.if the resistance of a voltmeter is infinite then it is called ideal voltmeter and an ideal voltmeter will not draw any current while it is under operation.
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