EDUCATION

Definition of galvanometer / Galvanometer vs Ammeter / Galvanometer to Ammeter and Voltmeter

A galvanometer is an electromechanical instrument used to detect the passage of current its working depends upon the fact. That when a conductor is placed in a magnetic field its experiences force as soon as a current passes through it. Due to this force-torque (τ) acts upon the conductor if it is in the form of a coil or loop.

Formula in physics

   τ=NIBA cosα

Explanation of Galvanometer

Where N is the number of turns in the coil, A is its area, I is current passing through it, B is the magnetic field in which the coil is placed such that its plane makes an angle (α)  with the direction of the B. Due to the action of the torque, the coil rotates and thus it detects the current. The construction of a moving coil galvanometer is shown. (a)

Definition of galvanometer / Galvanometer vs Ammeter / Galvanometer to Ammeter and Voltmeter
(a)

A rectangular coil C is suspended between the concave-shaped poles N and S of a U-shaped magnet with the help of a fine metallic suspension wire. The rectangular coil is made of enameled copper wire. It is wound on a frame of non-magnetic material. The suspension wire F is also used as one current lead to the coil. The terminal of the coil is connected to a loosely wound spiral E which serves as the second current lead.  A soft iron cylinder D is placed inside the coil to make the field radial and stronger near the coil as shown figure. (b)

Definition of galvanometer / Galvanometer vs Ammeter / Galvanometer to Ammeter and Voltmeter
(b)

when a current is passed through the coil, it is acted upon by a couple which tends to rotate the coil. This couple is known as deflecting couple and is given by NI BA cosα. As the coil is placed in a radial magneic filed in which the plane of the coil is always parallel to the field. so α is always zero. this makes cos

when a current is passed through the coil, it is acted upon by a couple which tends to rotate the coil. This couple is known as deflecting couple and is given by NI BA cosα. As the coil is placed in a radial magnetic field in which the plane of the coil is always parallel to the field. so α is always zero. this makes cos

when a current is passed through the coil, it is acted upon by a couple which tends to rotate the coil. This couple is known as deflecting couple and is given by NI BA cosα. As the coil is placed in a radial magnetic field in which the plane of the coil is always parallel to the field. so α is always zero. this makes cos α=1 and thus.

Deflecting couple=NI BA

As the coil turns under the action of the deflecting couple, the suspension wire (a) is twisted which gives rise to a torsional couple. It tends to untwist the suspension and restore the coil to its original position. This couple is known as a restoring couple. The restoring couple of the suspension wire is proportional to the angle of deflection θ as long as the suspension wire obeys Hooke’s law Thus.

Restoring torque=cθ

Ammeter

An ammeter is an electrical instrument that is used to measure current in amperes. This is basically a galvanometer. The portion of the galvanometer whose motion causes the needle of the device to move across the scale is usually known as the meter – movement. Most meter movements are very sensitive and full-scale deflection is obtained with a current of a few milliamperes only. So an ordinary galvanometer cannot be used for measuring large currents without proper modification.

Suppose we have a galvanometer whose meter movement coil has resistance Rg and which gives full-scale deflection when the current (Ig) passed through it. From Ohm’s law, we know that the potential difference (Vg) which causes a current (Ig) to pass through the galvanometer is given by

If we want to convert this galvanometer into an ammeter which can measure a maximum current I it is necessary to connect a low-value bypass resistor called a shunt. The shunt resistance is of such a value so that the current (Ig) for full-scale deflection of the galvanometer passes through the galvanometer and the remaining current ( I-Ig) passes through the shunt in this situation.

Voltmeter

Definition of galvanometer / Galvanometer vs Ammeter / Galvanometer to Ammeter and Voltmeter
Voltmeter Diagram

The voltmeter is an electrical device that measures the potential difference in volts between two points and has very high resistance so that it will not short the circuit across which the voltage is measured. this is achieved by connecting a very high resistance Rh placed in series with meter movement.

Suppose we have a meter movement whose resistance is Rg and which deflects full scale with a current Ig . in order to make a voltmeter from which has an arrangement of V volts, the value of the high resistance Rh should be such that full-scale deflection will be obtained when it is connected across V volt . under this condition the current through the meter movement is  Ig. Appling ohms law we have

v= Ig (Rg+Rh)

The scale of the galvanometer is calibrated from 0 to volts, the combination of the galvanometer and the series resistor act as a voltmeter with a range to 0 V volts. By properly arranging the resistance Rh any voltage can be measured. Thus we see that a voltmeter possesses high resistance.

It may be noted that a voltmeter is always connected across the two points between which potential difference is to be measured. Before connecting a voltmeter, it should be measured that its resistance is very high in comparison with the resistance of the circuit across which it is connected otherwise it will load the circuit and with altering the potential difference which is required to be measured.

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