Difference Between Ammeter And Galvanometer

An ammeter and a galvanometer are two important tools used in electrical circuits to measure current, but they have different functions and operate in unique ways. An ammeter is specifically designed to measure the current flowing through a circuit, giving you direct readings in amperes, which makes it pretty straightforward to use. 

1.0Definition of Ammeter

• An ammeter is connected in series with a current carrying wire to measure current passing through it. Since it is connected to the wire and has a finite amount of resistance, there will be some potential difference across it. This will lead to the change in actual flow of current. In an ideal situation where the ammeter can measure the actual current passing through a wire, there should be no voltage drop across it and hence its resistance should be zero.
• An Ammeter is always connected in series in the circuit in which current is to be measured.
• An Ammeter is a modified form of Galvanometer having a very low value of resistance.

Ammeter Diagram and Its Symbol

Symbol of an Ammeter

Construction And Working of Ammeter

Construction

• The ammeter is primarily composed of several key components. At its core is a U-shaped magnet featuring concave poles, which create a strong magnetic field. encased around a soft iron core is a rectangular copper coil. 
• The presence of this iron core enhances the magnetic field strength between the poles and ensures that the field lines remain parallel, improving the accuracy of the readings. 
• Additionally, a shunt, which is a low-value resistor, is connected in parallel to the galvanometer to manage the current flow. Finally, the pointer is attached to the coil in a pivoting manner, allowing for precise measurement of deflection to indicate the current reading.
  

Working

• On connecting an ammeter in series with a circuit, the current that passes through the ammeter is the same as the current flowing throughout the entire circuit. 
• This current causes the magnetic coils inside the ammeter to deflect, which moves the pointer across the graduated scale and enables you to measure the load current. 
• The way an ammeter operates is quite similar to a galvanometer. In fact, you can convert a galvanometer into an ammeter by adding a shunt resistance in parallel, which increases its capacity to measure higher currents.

2.0Definition of Galvanometer

• It is a device used to measure/detect small electric current flowing in the electric circuit.
• It detects whether current is flowing or not and whether the direction of current is reversed or not. 

Galvanometer Diagram

Principle Of Galvanometer

It is based on  the fact that when a current carrying loop or coil is positioned in the uniform magnetic field, it encounters a torque.

Construction And Working of Galvanometer

• It consists of a coil wound on a non-metallic frame, usually brass.The coil is suspended between two poles of a permanent magnet which are cylindrical in shape.
• The coil is suspended with a movable torsion head $\text{ H }$ by a strip or wire made of phosphor bronze which acts as path for the current to the coil also. End of the wire is connected to the terminal $T_2$ of the galvanometer. The other end of the coil is connected to a light spring (S) which is finally connected to terminal $T_1$.The spring exerts a very small restoring torque on the coil
• A soft iron core is placed within the frame of the coil. A plane circular mirror(M) is attached on the wire or strip to note the deflection of the coil using lamp and scale arrangement. The whole system is enclosed in a non-metallic $\text{ (say wooden) }$ case to avoid disturbance to air
• When current through the coil, it experiences a torque

$\tau =NIAB\mathrm{Sin}\theta$

$\Rightarrow 1$ Where is the angle made by the perpendicular to the plane of the coil with the direction of the magnetic field.

$\Rightarrow \theta =90^{\circ }\Rightarrow \mathrm{Sin}90^{\circ }=1$

It is possible when cylindrical poles of permanent magnet are used which produces a radial magnetic field.

$\tau =NIAB\Rightarrow$ This is called deflecting torque……(1)

• As the coil gets deflected, the spring is twisted and a restoring torque is developed in it. If k is the restoring torque per unit twist then the restoring torque for the deflection is given as,

${\tau }^{\prime }=k\varphi$ …………………….(2)

$\text{ For Equilibrium, Deflecting torque }=\text{ Restoring torque }$

$NIAB=k\varphi$

$I=\left(\frac{k\varphi }{NAB}\right)$

$I=G\varphi$

$I\propto \varphi$

$G=\frac{k}{NAB}\Rightarrow \text{ Galvanometer Constant }$

• Deflection in the coil is directly proportional to the current flowing through it, linear scale in the galvanometer to detect the current in the circuit.

3.0Sensitivity of Galvanometer

It is said to be sensitive if a small current flowing through the coil of the galvanometer produces a large deflection on it.

1. Current Sensitivity(C.S)- Deflection produced in the galvanometer per unit current flowing through it.

$\text{ C. }S=\frac{\varphi }{I}=\frac{\varphi (NAB)}{k\varphi }=\frac{NAB}{k}\left(I=\frac{k\varphi }{NAB}\right)$

2. Voltage Sensitivity(V.S)- Deflection produced in the galvanometer per unit voltage applied to it.

$V.S=\frac{\varphi }{V}=\frac{\varphi }{IR}=\frac{NAB}{kR}\left(I=\frac{k\varphi }{NAB}\right)$

4.0Difference Between Ammeter and Galvanometer