How Does Galvanometer, Voltmeter and Ammeter Work?

How Does Galvanometer, Voltmeter and Ammeter Work?

Galvanometer, Voltmeter and Ammeter are three most fundamental measuring devices we will encounter during our course in current electricity and one should have a sound understanding before using them properly. In this article, we will thoroughly discuss their working principle and differences. A galvanometer is the most basic electrical measuring device and other two, namely Voltmeter and Ammeter are slight modifications of a Galvanometer to measure the potential difference and electrical current respectively. In order to understand their working principle properly we have to start with the working principle of a galvanometer.

How does a Galvanometer work?

galvanometer

A galvanometer is an electrical instrument for detecting and measuring electric current. Galvanometers work according to the observations made by Hans Oersted. He showed that the needle of a magnetic compass deflects near a wire that has electric current flowing through it. Modern galvanometers are constructed with a small moveable pivoting coil of wire in the placed in the field of a permanent magnet. The coil is attached to a thin pointer that can traverse a calibrated scale. A tiny torsion spring pulls the coil and pointer to the zero position. When a direct current flows through the coil, the coil generates a magnetic field according to the direction of the current. This field acts against the field due to permanent magnet to twist the coil against the spring and moves the pointer. The coil comes to an equilibrium when this restoring torque balances the torque due to the magnetic field. If  \Phi be the deflection angle then

    \[ K\Phi = INBA\]

    \[ \Phi = \frac{INBA}{K}\]

Main purpose of a galvanometer is to detect electric current in a circuit.

How does an Ammeter work?

internal construction of ammeter

An ammeter is an electrical instrument to measure current in a circuit. An ammeter is a modified form of a Galvanometer where a small resistance, known as shunt is connected in parallel to the galvanometer to form the ammeter. In the diagram above, we have shown the internal construction of an ammeter. Shunt resistance is labelled as Shunt.

ammeter

Suppose we have constructed an ammeter with a galvanometer having an internal resistance of r_G and a shunt resistance r_S. We have shown the construction in the diagram above. As the voltage across the galvanometer and the shunt resistance is same, we can write

    \[ I_G\times r_G=I_S\times r_S\]

Again total current I is divided into two parts, so that,

    \[ I=I_G+I_S\]

or

    \[ I_S=I-I_G\]

Thus shunt resistance is

    \[ r_S=\frac{I_G\times r_G}{I-I_G}\]

How does a Voltmeter work?

A voltmeter is a modified form of galvanometer especially used to measure the potential difference. Instead of using a small resistance in parallel to the galvanometer as in case of an ammeter, we use a large resistance in series. The construction of a voltmeter is shown below.

Voltmeter

Suppose we have constructed a voltmeter with a galvanometer having an internal resistance of r_G and a large resistance r_S in series. The current I flowing through these two resistances are same as they are in series. If the potential drop across the voltmeter is V and that across the galvanometer is V_g then potential drop across the large resistance becomes V-V_g. Thus series resistance r_S is

voltmeter

    \[ r_S=\frac{V-V_g}{I}\]

    \[ r_S=\frac{V}{I}-r_G\]

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