## Ammeter

- An ammeter is a low resistance galvanometer used to measure strength of current in an electrical circuit.
- An ammeter is always connected in series in a circuit because when an ammeter is connected in series it does not appreciably change the resistance of circuit and hence the main current flowing through the circuit.
- In ideal ammeter has zero resistance.
- The reading of an ammeter is always less than actual current in the circuit because all practical ammeters have low finite resistance.
- Smaller is the resistance of an ammeter more accurate will be the reading.
- A galvanometer can be converted to an ammeter by connecting a low resistance shunt in parallel to coil of galvanometer. Ammeter Circuit Diagram: Here $\mathrm{I}_{9} \mathrm{G}=\left(\mathrm{I}-\mathrm{I}_{\mathrm{g}}\right) \mathrm{S}$ or $\quad S=\frac{I_{g}}{I-I_{g}} G$ Here G is resistance of galvanometer and $I_{g}$ is current required to produced full scale deflection of current.
**Shunt (S) :**It is a low resistance connected in parallel to coil of galvanometer to convert it to ammeter. It protects a galvanometer from strong currents. It is also used to alter range of an ammeter.- The resistance of converted ammeter is $R_{A}=\frac{G S}{G+S}$
- The range of an ammeter is increased by reducing shunt resistance S. If $\mathrm{I}=\mathrm{N} \mathrm{I}_{\mathrm{g}}$ then $S=\frac{I_{g}}{N I_{g}-I_{g}} G=\frac{G}{N-1}$ The range of an ammeter can be increased N times by reducing shunt to S = G/N-1.
- It is possible to increase the range of an ammeter because it lowers the resistance of ammeter further.
- The length of the shunt required $\ell=\pi r^{2} S / \rho$ where r is radius of shunt wire and is specific resistance of material of shunt wire.
- Reducing the shunt resistance may increase range but it reduces the sensitivity.

## Voltmeter

- A voltmeter is a high resistance galvanometer used to measure potential difference.
- A voltmeter is connected in parallel to a circuit element because when connected in parallel it draws least current from the main current. So it measures nearly accurate potential difference.
- An ideal voltmeter has infinite resistance.
- The reading of a voltmeter is always less than actual value because all practical voltmeter may have large but finite resistance.
- Greater is the resistance of voltmeter more accurate is its reading.
- A galvanometer is converted to a voltmeter by connecting a high resistance in series with coil of galvanometer. $V=I_{g}(R+G) \quad$ or $\quad R=\frac{V}{I_{g}}-G$
- The resistance of converted voltmeter is $\mathrm{R}_{\mathrm{v}}=\mathrm{R}+\mathrm{G}$
- The range of a voltmeter is increased by increasing the series resistance. If $\quad \mathrm{V}=\mathrm{NV}_{9}=\mathrm{NI}_{9} \mathrm{G} \quad$ then $\quad R=\frac{N I_{9} G-G}{I_{9}}=(N-1) G$ The value of resistance required to increase range $N$ times is $R=(N-1) G$

**Ex. What is the value of shunt which passes 10% of main current through a galvanometer of $99 \Omega ?$**

**Sol.**

$S =\frac{ I _{ g } G }{ I - I _{ g }}$

where $I_{9}=\frac{10}{100} I=0.1 I$

So $S=\frac{0.1 I \times 99}{I(1-0.1)}=\frac{9.9}{0.9}=11 \Omega$.

**Also Read:**

- Types and Effects of Electric Current
- Ohm’s Law and Resistance
- Combination of Resistances
- EMF and Internal Resistances of a Cell
- Cells Connected in Series, parallel and Mixed
- Kirchhoff’s Circuit Law
- Electric Currents in Conductors
- Wheatstone Bridge
- Post office Box
- Wheatstone Meter Bridge
- Moving Coil galvanometer
- Ammeter and Voltmeter
- Potentiometer Working Principle