Difference Between Ammeter And Voltmeter

An ammeter and a voltmeter are both key tools for electrical measurements, but they each have distinct functions. An ammeter is used to evaluate the current flowing through a circuit, whereas a voltmeter estimate the voltage  between two points in the circuit.Both instruments are typically based on a galvanometer (a device that detects small currents) but are modified with additional components (like a shunt resistor for the ammeter and a high resistance for the voltmeter) to suit their specific functions.

1.0Ammeter

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.

2.0Conversion of Galvanometer into Ammeter 

• To convert a galvanometer into an ammeter a very small resistance is connected in parallel to the galvanometer called SHUNT.
• Resistance of an ammeter is very small and it is zero for ideal ammeter i.e. ideal ammeter behaves like conducting wire.
• Value of shunt.

$V_S=V_G⟹(i-i_g)S=i_{g}G$

$⟹S=\left[\frac{i_g}{i-i_g}\right]G$

$⟹S=\left[\frac{1}{\frac{i}{i_g}-1}\right]G⟹S=\left[\frac{G}{n-1}\right],\text{where }n=\frac{i}{i_g}$

G –resistance of galvanometer

$i_g$ -range of galvanometer or current required to produce full deflection

I -Range of ammeter [Max. current can be measured.

NOTE:

1. To increase the range of an ammeter, a shunt is joined in parallel with the galvanometer.
2. To convert an ammeter of range I amperes and resistance $R_g\Omega$ into an ammeter of range nI amperes, the value of resistance to be connected in parallel will be $\frac{R_g}{n-1}\Omega$
3. Resistance of ideal ammeter is zero. 

3.0Voltmeter

A voltmeter is joined in parallel to the current carrying wire, to measure the potential difference between two points. Since it is connected to the wire, a finite amount of current will pass through it. This will lead to the change in actual flow of current and hence change in the actual potential difference. In an ideal scenario where the voltmeter accurately measures the voltage between two points, no current would flow through it. Therefore, its resistance must be infinite.

4.0Conversion of Galvanometer into Voltmeter

• To convert a galvanometer into voltmeter, a high resistance is linked in series with a galvanometer.
• The resistance of the voltmeter is very high and it is infinite for an ideal voltmeter.
•  So ideal voltmeter open circuit

Resistance R and G+H are in parallel between points A and B ,it may be represented as,

$$\begin{gathered} i_g(G+H)=(i-i_g)R \\ {i}_{g}G+i_{g}H=(i-i_g)R \end{gathered}$$$$\begin{gathered} V_g+V_H=V⟹V_H=V-V_g \\ {V}_H=V-V_g \end{gathered}$$$$\begin{gathered} i_{g}H=V-V_g \\ \frac{V_g}{G}H=V-V_g \end{gathered}$$$$\begin{gathered} V_{g}H=(V-V_g)G \\ H=\left(\frac{V-V_g}{V_g}\right)G=\left(\frac{V}{V_g}-1\right)G \\ H=(n-1)G\text{where }n=\frac{V}{V_g} \end{gathered}$$

V=Range of Voltmeter

$V_g$ =Range of Galvanometer

NOTE:

1. To increase the range of a voltmeter a high resistance is connected in series with it.

2. To convert a voltmeter of resistance $R_g\Omega$ and range V volts into a voltmeter of range nV volts, the value of resistance to be connected in series will be $(n-1)R_g\Omega$

3. Resistance of the ideal voltmeter is infinite.