# Ammeter: Definition & Working Principle

An ammeter is a measuring instrument which is used for the measurement of electric current in an electric circuit.

An ammeter has a low resistance. The ammeter is connected in series with the load to measure the electric current flowing through it. As, the ammeter has a very low resistance, therefore when it is connected in series with any circuit, it does not change the current.

## Working Principle

When the ammeter is connected in series with the load, the same current flows through the ammeter that flows through the load, this is because the current in a series circuit remains same through all the circuit elements. Therefore, the deflection in the ammeter coil is according to the amount of load current. In such a way, the ammeter measures the load current in an electric circuit.

### Explanation When a shunt (parallel) resistance of very low value is connected in parallel with a basic galvanometer, it becomes an ammeter and can be used to measure current in the circuit. The shunt resistance can calculated as follows −

Refer the circuit,

$$R_{m}=internal \:resistance\:of\:meter$$

$$R_{sh}=Shunt\:resistance$$

$$I_{sh}=Shunt\:current$$

$$I_{m}=Meter\:current$$

$$I=Current\:of\:the\:circuit\:to\:be\:measured$$

Therefore,

$$I=I_{sh}+I_{m}$$

$$\Rightarrow\:I_{sh}=I-I_{m}$$

As the meter and shunt are in parallel, thus voltage drop across both is same, i.e.

$$I_{m}R_{m}=I_{sh}R_{sh}$$

$$\Rightarrow\:R_{sh}=\frac{I_{m}R_{m}}{I_{sh}}=\frac{I_{m}R_{m}}{(I-I_{m})}$$

## Types of Instruments used as Ammeters

The following instruments can be used as an ammeter in an electric circuit −

### Permanent Magnet Moving Coil (PMMC)

• It can be used for DC current measurements only.

• PMMC is also known as D’Arsonval meter movement.

### Moving Iron (MI)

• It can be used for both AC and DC current measurements.

### Electrodynamometer

• It can also be used for both AC and DC current measurements.

### Hot wire & Thermocouple

• These are calibrated for AC and DC current measurements.

• These instruments are particularly suitable for AC current measurements because deflection of meter depends upon the heating effect of AC i.e. RMS value of current.

• The readings of these instruments are independent of the frequency.

### Rectifier Type Ammeter

• These are used for AC current measurements.

• These can be used for high frequencies.

Note – Now a days, Digital Ammeters are also used for the current measurements in the electric circuits. These consist of a seven segment display screen to show the readings. The below picture shows a typical digital ammeter. 