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.