#### There are different types of ammeters used in measuring system to measure current. They are,- Moving Iron Type - Used for measuring both direct and alternating currents
- Permanent Magnet Moving Coil (PMMC) Type - Used for dc measurements only
- Electro-dynamometer Type - Used for measuring both direct and alternating currents
- Rectifier Type - Used for measuring both dc and ac measurements

But every instrument has its measuring range capacity. For example, a moving coil ammeter can measure currents 0-100mA. Thus it is impossible to measure high currents which are more than the measuring range of an instrument. Therefore, it is necessary to bring up these large currents to a value within the range of the instrument.

For extending the range of ammeters, the devices used are shunts or current transformers. Shunts are used for dc and ac measurements while the current transformers are used for ac measuring instruments only. From this, a moving coil ammeter range can be extended by using shunts, and a moving iron ammeter range can be extended by either shunts or current transformer.

## Extension of Range of Ammeter using Shunts :

#### In measuring instrument, shunt refers to a low resistance which is connected in parallel to the basic meter as shown below. It acts as a diverter, and the maximum percent of current measured will be passed through it.

### Essential Requirements for the Construction of Shunts :

#### The essential requirements for the construction of shunts are as follows,- The coefficient of the temperature of the shunt and the instrument coil should be equal as far as possible and should be very low.
- As the maximum percent of current is being diverted through the shunts, they should not produce excess heat.
- Their resistance should be time-invariant.
- In the case of ac instruments, the time constant of the shunt and the instrument coil should be equal.
- They should have low thermoelectric emf with copper.

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Let,- I
_{m} = Full deflection current of the basic meter - I
_{sh} = Current through shunt - R
_{m} = Internal resistance of the basic meter - R
_{sh} = Resistance of the shunt - I = Current being measured
- m = Multiplying factor

As the shunt is connected in parallel to the basic meter, the voltage drops across the shunt will be equal to that of a basic meter.*∴ I*_{m} R_{m} = I_{sh} R_{sh} ...(1)*I = I*_{m} + I_{sh} (From figure)*I*_{sh} = I - I_{m} ...(2)

Substituting equation 2 in 1 we get,
The ratio of I and I_{m} is called the multiplying factor m.*i.e., m = I/I*_{m} ...(4).

Substituting equation 4 in 3 we get,
The meter deflection Î¸ is directly proportional to the meter current I_{m}.*i.e., Î¸ ∝ I*_{m} ∝ I

So, the meter deflection is directly proportional to the total current being measured. Hence, by connecting a proper value of shunt and calibrating the meter scale in terms of m × I_{m} = I the range of an ammeter can be extended.

_{m}= Full deflection current of the basic meter_{sh}= Current through shunt_{m}= Internal resistance of the basic meter_{sh}= Resistance of the shunt*∴ I*

_{m}R_{m}= I_{sh}R_{sh}...(1)*I = I*

_{m}+ I_{sh}(From figure)*I*

_{sh}= I - I_{m}...(2)*i.e., m = I/I*

_{m}...(4).*i.e., Î¸ ∝ I*

_{m}∝ I