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** **Ammeter is a simple pivoted type
moving coil galvanometer with suitable modification.

** Principle**

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__An ordinary galvanometer usually has some fixed
resistance R of its coil and it gives full scale deflection when a certain maximum current
I passes through it. This maximum current is called the range of the unaided galvanometer.
A current that lies with in this range can be measured directly with galvanometer thus it
can serve as an ammeter.

An Ammeter is always placed in series with resistance or
other circuit components through which the current is to be measured.Ammeters should
therefore have a very low resistance ( about zero )compared with that of the rest of the
circuit, so that they do not reduce the amount of current.

**Construction**

** **Moving Coil instruments are not
made to take currents of more than a few milliamperes. If designed for larger currents the
coil would have to be wound with much thicker wire and the resultant instrument would be
both clumsy and expensive.

**Working and Use of Shunt**

** **When currents of several amperes
have to be measured a low resistance which by passes the greater part of the current is
placed in parallel with a milliammeter and then only a small known fraction of the total
current passes through the meter itself. A resistance used this way is called 'Shunt'.

Suppose for example a milliammeter of resistance 5
ohms and full scale deflection 15 mA is to be used for the purpose of measuring currents
to 1.5 A it would be necessary to use a shunt which passes (1.5-0.015) = 1.485 A while the
meter carried only 0.015 A.

**How to calculate the value of Shunt**

** **Let the resistance of the shunt be
'R' Since the shunt and the meter are in parallel , there will be the same potential
difference V across each. We can use this fact to obtain two equation from which R may be
calculated.

Remembering that P.D = Current X Resistance (OHM 'S
LAW) we may write:

for the millimeter V = 0.015 * 5 and for
the shunt V =1.485 * R equating the right hand side of there equations. There equations:

1.485 * R = 0.015 * 5

R = 0.015 * 5 / 1 .485 = 0.0505 ohms