Ammeter is a simple pivoted type moving coil galvanometer with suitable modification.
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.
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
When the shunt is in use the scale readings of the instruments are divided by 10 and called amperes instead of milliamperes.