Moving coil ammeters are not made to take currents of more than a few milliamperes. If it were required for one to measure much larger currents the coil would have to be wound with much thicker wire, making the instrument heavy and clumsy. We can measure higher currents by putting a resistance in parallel with the ammeter. The resistance of the resistor is selected to be much higher than the resistance of the ammeter so only a small part of the current will pass through the ammeter. A resistance used in this way is called a shunt.
Suppose a milliameter of resistance
and full scale deflection
is to be used to measure currents up to
Since the ammeter can only pass
the shunt must take
Let the resistance of the shunt be
Since the shunt is in parallel with the ammeter, there will be the same potential difference across both.
For the ammeter
For the resistor
When the shunt is in use the ammeter readings are multiplied by 100.
Many ammeters now come with a selection of shunts inside the case, and a switch to change between them. This makes it easy to read different ranges of currents.