- An ammeter is required to measure the current in a circuit and it is therefore connected in series with the components carrying the current.
- For higher range ammeters a low resistance made up of manganin (low temperature coefficient of resistance) is connected in parallel to the moving coil (see and instrument may be calibrated to read directly to the total current.
- Both the magnetic field strength and spring tension decrease with an increase in temperature. On other side, the coil resistance increases with an increase in temperature. These changes lead to make the pointer read low for a given current with respect to magnetic field strength and coil resistance.
- Use of Manganin resistance (known as swamping resistance which has a temperature coefficient practically zero) in series with the coil resistance can reduce the error due to the variation of resistance of the moving coil. The total resistance of the coil and swamping resistor increases slightly with a rise of temperature, but only just enough to counteract the change of springs and magnet, so that the overall temperature effect is zero.
- The swamping resistance is usually three times that
of coil thereby reducing a possible error of, say, 4% to 1%. One
disadvantage of use of swamping resistors is reduction in the full-scale
sensitivity of the movement, because a higher applied voltage is necessary
to sustain full-scale current.
The range can be extended with
the addition of shunt. The resistance of the shunt can be calculated by
applying conventional circuit analysis where
Since
shunt resistance is in parallel with the meter movement, the voltage drops
across the shunt and the movement must be the same and we can write
For
each required value of full-scale meter current we can then solve for the value
of shunt resistance required. External shunts of this type are normally used
for measuring very large currents.
