Ammeter shunt resistor operation is well-understood. Used extensively for current measurement in the past, the resistors are used to “shunt” most of the current around the measurement instrument. This technique allows measurement of greater currents than is possible using normal ammeters. The low-value resistor is situated in series with the load, creating a small voltage drop across the resistor, which can be measured using a voltmeter. The current is then calculated using Ohm’s law.

 
Modern ammeter shunts are a bit more complex. The value of the shunt resistor works with the voltmeter to provide the maximum deflection at the maximum-rated current of the shunt. At that current, the design normally has a voltage drop of 50, 75 or 100 mV. In a typical example, a resistor that would measure a current of up to 100 A, with a 100-mV voltage drop, would have a value of 1 mΩ. For higher currents, a lower value of resistor would be required, with lower voltage drops.Microhm Electronics' ammeter shunt resistor FL-2 type are widely used in meters.
 
With such a low resistance, it has to be defined precisely in advance, and any other elements in the systems that could cause errors must be eliminated. One way to reduce potential erroneous measurements is to use a four-terminal resistor to keep the load connections separate from the measurement terminals. MVR2321-4 is a 4-terminal precison resistors used for current measurement for meters as well.

 
Many materials used to manufacture resistors are susceptible to temperature variation. When trying to measure with an accuracy of 0.1%, the resistor material must have a low temperature coefficient of resistance (TCR). For example, Manganin alloy can be used for shunt ammeter resistors because its TCR is stable over a temperature range of −40 to +60ºC.