A device to measure electric current is called an ammeter. Most modern ammeters measure the voltage drop over a precision resistor with a known resistance. The current flow is calculated by using Ohm’s law:
 
Ohm's Law I=V/R
 
Most ammeters have an inbuilt resistor to measure the current. However, when the current is too high for the ammeter, a different setup is required. The solution is to place the ammeter in parallel with an accurate shunt resistor.  Another term that is sometimes used for this type of resistor is ammeter shunt. Microhm Electronics' FL-2 series is popular in ammeter industry for its high precison and stability.

 
Usually this is a high precision manganin resistor with a low resistance value. The current is divided over the shunt and the ammeter, such that only a small (known) percentage flows through the ammeter.  In this way, large currents can still be measured. By correctly scaling the ammeter, the actual amperage can be directly measured. Using this configuration, in theory the maximum amperage that can be measured is endless. However, the voltage rating of the measurement device must not be exceeded. This means that the maximum current multiplied by the resistance value, cannot be higher than the voltage rating. Also, the resistance value should be as low as possible to limit the interference with the circuit. On the contrary, the resolution gets smaller the smaller the resistance and thus the voltage drop is.
 
Example of calculation
As an example a shunt resistor is used with a resistance of 1 mOhm. The resistor is placed in a circuit, and a voltage drop of 30 millivolts is measured across the resistor. This means that the current is equal to the voltage divided over the resistance, or: I = V / R = 0.030 / 0.001 = 30 A. The same calculation could be made, but now with the resistance value unknown and the voltage and current known. This is used to calibrate shunt resistance. Microhm Electronics is focusing on this filed and trying to provide more powerful shunt resistors besides FL-2 series.