A shunt resistor is a precise, low Ohm resistor that is temperature stable. It is used as a current "sensor" by using a millivolt meter to measure the voltage drop across it. Large current shunts are commonly made of one or more strips of manganin, a copper alloy capable of carrying high currents, that are soldered between machined blocks of brass with connecting bolts.
Shunts are rated according to the number of Amps it is capable of carrying and the mill votage which is generated across the shunt when the rated current is being passed through it. Common shunt ratings include 100A 100mV or 500A 50mv. The resistance can be calculated by using Ohms Law, V=IR, 50mV=500A(R), therefore R=0.1m, or 0.0001. This is a very small value of resistance; it must be in order to minimize the power loss when large currents are flowing.
The shunt normally has two separate screws on to which the sense leads are attached. It is important to realize that the integrity of these connections are critical to accurate measurement and should not be used as current carrying connections.