An ammeter is a device designed to accurately measure and display electrical current in a readable form, which could be a moving coil meter, a LED bargraph display or a digital panel meter. The basic unit of electrical current is Amperes, usually shortened to Amps.  Milliamps, is a more convenient measurement for lower power circuits When designing an ammeter, external resistors, usually referred to as shunt resistors (or sometimes current resistors), are connected in parallel with the meter input to extend or convert the range. This arrangement divides the current being measured so the majority flows through the shunt resistor and a small portion goes to the meter.
 An ammeter shunt is a special type of current-sensing resistor, having four terminals and a value in milliohms or even micro-ohms. Microhm Electronics FL-2 series is a kind of typical ammeter shunt. Current-measuring instruments, by themselves, can usually accept only limited currents. To measure high currents, the current passes through the shunt across which the voltage drop is measured and interpreted as current.
 
A typical shunt consists of two solid metal blocks, sometimes brass, mounted on an insulating base. Between the blocks, and soldered or brazed to them, are one or more strips of low temperature coefficient of resistance (TCR) manganin alloy. Large bolts threaded into the blocks make the current connections, while much smaller screws provide volt meter connections. Shunts are rated by full-scale current, and often have a voltage drop of 50 mV at rated current. Such meters are adapted to the shunt full current rating by using an appropriately marked dial face; no change need to be made to the other parts of the meter.

 
Microhm Electronics has expanded its ammeter shunts to include current ratings up to 10,000 amps. FL-2 series of off-the-shelf, base-mounted and bus-bar shunts now offers current ratings from 100 A to 10,000 amps and a number of mounting configurations to meet various installation requirements.