Generally, it is possible to directly measure currents in the order of microamps or milliamps. To measure larger currents, the ammeter should have a parallel resistor (also known as a shunt). The measurement mechanism mainly uses a magnetoelectric electric meter. The resistance value of the shunt is such that when the full range current passes through, the ammeter deflects fully, i.e. the ammeter indication reaches the maximum. For a few amperes of current, a dedicated shunt can be installed in the ammeter. For currents above a few amperes, an external shunt is used. The resistance value of a large current shunt is very small. To avoid errors caused by the addition of lead resistance and contact resistance to the shunt, the shunt should be made into a four-terminal form, that is, with two current terminals and two voltage terminals.