Ammeter connected directly across a voltage source since their internal resistance is very low and excess current would flow. Ammeters are designed for a low voltage drop across their terminals, much less than one volt; the extra circuit losses produced by the ammeter are called its "burden" on the measured circuit. Ordinary Weston-type meter movements can measure only milliamperes at most, because the springs and practical coils can carry only limited currents. To measure larger currents, a resistor called a "shunt" is placed in parallel with the meter. The resistances of shunts is in the integer to fractional milliohm range. Nearly
Conventional electrical unit in this way, and as such, represent a significant step towards using "natural" fundamental physics for practical measurement purposes. However, the conventional electrical units are unlike other systems of natural units in that some physical constants are not set to unity but rather set to fixed numerical values that are very close (but not precisely the same) to those in the common SI system of units. Four significant steps were taken in the last half century to increase the precision and utility of measurement units. In 1967 the Thirteenth General Conference on Weights and Measures (CGPM) defined the second of