milliampere

The milliampere is the current unit of the electronics bench. A multimeter's most exercised range is mA, datasheet operating currents are quoted in it, and a USB 2.0 port's original budget of 500 mA shaped a decade of gadget design. On this site it feeds the Ohm's law and electrical power solvers directly, and the charge solver turns a milliamp held for some seconds into millicoulombs.

Industrial instrumentation settled on the 4 to 20 mA loop for a reason worth repeating: the zero of the scale is 4 mA, not 0. A transmitter reporting its lowest possible reading still drives 4 mA, so a measured 0 mA can only mean a broken wire or a dead instrument. That live zero turns the most common failure in a plant into an unambiguous alarm rather than a plausible-looking measurement, and the 4 mA floor also carries enough power to run the transmitter itself over the same two wires.

This is also the decade where protection hardware pays attention. A North American GFCI outlet is required to trip on a ground fault between 4 and 6 mA, while the residual current devices used across most of the rest of the world trip at 30 mA. Both figures are printed on the device face, and both are milliampere-scale, which says something about how little imbalance the codes are willing to leave flowing.