siemens

The siemens is the ohm read upside down, and it exists as more than a curiosity because some problems are additive in conductance rather than resistance. Resistors in series add ohms; resistors in parallel add siemens, which turns the awkward reciprocal formula in the site's parallel-resistor solver into plain addition. The same logic makes the siemens the working unit of admittance in power system analysis, of transconductance on transistor datasheets, written in mS or µS, and of electrolytic conductivity, where the microsiemens entry on this site carries the whole water-quality story.

Two symbol traps. Capital S is the siemens and lower-case s is the second, an unusually dangerous single-letter pair because both appear in the same equations. And the older name mho, with its upside-down omega symbol, still marks meters and texts; the siemens itself has been official only since 1971, when the CGPM adopted it into the SI, a young unit wearing a nineteenth-century name.

The unit's best fact comes from quantum mechanics. Conductance has a natural quantum in a way bulk resistance does not: squeeze a metallic contact down to its last few atoms, as happens fleetingly whenever a contact breaks, and its conductance steps down in units of $2e^2/h$, about 77.5 µS. Since the 2019 SI fixed both $e$ and $h$, the conductance quantum is now an exact number, one of the few electrical properties of matter known with no uncertainty at all.