Ω·mm²/m
ohm square millimeter per meter
Electrical resistivityexact by definition
The ohm square millimetre per metre is the same size as the microohm metre, exactly 1e-6 ohm metres, written the way a cable schedule works: resistance equals resistivity times length in metres divided by conductor area in square millimetres. Copper is taken as 0.01724 Ω·mm²/m at 20 degrees Celsius in IEC practice and 0.0175 in older European tables. The conversion is exact, following from the exact definition of the millimetre.
| 1 Ω·mm²/m | 0.000001 Ω·m |
Where the unit came from
The form survives because cable is sold by cross-sectional area in square millimetres. Keeping the unit in the shape of the calculation means a 2.5 mm² conductor 30 m long comes out in ohms with no prefix arithmetic at all.
Solvers that use electrical resistivity