- To direct current, the potential difference divided by the current when there is no electromotive force in the conductor.
- To alternating current, the real part of impedance.
Green Book, 2nd ed., p. 15 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 971 (https://doi.org/10.1351/pac199668040957)