What is the basic unit of electrical resistance?

• A. Volt
• B. Siemens
• C. Ohm
• D. Ampere

Answer: (C)

Resistance is measured in ohms. An ohm is defined as the resistance between two points of a conductor when a constant potential difference of one volt, applied to those points, produces a current of one ampere, with no electromotive force in the conductor. In other words, R equals V divided by I, so when you apply a voltage and observe the current, the resistance comes out in ohms (Ω). For example, if 10 volts drive 2 amperes through a component, the resistance is 5 ohms.

Other units describe different quantities: volts measure electrical potential difference, amperes measure current, and siemens measure conductance (the reciprocal of resistance). So the unit that directly represents resistance is the ohm.