Figure 1. A carbon resistor with the resistance of 2 MΩ, the colour code gives the resistance in Ohms.[1]

The Ohm (Ω) is a unit of electrical resistance, measured in volts/ampere, to quantify the resistance in resistors, conductors, or even electrical insulators..[2]

Measurements of resistance go from 0 Ω for superconductors to ~[math]10^{-6} \Omega [/math] for a cm block of metal like copper or aluminum and up to ~[math] 10^{19} \Omega [/math] for a block of sulfur.[3][4] Very few physical measurements have spanned 25 orders of magnitude like this (for length, that's a ratio of the thickness of a fingernail to the size of a galaxy)!

To learn more about the Ohm please see Dr. Rowlett's dictionary of units.


  1. Hyperphysics. (August 28, 2015). Carbon Resistors [Online], Available:
  2. Hyperphysics. (August 28, 2015). Ohm's Law [Online], Available:
  3. Hyperphysics. (August 28, 2015). Table of Resistivity [Online], Available:
  4. PeriodicTable. (August 28, 2015). Technical data for the element sulfur [Online], Available: