Resistivity is usually symbolised with the greek letter 'rho' (ρ) and is measured in ohm metres.

What does the (electrical) resistance (R) of a sample substance depend on?

  1. The Type of Material
  2. The Cross-Sectional Area (a) of the sample
  3. The Length (l) of the sample

Resistance is proportional to length
Resistance is inversely proportional to cross-sectonal area
Combine these two and you get:
Resistance is proportional to length divided by cross-sectonal area.

Resistance = (ρl)/a
ρ = resistivity constant for the material

ie. The resistance of the sample equals the length divided by the cross-sectional area, all multiplied by the resistivity constant for the material.