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?

- The Type of Material
- The Cross-Sectional Area (a) of the sample
- 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.