Resistance. Rheostats

The quantity that describes how a conductor opposes the passage of current in a circuit is called electrical resistance; it is denoted R and measured in ohms (Ω); 1 kΩ = 10³ Ω, 1 MΩ = 10⁶ Ω. In metals resistance comes from the collisions of free electrons with the ions of the crystal lattice. Resistance is measured with an ohmmeter. Experiments show that R is directly proportional to the length l of the conductor, inversely proportional to its cross-section S and depends on the substance: R = ρ·l/S. Here ρ is the resistivity of the substance, in Ω·m; for example copper 0.017·10⁻⁶ Ω·m, aluminium 0.028·10⁻⁶, nickelin 0.4·10⁻⁶, nichrome 1.1·10⁻⁶ Ω·m. Since 10⁻⁶ Ω·m = 1 Ω·mm²/m, we can take S in mm² and l in m in calculations. Nichrome and nickelin have high resistivity, so they are used in heaters and resistors.