The electrical resistance (symbol R) is the physical property of (mostly metallic) conductors to oppose the current flow. The electrical resistance is the reciprocal value of the conductance of a material and is measured in Ω (Ohm). A conductor has a resistance value R =1 Ω when an applied voltage U = 1 Volt results in a current flow of I = 1 ampere. A given material (preferably metal) is a good conductor the better its conductance, i.e. the smaller its resistance. The resistance value is directly proportional to the specific resistance ρ (rho) of the conductor and to its length l and inversely proportional to its diameter A: R = ρ ∙ l / A. The current flowing through the resistor generates heat, which the resistor releases to the atmosphere via its surface. The heat generated in the resistor is equal to P and proportional to its ohm value and the square value of the current flow: P = i²R. Good conductors are gold, silver, copper and aluminum. Poor conductors are iron, steel, chromium, nickel and their alloys. The poor conductors are used as resistor materials because this property is required for the manufacture of resistors. It is the task of the resistor manufacturer to design the resistor conductor, provide it with terminals and fix it mechanically such that the specific data required for application are met. The key dimensions of a resistor are its resistance value in Ohm and its possible power output in Watt. Other important characteristic values are the resistor tolerance, storage capacity, type of cooling and protection.