The electrical resistance (symbol R) is the physical property of (mostly metallic) conductors to resist an electric current. The electrical resistance is the reciprocal value of the conductance of a material and is measured in ohm. A conductor has a resistance value of R =1 Ohm when an applied voltage of U = 1 volt results in a current flow of I = 1 ampère. A given substance (preferably a metal) is a good conductor the better its conductance, i.e. the smaller its resistance. The resistance is directly proportional to the specific resistance rho of the conductor and to its length l and inversely proportional to its diameter A : R = (rho . l )/ A. The current flowing through the resistor generates heat, which the resistor releases to the environment via its surface. The heat generated in the resistor is its power rating P and proportionate to its ohm value and the square 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 energy output in watt. Other important characteristic values are the resistor tolerance, storage capacity, type of cooling and protection. Resistors are either named according to their design as layered resistors, thin film and thick film resistors (not considered here), wire-wound resistors, ribbon-wound resistors, sheet resistors, steel grid resistors and cast resistors, or according to their application, among others as starting, braking, load, grounding resistors, etc. Some designs are particularly suited for certain applications (e.g. cast iron resistors as starting resistors) but this does not exclude that a different design may be used for the same application depending on the respective conditions. The expert who selects the resistor for a given application will always try to select the optimum design in both technical and economic terms.

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