The primary aim of a resistor is to oppose current in a circuit. The particular degree of opposition to electric current is referred to as the resistor resistance. The unit in which the level of resistance is expressed is Ohms (Ω). The resistive material utilized and the size have to be taken into consideration when you want to analyze the level of resistance.
The materials used in producing resistors are various and some of them have been applied over the years. In the start of radio building, amateur constructors would work with a pencil line on a wooden table as a resistor of a high value, making connections using a bolt and washer touching the line ends. The mix that made up the pencil, consisting of a conductor (graphite) and a non-conductor (clay) was the resistor which was relied on. The resistance of the line was larger when the clay content of the pencil was higher. The clay content is indicated by the firmness of the pencil. If the resistors to be utilized should be of low value, metal wire with a silk or cotton insulation was applied and the covered cable was wound around a covered rod.
Resistivity is a measure of the relative resistance generated by the resistor material. If we need to figure out the level of resistance of a sample, the dimensions of the material have their own contribution the resistance of a resistor, and the sample material resistance will depend on its length (the longer the sample the higher the resistance) and inversely on its area of cross-section (the bigger the cross-sectional area the smaller the level of resistance. Assuming that the resistor features a uniform shape, the formula R = p * l / a indicates the material resistance where p (Greek rho) indicates resistivity, l denotes the length and a is used to denote the area of the cross-section.
Resistivity is only based on the material and unaffected by size. The units of resistivity are ohms multiplied by metres, not ohms per metre (ohms x metres) and good conductors such as metals have values which are really small, in the order of 10^-8 Ohms. Resistivity of insulators can be very high, with values that can be as high as 10^10 Ohm. Many materials, notably some metals at temperatures near to the absolute zero (O K), do not have resistivity and as such zero resistance. They are known as superconductors, and so are beyond the scope of this article.
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