Resistors' one important parameter is electrical resistivity which is a measure of a material’s property to oppose the flow of electric current. This is expressed in Ohm-meters (Ω⋅m). The symbol of resistivity is usually the Greek letter ρ (rho). A high resistivity means that a material does not conduct well electric charge.


 
Electrical resistance is expressed in Ohms, and is not the same as resistivity. While resistivity is a material property, resistance is the property of an object. The electrical resistance of a resistor is determined by the combination of the shape and the resistivity of the material. For example, a wirewound resistor with a long, thick wire has a higher resistance then with a shorter and thinner wire. A wirewound resistor made from a material with high resistivity has a higher resistance value then one with a low resistivity. An analogy with a hydraulic system can be made, where water is pumped through a pipe. The longer and thinner the pipe, the higher the resistance will be. A pipe full with sand will resist the flow of water more than a without sand (resistivity property).
 
The resistivity of a material is dependent on the temperature and is normally given for room temperature (20°C). The change in resistivity as a result of temperature change is described by the temperature coefficient. For example thermistors make use of this property to measure temperature. On the other hand, in precision electronics this is usually an unwanted effect. Metal foil resistors have excellent properties for temperature stability. This is reached not only by the low resistivity of the material, but also by the mechanical design of the component.