Resistivity and conductivity are both properties of conductors. Conductors are substances that allow the flow of electrical current or thermal energy through them. The most common and well-known conductors of electrical current are metals. The most common and well-known conductors of thermal energy are metal and glass.
Resistivity is the electrical resistance of a conducting material per unit length. In other words, it is the degree to which a conductor opposes the flow of electricity through itself, instead allowing the energy to flow out of the electrical circuit, most often as heat. Resistivity is useful in comparing various materials based upon their ability to conduct electrical currents. The unit of resistance is the ohm.
Conductivity, by contrast, is the degree to which a conductor allows the flow of electricity through itself. The unit of conductivity is the siemens (S). It was formerly called the mho. Good conductors retain heat, minimizing the loss of energy from the electrical circuit. Copper wires, for example, are a material with excellent conductivity. Materials such as air, cloth, or rubber have very poor conductivity.
Conductivity is resistivity’s reciprocal. A number and its reciprocal’s product is always 1. For example, the reciprocal of 4 is ¼. This means that as conductivity increases, resistivity decreases. Likewise, as conductivity decreases, resistivity increases. In practical terms, this means that a material cannot have high conductivity and high resistivity, but can have one or the other.
Conductivity has a myriad of uses. It determines what materials to make electrical parts from. It can also be used to test the purity of water (impure water conducts more readily). It can be used to sort materials by type as well. Resistivity has its uses as well. Of these, perhaps the best known is the use of rubber as an electrical insulator. Insulators are materials used to surround conductors to prevent the passage of electricity or heat out of the circuit.