Conductivity

• The conductivity of a given substance is a measure of how well it carries a charge. Copper has long been the standard by which conductivity is judged, which is why the conductivity of a material is often given in IACS (International Annealed Copper Standard). The conductivity of copper of the time when this standard was created was rated 100%, and the conductivity of other materials is given as a percentage of this standard. Iron, for instance, is rated at about 18% IACS because it is 18% as conductive as traditional copper wire. However, advances have allowed us to better purify copper, so there are forms of copper today that are rated at better than 100%.
  
  

Resistivity

• Resistivity is the measure of how resistant a given material is to the flow of an electrical current. Knowing the conductivity of a given substance isn't sufficient, as insulating against an electrical charge is also an important aspect of working with electricity. Being able to fully control where an electrical current will go and where it will not is vitally important to the safe use of electricity. If a material has a low resistivity it is a good conductor, and if it has a high resistivity it is a good insulator.
  
  

Temperature

• Temperature affects the conductivity of a material because of the change in the material when it is exposed to different temperatures. Most materials contract when they are cold, which slows down the movement of atoms and minimizes the impact of the material's structural imperfections, allowing a charge to move through the object more easily. When a given material is warmed, the atoms which create the structure of a material move more quickly, and are more likely to be out of their ideal configuration, which increases a material's resistivity. In general, lower temperatures create better conductivity, while higher temperatures increase most material's resistivity.
  
  

Electronic vs. Ionic Conduction

• Electricity is the transfer of energy from one atom to the next through negatively charged electrons; however, this is not the only way energy is carried. When the conductor is an ionic crystal the charge is carried not by electrons, but by ions. Because of this, there are two forms of conductivity: electronic and ionic. A substance that is conducive to the flow of ions may not be as conducive to the flow of electrons and vice-versa.