Temperature

• Electricity flows best when the atoms in a conductor remain still. Because heat makes atoms vibrate, it increases resistance. Generally, the hotter an object becomes, the more resistance it has. For some materials, such as silicon, this rule works backwards to an extent; for a certain range of temperatures, heat reduces resistance.
  
  

Materials

• Materials with tightly-bound electrons, such as plastic and wood, make poor electrical conductors and have high resistance. Scientists do not consider these materials conductors at all; instead they call them "insulators." Among conductors, carbon and silicon have high resistance. The resistance of metals such as copper and nickel is much lower.
  
  

Size and Shape

• Thin and small conductors have higher resistance than large, thick ones --- much as a narrow pipe resists the flow of a liquid more than a large-diameter pipe. Conductors for powerful, high-current industrial machines are much larger than those for low-power consumer electronics. The filament in an incandescent light bulb is a very fine wire designed to produce heat through high electrical resistance.
  
  

Current

• Ideally, the amount of current does not affect the resistance in a material. In reality, however, materials become warm with increasing electrical currents, driving up resistance. Scientists call this resistance "non-ohmic." Electronic components called "resistors" have a constant resistance for a range of currents, though these, too become hot when forced to carry excessive current.