Resistance is the property of an object, and resistivity is the property of the substance of which the object is made. Resistivity varies between materials. Copper, silver and aluminum all have very low electrical resistivity; therefore, they are excellent conductors. Salt water also makes a good conductor. Air, rubber, glass, stone and ceramic have very high resistivity and thus make good insulators.
When current is passed through a wire, the negative charges repel free electrons through the wire from one atom to the next until the reaction reaches the other end of the wire. Electrical resistance occurs when substances have few or no electrons that may travel. Heat also affects electrical resistance. In most metals, more heat means more resistance. In wire, thicker wires carry more current because there is more space for electrons to travel without rubbing against one another. That’s why thicker wires are used for high-power applications.