A conductor has very low resistance. All metals are good conductors; the best are silver, copper, and aluminum. Copper is generally used for wire conductors due to its lower cost.
The sizes for copper wire are specified by the American Wire Gage. Higher gage numbers mean thinner wire. Typical sizes are No. 22 gage hookup wire for electronic circuits and No. 12 and No. 14 for house wiring.
The cross-sectional area of round wire is measured in circular mils. One mil is 0.001 in. The area in circular mils equals the diameter in mils squared.
The resistance R of a conductor can be found using the formula R = ρ(l/A), where ρ is the specific resistance, l is the length of the conductor, and A is the cross-sectional area of the conductor. Wire resistance increases directly with length l, but decreases inversely with cross-sectional area A.
The voltage drop across a closed switch in a series circuit is zero volts. When open, the switch has the applied voltage across it.
A fuse protects circuit components and wiring against overload; excessive current melts the fuse element to open the entire series circuit. A good fuse has very low resistance and practically zero voltage across it.
Ionization in liquids and gases produces atoms that are not electrically neutral. These are ions. Negative ions have an excess of electrons; positive ions have a deficiency of electrons. In liquids and gases, electric current is a result of the movement of ions.
The resistance of pure metals increases with temperature. For semiconductors and liquid electrolytes, resistance decreases at higher temperatures.
An insulator has very high resistance. Common insulating materials are air, vacuum, rubber, paper, glass, porcelain, shellac, and plastics. Insulators are also called dielectrics.
Superconductors have practically no resistance.
Common circuit troubles are an open in wire conductors; dirty contacts in switches; and dirt, oxides, and corrosion on connectors and terminals.
