Conductors may be solid, (such as copper, aluminum, silver, iron etc.) liquid (such as electrolytes and molten metals and gaseous (such as ionized air), Most commonly used metal conductors (silver, copper, aluminum, and iron) in engineering practices are described below :

1.    Silver : It is the best conductor of heat and electricity, malleable, ductile and resistive to corrosion effect. Its use is limited due to its high cost and is only used of special purposes such as in contact surfaces of switch-gear etc. as it requires low contact surface.

2.    Copper: It is the next best conductor and is most commonly used in electrical engineering. It is malleable, docile, highly resistant to corrosion, good conductor of heat, reasonably cheap and readily available in almost all shapes. Its resistively is very low (172 to 177 x 108 Ω-m) and mechanical strength is good (an ultimate tensile strength of 25 to 40 kg / mm). It can be easily and efficiently jointed by variety of methods including soldering and welding.

The copper conductor mostly used is of two types namely (i) hard drawn (ii) at nealed.

Hard drawn copper wire is made by drawing it cold through dies. It is stronger in mechanical strength but less ductile and is used as transmission line conductors, overhead contact wires, switch-gear, busywork, commentator segments for electric machines.

The annealed copper is made by heating hard drawn copper to high temperature of 400⁰C and then cooling slowly. It is more soft and ductile. Annealed copper is used in the manufacture in of around and square wires, cables cores etc.

3.    Aluminum : It is third in rank in electrical conductivity after silver and copper. Its resistively is 2.8 x 10^-8 Ω-m i.e. about 1.7 times of that of copper, It is lighter in weight and cheap in cost It is mostly used in overhead transmission lines. Its mechanical strength is comparatively low (tensile strength of 8 to 9 kg /mm² for annealed aluminum and 18 kg / mm² for hard aluminum ) which can be increased by allotting it with silicon, iron or manganese or by having an envelop of aluminum wires around a core of one of more high tensile galvanized steel wires. Aluminum is also used in manufacturing of wires, toured or square bus-bars, capacitor foils and cable sheaths.

4.    Iron: Iron is mechanically stronger and cheaper in cost in comparison with copper. It is used for overheads transmission lines of very long spans where copper can not be used due to comparatively low mechanical strength. Its conductivity is one sixth-oneseveth of that of copper. It is less resistive to corrosion but this short coming can be overcome by coating it with a more carrion resistant metal such as zine.Being a ferro-magnetic material it is subject to skin effect when carting alternating currents, hence its resistance is higher to alternating currents than that to direct currents. Hysteretic loss is inevitable in still conductors when used in a.c. circuits. I is used in manufacturing of wires, busbares, earthling matis, railway and tramway rails. It is also used as overhead ground wire. 

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