Although silver is the best conductor, its cost limits its use to special circuits. Silver is used where a substance with high conductivity or low resistivity is needed.

The two most commonly used conductors are copper and aluminum. Each has positive and negative characteristics that affect its use under varying circumstances.

A comparison of some of the characteristics of copper and aluminum is given in table 1-4.

Table 1-4. - Comparative Characteristics of Copper and Aluminum

Copper has a higher conductivity than aluminum. It is more ductile (can be drawn out). Copper has relatively high tensile strength (the greatest stress a substance can bear along its length without tearing apart). It can also be easily soldered. However, copper is more expensive and heavier than aluminum.

Although aluminum has only about 60 percent of the conductivity of copper, its lightness makes long spans possible. Its relatively large diameter for a given conductivity reduces corona. Corona is the discharge of electricity from the wire when it has a high potential. The discharge is greater when smaller diameter wire is used than when larger diameter wire is used. However, the relatively large size of aluminum for a given conductance does not permit the economical use of an insulation covering.

Q.14 State two advantages of using aluminum wire for carrying electricity over long distances.
Q.15 State four advantages of copper over aluminum as a conductor.

TEMPERATURE COEFFICIENT

The resistance of pure metals, such as silver, copper, and aluminum, increases as the temperature increases. However, the resistance of some alloys, such as constantan and manganin, changes very little as the temperature changes. Measuring instruments use these alloys because the resistance of the circuits must remain constant to get accurate measurements.

In table 1-1, the resistance of a circular-mil-foot of wire (the specific resistance) is given at a specific temperature, 20C in this case. It is necessary to establish a standard temperature. As we stated earlier, the resistance of pure metals increases with an increase in temperature. Therefore, a true basis of comparison cannot be made unless the resistances of all the substances being compared are measured at the same temperature. The amount of increase in the resistance of a 1-ohm sample of the conductor per degree rise in temperature above 0C is called the temperature coefficient of resistance. For copper, the value is approximately 0.00427 ohm.

A length of copper wire having a resistance of 50 ohms at an initial temperature of 0C will have an increase in resistance of 50 X 0.00427, or 0.214 ohms. This applies to the entire length of wire and for each degree of temperature rise above 0C. A 20C increase in resistance is approximately 20 X 0.214, or 4.28 ohms. The total resistance at 20C is 50 + 4.28, or 54.28 ohms.

Q.16 Define the temperature coefficient of resistance.
Q.17 What happens to the resistance of copper when it is heated?