SOLUTION

We know that metals contain a large number of free electrons and thus electric current can pass through them easily. For example, metals offer a little resistance to the flow of free electrons through them, i.e., they have low resistance. Substances like manganin, constantan, nichrome, tungsten, etc., offer high resistance to the flow of electric current through them. Hence, they have high resistance. Constantan (or Eureka) is an alloy of copper (55%) and nickel (45%). It is used in the construction of pyrometers and thermocouples. Manganin is an alloy of copper (83%), manganese (13%), and nickel (4%). It is used in the construction of rheostats and resistors. Nichrome is an alloy of nickel (60%) and chromium (40%). It is used in the construction of the heating element.

The heating elements (or heating coils) of electrical heating appliances such as electric iron and toaster, etc., are made of an alloy rather than a pure metal because

(i) the resistivity of an alloy is much higher than that of pure metal

(ii) an alloy does not undergo oxidation (or burn) easily even at high temperatures when it is red hot.

For example, nichrome alloy is used for making the heating elements of electrical appliances such as electric iron, toaster, electric kettle, room heaters, water heaters (geysers, and hair dryers, etc., because:

(i) Nichrome has very high resistivity (due to which the heating element made of nichrome has a high resistance and produces a lot of heat on passing current).

(ii) Nichrome does not undergo oxidation (or burn) easily even at high temperatures. Due to this nichrome wire can be kept red-hot without burning or breaking in the air.

The resistivity of conductors (like metals) is very low. The resistivity of most of the metals increases with temperature. On the other hand, the resistivity of insulators like ebonite, glass, and diamond is very high and does not change with temperature. The resistivity of semiconductors like silicon and germanium is in-between those of conductors and insulators and decreases on increasing the temperature.