The resistivity of a pure metal increases at a particular temperature when we add more and more impurities or when we make alloy. So the resistivity of alloys depends upon the percentage of impurity material in the base material at a particular temperature.
According to Nordheim's rule, at a particular temperature, the residual or ideal resistivity of an alloy can be written as
High resistivity alloys
High resistivity alloys is made by mixing of various metals with appropriate percentage and temperature.
1. Nichrome
79.80% Ni, 19.20% Cr, 1.15% Mn and small amount of iron, used as heating elements in heaters and furnaces.
2. Manganin
86% Cu, 12% Mn and 2% Ni used as standard resistances.
3. Constantan
60% Cu and 40% Ni used as thermocouples, rheostats and starters for electric motors.
4. Kanthal
69% Fe, 23% Cr, 6% Al and 2% Co, used as heating elements in heaters and furnaces.
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