Explain why nichrome and not copper is used as heating element whereas manganin is used as standard resistance?

For heating elements, primary requirements are high melting point, high electrical resistance and low thermal expansion. Last two requirements help in reducing thermal fatique because of repeated heating and cooling. Heating elements must be designed in a way as to allow unhindered expansion and contractions for instance, in the form of a coil of wire. Nichrome (80% nickel & 20% chromium) has all above, mentioned requirements and can be used up to 1300°C. So, nichrome and not copper is used as heating element. Additionally melting point of copper is 1083°C, which is less than nichrome.

For resistor applications, primary requirements are uniform resistivity, small temperature coefficient of resistance, stable resistance and low thermo electric potential with respect to copper.  A small α minimizes erron in measurement because of variations in ambient temperature. α is  defined  as  α =  1/R=  dR/dT  where  R is  resistance  of  the  alloy  at temperature T. For Manganin alloy (87% copper and 13% manganese) α is only 20 x 10^-6/K as against 4000x10^-5/K for pure copper. Therefore manganin is used as standard resistance.