Explain the following.
$(a)$ Why is tungsten used almost exclusively for the filament of electric lamps?
$(b)$ Why are the conductors of electric heating devices,such as bread-toasters and electric irons,made of an alloy rather than a pure metal?

(N/A) The melting point and resistivity of tungsten are very high. It does not oxidize or burn readily at high temperatures. Electric lamps glow at very high temperatures,and tungsten can withstand these temperatures without melting. Hence,tungsten is used as the filament of electric bulbs.
$(b)$ The conductors of electric heating devices,such as bread-toasters and electric irons,are made of an alloy because the resistivity of an alloy is significantly higher than that of pure metals. Furthermore,alloys do not oxidize (burn) readily at high temperatures even when they are red hot. This property allows them to produce a large amount of heat efficiently.