Explain the following:
(i)  Why is tungsten used almost exclusively as filament for incandescent lamps?
(ii) Why are the conductors of electric heating devices, such as toasters and electric irons, made of an alloy rather than a pure metal?
(iii) Why is the series arrangement not used for domestic circuits?
(iv) How does the resistance of a wire vary with its cross-sectional area?

(i) A thin wire of tungsten has high resistance and does not get oxidized easily at high temperatures. Hence a tungsten filament is used for making filaments of electric bulbs.
(ii) The resistance of all pure metals increases on raising the temperature and decreases on lowering the temperature. But the resistance of alloys like manganin, constantan and nichrome is almost unaffected by temperature.
(iii) If we connect all the electrical appliances like bulbs, fans and sockets, etc., in a series, then if one appliance is switched off or gets fused then all the appliances will also stop working because their electricity supply will be cut-off.
(iv) When the area of a cross-section of a wire is doubled, its resistance gets halved; and if the area of a cross-section of wire is halved, then its resistance will get doubled. Hence a thick wire which has a greater cross-section has less resistance and a thin wire which has a smaller area of cross-section has high resistance.