Different metals react differently to the flow of electric current. If a metal offers high resistance to the flow of electric current, it is not a good conductor. Because of the resistance of the wire, electric current flowing through the wire causes heat. The best metals of high resistance are alloys - combinations of several metals - like nickel-chromium, better known by its trade name Nichrome.
A toaster's heating element usually consists of Nichrome ribbon wound on mica strips (fig. 3), although some toasters use Nichrome wire coils (fig. 2).