In metal conductors, electrical current flows due to the exchange of electrons between atoms. As electrons move through a metal conductor, some collide with atoms, other electrons or impurities. These collisions cause resistance and generate heat. Heating the metal conductor causes atoms to vibrate more, which in turn makes it more difficult for the electrons to flow, increasing resistance.
This is illustrated in the tutorial above. A circuit connects a 3V Lamp with Iron Wire to two 1.5V Batteries. The circuit also contains a Ceramic Spool around which a section of the wire is wrapped. Underneath the spool is a Bunsen Burner to heat the wire. Click the Turn On button to light the Bunsen burner.
As the wire heats, the lamp dims, then ceases to glow. Heating the wire increases resistivity. Increasing resistivity decreases current flow to the lamp, eventually causing it to stop lighting. Click the Turn Off button to shut off the Bunsen burner. As the wire cools, resistance decreases and current flow resumes, allowing the lamp to relight.