Just a couple of guys doing fun things with a DIY low voltage very high current transformer. One of the things is putting a spanner on a brick and touching the two ends with an extremely thick copper cable carrying several thousand amps.
The spanner then becomes red hot and melts. And here we come to the question:
Why does the spanner turn red hot at the ends first and then later towards the center? I would have thought uniform current would have heated it evenly
Best Answer
There is heating from the contact points, but not enough to make them turn red. More heat comes from the thin section. Where both sources heat the metal it gets hotter than the rest of the thin section, causing the resistance to rise as it heats, yielding more localized heating (positive feedback), and so on, so the ends of the thin section get hot first and the hot area propagates toward the center of the thin section.
It may only take a relatively small temperature difference to start the positive feedback in a given section. See, for example, this curve.