1. The problem statement, all variables and given/known data As graphite heats, the resistance of electrons flowing through it in a circuit decreases. 12 volts of electricity was run through a rheostat and through a small strip of mechanical pacer 'lead' to observe the state of the 'lead' as the resistance of the rheostat was decreased. When the graphite ('lead' strip) heated up it would release light from the center of the graphite. This light intesified and spread outwards towards the end of the graphite. After a while the light would retract towards the center right before the graphite melted in the center. 2. Relevant equations Why does the graphite heat up in the center of the graphite first? Why does the light retract towards the center of the graphite right before the center melts? 3. The attempt at a solution I have hypothesised that because gravity is acting upon the center of the graphite, that it is weakest there as the ends of the graphite are supported. So when it heats, the graphite begins to break slightly, which then decreases the cross-sectional area at that point. This is why the light begins to dim and retract until the graphite completely breaks killing the circuit. This does not explain why it heats in the center first. I would greatly appreciate any oppinions or help.