As we increase the voltage, the temperature of the wire is increasing. Due to tiny temperature variations, the phase transition threshold is not exceeded simultaneously along the whole wire, but at a single point. At that point, the heat dissipation rate is suddenly increased so that not only will the temperature of that point start raising, but the heat will propagate to the neighboring regions triggering there a phase transition, too. This process will continue until a thermal quasi-equilibrium is reached: with an equilibrium temperature profile along the wire, the temperature at the phase separation point equals to the critical temperature, corresponding neither to an expansion nor to a contraction of the high-resistivity region.
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Please submit the solution to this problem via e-mail to physcs.cup@gmail.com. The next hint for the Problem 1 will be published at 13:00 GMT, 15th January 2023, together with the next updates of the intermediate results.