Explanation: When a DC voltage is applied to a filament, a slight voltage gradient exists along its length. This gradient, combined with the high operating temperature, causes a phenomenon known as electromigration. Filament material (e.g., tungsten) atoms slowly migrate from the more negative end of the filament toward the more positive end. Over extended periods, this continuous migration can cause the filament to thin at the negative end and thicken at the positive end. The thinned section becomes a "hot spot" and is more susceptible to burning out prematurely, leading to tube failure. Periodically reversing the DC polarity counters this migration by changing which end is negative. This helps redistribute the filament material more evenly along its length, preventing localized thinning and significantly extending the operational life of the filament. The polarity of the filament supply has no bearing on the lifespan of the power source itself.