In the early days, the science wasn't yet refined, so they used huge battery plates beyond what was needed. The main wear-out factor is going to be the water in the electrolyte. When the water evaporates away, the reaction stops. The giant glass cover is probably why it survives so long... humidity is trapped inside, so the electrolyte is not allowed to dry out.<br><br>But the bell jar base is probably not sealed. There will likely be a small amount of "breathing" of the air inside the jar, and moisture will be lost through the slot in the base where the bell sits, due to atmospheric air pressure changes (highs and lows), and thermal expansion and contraction if sunshine warms the apparatus, or the room temperature is not held constant.<br><br>If they want to keep it going as long as possible, it might actually be for the best to put a small glass beaker of distilled water inside the bell jar to help maintain the high humidity, and to periodically top off the beaker if the water level drops due to evaporation.