Dr. Stone › Saison 1 Dr. Stone : Stone World

Chapter 21 — Episode 21

First, the Kingdom of Science celebrates a major milestone as Senku unveils a completed waterwheel and generator, announcing the dawn of a "power age" that eliminates the need for manual labor. Senku then explains that the electricity must be stored, and the group begins constructing lead-acid batteries by submerging lead plates in sulfuric acid — a process that takes effort but successfully stores power. With the batteries working, Kinro and Ginro are relieved of their exhausting manual generator duties, freeing the fighters to focus on combat training under Kohaku in preparation for the coming battle against Tsukasa and Hyoga. Then, Senku pushes the team further by using the waterwheel's gear system to attempt building a vacuum light bulb, repeatedly failing as the glass cracks during production. After two months of grueling work, the light bulbs are finally completed, and on Christmas Day, Senku switches them on, filling the village with glowing light that moves everyone deeply. Senku reveals the next goal is building a vacuum tube — the "egg of a computer" — requiring tungsten for the filament, a material not yet in their possession. Finally, on New Year's morning, Gen wakes Senku to watch the sunrise, and during the dawn, Suika — who had spent the night searching for useful stones — presents a glowing blue mineral. Senku identifies it as scheelite (wolframite), which glows blue under ultraviolet light at sunrise and contains tungsten, the strongest metal in the universe and the key material needed for the filament. Excited by the discovery, Senku selects Magma to join the cave exploration team, forming a trio with Senku and Chrome to venture deep into the cave and mine the tungsten ore.