[Dr. STONE] Can the magnets created in episode 9 of the anime really be made?

2025年7月29日

Episode 9 of the anime Dr. Stone has been released. The enemy suddenly appears, and Senku is somehow curious about him. Meanwhile, this episode sees a rapid advancement in the times, from iron refining, magnets, and even hand-cranked generators to lightbulbs. This time, we'll introduce and explain the super-science behind Episode 9.

Magnets from Lightning! How Does It Work? ?

Senku began refining iron in Episode 8 of the anime. Thanks to the Level 2 Steelworks, he successfully produced iron. Then, he uses lightning to create a super-powerful magnet. But how does that work? ?

Magnetization by Lightning

Magnetization refers to the act of imparting magnetic force to minerals that become magnets. In principle, a magnetic field is generated by wrapping copper wire around a coil and passing an electric current through it. In this case, an iron rod is placed inside the coil, imparting magnetic force to the iron. Placing a magnet or iron inside a coil that generates magnetic force creates a magnet, which is essentially the same as an electromagnet. I'm sure it's still done in science experiments... (For example, if you wrap copper wire around a nail and pass an electric current through it, the nail will become magnetic while the current is flowing.) This magnetization requires a very large current, so we used lightning in this example. Furthermore, a magnetic field cannot be properly generated without an insulator between the copper wire acting as the coil and the iron that acts as the magnet. For this reason, we used lacquer as an insulator and coated the iron. This coil-type magnetization method can only produce magnets with a south and north pole. Incidentally, minerals with this magnetic force have a maximum value determined by the material, and a saturation point at which they no longer become stronger, no matter how strong the current applied.

There are actually minerals that have been magnetized by lightning.

Lightning has indeed caused magnetization in nature. Near the summit of Mount Koyama in Susa, Hagi City, Yamaguchi Prefecture, there is a rock mass known as "Magnetic Stone," designated as a national natural monument. It's believed that lightning is the cause of this rock mass's magnetic force. It's said that the magnets used in compasses made in China before the Age of Discovery were originally lightning magnets.

The Principle of a Hand-Cranked Dynamo

Arago's Disk

This hand-cranked dynamo revolves around two copper plates. This device is based on the Arago disk. Arago's disk involves placing a magnet close to a copper disk, and when the magnet is rotated, the copper disk also rotates. Today, this principle is used in water meters and other devices. (Although it's somewhat different.) This is due to eddy currents generated in the copper disk, which was later elucidated by Foucault.

Fleming's Right-Hand Rule

In Arago's disk, both the magnet and the copper disk rotated, but in this generator, only the copper plate rotates. This is how an electric current is generated. The flow of electricity is easily explained using Fleming's right-hand rule. (While Fleming's left-hand and right-hand screw rules are usually covered in class, Fleming's right-hand rule is easier to understand when it comes to electromotive force.) Magnetic force moves from north to south and is represented by the index finger. Force refers to the direction of rotation of the copper plate and is represented by the thumb. The electromotive force flows toward the middle finger. To achieve this, the copper plates on the right and left must rotate in opposite directions to generate electricity.