The creation of electricity is a pivotal event in Dr. Stone, and the first use of Senku's generator is a gleaming moment that marks a huge step towards rebuilding civilization. Senku, whose interest in science started with a book on Thomas Edison, did so with one of the first real inventions used to create electricity. It’s called a Faraday Disk, a type of Homopolar generator referred to as a dynamo. Senku’s build is actually a double Faraday generator, using two disks connected together in series. I built a small version of this generator, and you can see the building and testing process in this video:
I tried my best to replicate Senku’s generator but desktop-size (mostly limited by the cost of the copper). I really don’t understand much about electricity so through this whole process I’ve had a lot of help from MisterTalkingMachine who's working on recreating Senku’s cell phone. I’ve learned a lot about the science of electricity while making this, and I’ve been careful to make sure everything explained here is accurate, including edits from MisterTalkingMachine for the technical explanations. First I’ll talk about the science behind the generator, then I’ll get into how mine was built.
Michael Faraday is the one who discovered the concept of creating electricity with magnets in 1831. Based on his discoveries, he developed the first electric generator, called Faraday's disk. Senku's generator is built as a very large, twin version of this.
On a Faraday generator, a magnetic field passes through a conductive material such as copper.
The copper has electrons that are free to move through the disk. When the disk is spun, so are the electrons in it. As they pass through the magnetic field, the field exerts a force on them, pushing them towards the edge, and this movement of electrons constitutes an electric current, that will flow through an external circuit if it's connected to one.
The faster the magnetic field moves through the copper, the stronger this force becomes, and a larger current is generated. More energy put in translates to more energy put out. The high amount of energy from the drills I used is transferred to the disks, and the energy that turns the disks becomes an electric current.
If the magnet sits by the conductive material without moving, no current is generated, as the force is only produced when the electrons are in motion through the field.
Faraday’s previous generator was in the form of a copper coil with a loose magnet inside, when the magnet was shaken inside, it produced a pulsing current that changed direction as the magnet moved back and forth. With this rig, he discovered that electric currents could be produced by moving magnets around conductors. This device using a coil is much more similar to modern day generators than Faraday's disk generator that came later, and Senku’s generator.
Modern torches or flashlights that can be charged by shaking use the same type of generator nearly 200 years later.
Senku’s first cotton candy machine for making wire had the problem of not being able to form smooth threads because of the motion switching direction and stopping in between. Similar to this problem, moving the magnet back and forth inside Faraday's coil produced a current that wasn't continuous and also changed direction (which today we call Alternating Current, AC for short). He found inspiration from an invention called Arago’s disk which was a copper disk that spun and effected the magnet of a compass while it spun, making it spin along through an invisible, contact-less interaction.
Arago's disk worked by the production of electric currents within the disk as it spun through the magnetic field of the compass needle. These currents in turn, produce their own magnetic field that pulls the needle along making it spin.
Faraday fixed the magnet and made it larger and more powerful. Metal brushes touching the disk allow the currents to flow through an electric circuit connected to the brushes. This is how we get Senku’s design.
These early devices were “desktop” sizes like the one I built, only suitable for small electrical experiments. Senku must have done some math to conclude that not only did he need a huge copper disk (2.2 meters, or 7 feet), but he needed two of them to produce the amount of electricity he wanted. Senku’s generator is based on Faraday’s disk generator, which was actually not very effective but did demonstrate the possibility of using magnets to generate electricity by a smooth continuous motion.
When my finished replica was tested, I saw just how inefficient it really is. I first tested the output with an instrument that measures volts, and it generated up to 7 volts. However, I later tested the amps and it was almost nothing. Connecting various small light bulbs to the circuit proved that the generator wasn't actually a practical source of power when they didn’t light up. Maybe Senku knew this wasn't enough, and it’s why he made his generator so big and doubled it. It would have to be replicated true to size to find out just how much current Senku’s generator made.
To solve this problem and make it more efficient, more magnets are needed to cover more area of the disk. What happens is that these areas that are not between the magnets, and thus not generating power, provide a clear, solid metal path for the current to flow through the inside of the disk and not through the circuit connected to it. Passing the magnetic field through the entire disk (such as by sandwiching it between two ring shaped magnets) eliminates these areas where current is wasted. This is something I still want to try, to get more magnets and cover more of the disk with them, but for this post I’ll just focus on recreating what Senku actually made. I’ll update when I add the magnets and test it.
Senku usually explains at least part of how his re-inventions work, and I’m surprised that he didn’t explain anything about how the generator works, or about Faraday himself. Faraday’s work is exactly the step-by-step, keep trying different things kind of approach that Senku's all about. Imagine being in a time where the methods to generate electricity were few and very rudimentary, and you’re playing around with materials, and you find the puzzle pieces that lead to a generator that can turn movement directly into electric power. It’s a perfect example of the scientific and engineering processes. Use knowledge to find more knowledge, and use inventions to make better inventions.
So should Senku have done something different? A more useful generator to build with the resources they had would be a regular DC dynamo with coils and a commutator. It used to be a common generator used in modern times.
When building this generator I wanted to stay true to Senku’s design. Partly because I wanted to see how well it worked the way he built it, but also for aesthetics so that it would be recognizable as the generator from Dr. Stone. I carefully measured pictures of it from the anime and manga to get the proportions right. The legs were added last, after all the testing was done. The video shows how it was built, so here I'll mostly discuss the issues I ran into.
The biggest challenge was where the disks spin, for a couple of reasons. In Dr. Stone, the copper disk is attached to a wooden cylinder on a bamboo pole. The wooden cylinder is what spins in the groove in the structure. This would create a ton of friction. I decided to use ball bearings, the same that you'd find in a fidget spinner or roller skate wheels, to support the rod and let it spin freely.
I created braces for each of the four ball bearings to hold it in place. One or two of them still managed to come loose during testing. There's a dowel rod that goes through the ball bearings and the disk. Where the rod connects to the disk was another challenge. It needed to hold tight to the disk and the rod. I chose a rubber washer shaped like a top hat. It goes inside a hole in the middle of the disk, and the washer has a hole for the rod to go through, and they all fit tightly (friction's on my side this time).
It worked at first but the more I tested it, the more the friction gave way and the washer wasn’t holding onto the disk. So to fix it I would need something that actually screws into the disk and screws into the rod.
In general it's pretty unstable. In my video you can see that I'm constantly adjusting and fixing the parts that hold the disks. The disk doesn’t stay centered, the ball bearings come out from the brace, the rod slides out of place. And that’s with me already strayed pretty far from Senku’s build trying to make it operate smoothly. Ginro and Kinro would have had to exercise careful control of their movements to keep everything in place. Once it was hooked up to the waterwheel, it would have
needed structural upgrades or it would fall apart.
One other issue was the brushes at the end of the wire. It was difficult to get as much of the wire strands touching the disk as possible. The turning of the disk tended to warp the shape of the brush and make it curl away from the disk. The strands of these brushes would need some reinforcement to stay in place and have maximum contact.
My magnet source: https://appliedmagnets.com/
When I was a teacher I had to find a proper bar magnet to demonstrate the poles of a magnet. The way magnets are generally made today don’t have a clear north or south pole, or they have the poles on the flat sides. For example, if a coin was a magnet, one face of the coin would be north and the other face would be south. I found some true bar magnets at a local “magnet warehouse” (yes those exist, how exhilarating) that were also strong neodymium. These magnets from a couple years ago just happened to be perfect for this project. If you're wanting to make your own generator, you must carefully choose your magnets based on how the poles are, and the stronger the better.
If you have any questions, you can email us at aboardtheperseus@gmail.com or catch me on the discord server (Sammzor).
Materials:
3/8 inch plywood
L-braces x 6
ball bearings x 4
6-inch 18 gauge copper sheet x 2 cut into circles
scrap aluminum sheet metal
2 inch x 1/2 inch x 1/2 inch neodymium bar magnets
square and round wooden dowels
waxed twine (used for sewing leather)
thick rubber washer x 2
wire with many copper strands – at least 2 feet
wood stain
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