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Creating electricity in the era of cavemen
Let's say you want to reinvent electricity. The majority of the today’s electricity is generated by moving conductors through a magnetic field. So you need a magnetic field, a conductor and motion. A water wheel could provide motion, so then you just need the conductor (wire) and some magnets and you could generate electricity. This sounds great on paper but in reality it glosses over an interesting Catch-22 in using permanent magnets to generate electricity. It is true that the majority of the today’s electricity is generated by moving conductors through a magnetic field but the strength of the magnetic field needed to create any practical amount of electricity is far stronger than the field produced by a natural lodestone. Today’s magnets are manufactured by using large electric currents. So you need electricity to make a strong magnet but you need a strong magnet to make electricity. That’s the Catch-22. Luckily there is a way around this dilemma. (I’m skipping how to make a permanent magnet but if you really want to know see this video.
Instead of trying to making a permanent magnet, a much more practical approach would be to make a primitive battery and use it power an electromagnet. Then use the electromagnet in place of the permanent magnet in your generator. Some of the electricity generated is then fed back to the electromagnet and once the generator is going the battery can be completely disconnected. Of course if the generator ever stops then the whole process must be repeated. This is technically called using external-excitation to induce self-excitation in field coils in a generator. Self-excitation is how the majority of large generators, like the ones used in modern power plants, function. They don’t use permanent magnets but rather they power an internal electromagnet with some of the electricity they generate.
Let’s quickly look at how an electromagnet works and why not just any copper wire is going to work and how electricity is generated by moving conductors through a magnetic field.
Moving electrons create a magnetic field. It is much like how a moving boat creates a wake.
So any conductor that has electrons flowing through it has a magnetic field around it. Normally this field is very weak (compared to modern magnets) but it can be intensified by wrapping lots of coils of wire in a small space and if the coils are wrapped around something ferromagnetic, like a piece of iron, then the magnetic field is focused. This is how you create an electromagnet. But it can’t just be any copper wire - bare copper wire wont’ work. The wire needs a thin coat of insulation. Insulation is what keeps the electrons flowing through all the coils instead of taking the short cut from the first coil to the last coil. Modern copper wire for use in windings has a thin layer of enamel that insulates the wire and avoids short circuits. Any non-conduction insulation will work but it needs to be thin because a thick insulation will limit how many coils of wire you can wrap in a small space. Have you ever “burned up” an electric motor? This happens when the motor gets too hot and the insulation melts and allows the electricity to take a short cut. This short cut reduces the strength of the magnetic field so much that the motor no longer functions.
Rather than go into the details of how generators and motors work, I will just direct you to this excellent video that every time traveler should watch. If you don’t understand how to build a decent DC motor or generator after watching it, then you probably shouldn't be messing around with time machines.