In this question: How far can a time traveller go into the past before his electrical equipment becomes unchargeable?

@Mark assures us that "Building a generator can be done at any time:" (just wind copper).

According to an eHow article, after obtaining an engine (we'd use a windmill, waterwheel, bicycle/animal motive power, or some other type of engine; simple exercises left to the reader), you need:

"2: Choose an AC generator head. This head will use an internal magnet to create electricity when the shaft mounted magnet is spun by the external engine."

"3:" (Alternator)

And on another site, a guy created an wooden (armature) alternator (which contrary to @Youstay-Igo's assertation, does not seem to need that much precision), but the components?:

  • "18 surplus NdFeB rare earth magnets"
  • (and epoxy, and nice wire)

These magnets are not easy to come by because rare-earths are... well, rare. (and I'm assuming higher-power magnets are what makes this alternator as effective as it is, versus say, lower power magnets).

From the original question, one of the comments is:

@AdamDavis: Building a generator is considerably more difficult than one might assume. Lodestone, naturally occurring magnetic rock, is not only rare, but fairly weak. Generating 5W out of a motor built out of lodestone and crude wire is going to be an exercise in futility. The fabrication of stronger magnets requires electricity, thus a chicken and egg style problem.

I understand that non-mass-produced wire in the past (hand-drawn, ouch!) is a problem. As well as wire coatings, which are at least as problematic (for any type of longer-term use; and/or preventing the melting of your copper wire into a copper lump while under load). But the magnets seem to be the real sticking-point.

Obviously, a time-traveler - like in the existing question - caught in such a situation would begin to assess his problems, and could get magnets and wire and/or a solution anytime before going further back than 1880s. And probably some solutions further back than that (Judean batteries, etc).

But, if a time-traveler jumped back pretty far in his first jump, how would he make magnets/wire to make a generator; or, how would he make a generator (without those)?

Granted, that in the past there's plenty of resources, that you don't have to "mine": native copper, deposits that're still on the Earth's surface, etc. Lodestone is more rare however. Assuming he can prospect or talk to people who know where such resources are, we will assume he can get to it (ie: he doesn't have logistics issues (he's eating Shmoo), and can boop around using his (sealed) antigrav flitter to get to any location).

He's got some limited amount of power in his laptop/cell/etc, so if he's careful, he could set something up (what would he need to get that power out?) to jump-start his chicken-and-egg problem of creating a strong magnet. Maybe. (ie: how much power do you need to make a magnet, for how long?)

But, how would you do that (make a magnet)?

We're leaving the rectification of DC and voltage regulators (capacitors, diodes, etc) and stuff alone. I'm going to assume he has a robust laptop plug that handles a fairly dirty/unregular AC power supply input (ie: about how effective your laptop power cord/block is), as I don't see how he could easily make those, but bonus points if you want to tackle that.

In case it wasn't clear: Previous question assumed (and I am to), that he wasn't well prepared to do this; he just grabbed a few things and jumped in his time machine. Otherwise, obviously, you'd take a solar battery unit/hand-crank generator/@Youstay-Igo recommends a dynamo (and you've got one of those lying next to your briefcase/laptop, don't you? /snark), and a set of tools to make new things and spare parts: rectifier to run off of batteries, magnets, wire, capacitors, diodes, voltage regulators, interfaces, etc. Spare time-machine, etc, etc.

ie: What do you have right next to your laptop/cellphone? Probably not any of those things, nor a large-enough, powerful enough, set of magnets (how large is needed?)

Sounds like we might be using electromagnets (iron with wire wrapt around it) to be our magnets for the first iteration... if there's a way to power it. I'm thinking USB out of the laptop - which avoids the rectification problem. If not, why not power the electromagnets with hand-made chemical batteries; potatoes if nothing else?

  • $\begingroup$ How much electrical equipment is this time traveler carrying? Can't he just take a small human powered (e.g. wind-up) generator along too? Or just carry a pocketful of magnets. $\endgroup$ Commented Sep 26, 2015 at 19:24
  • $\begingroup$ Previous question assumed (and I am to), that he wasn't prepared to do this; grabbed stuff and went. Otherwise, obviously, you'd take a solar battery unit, and a set of tools to make new things. $\endgroup$
    – user3082
    Commented Sep 26, 2015 at 20:32
  • $\begingroup$ The problem with USB is that port is limited to 500 milliamps at 5 volts, and it won't do anything meaningful to the coil. The problem with potatoes is similar, a low voltage that will disappear when you connect the coil. $\endgroup$
    – user8827
    Commented Sep 27, 2015 at 16:28
  • $\begingroup$ So, what's the minimum voltage/power to make an electromagnet that could be used in an alternator armature? ie: as substitutes for the NdFeB in the DIYer's setup? USB makes those electromagnets work, and the mechanical force (engine) delivers force that those magnets turn into electricity. $\endgroup$
    – user3082
    Commented Sep 27, 2015 at 16:29
  • 1
    $\begingroup$ I would say we've established any experienced time traveler should have the components for a basic generator in his emergency bag. Bonus points if he can hook it up to a water wheel (or has the lightweight components for a water wheel in his bag as well). $\endgroup$ Commented Jul 1, 2016 at 20:55

7 Answers 7


For the record - my entire Navy career was as an electrician, and my degree is in power electronics.

Any first year EE student has observed that you really can make a (fairly poor) voltage by simply waving a coil around in the air. This works in the usual way, because the earth has a magnetic field - nothing weird. It is weak - maybe even weaker than you think it is. But, it's enough for what we need. The biggest problem now is that the time traveler absolutely must have a way of rectifying current flow or else fail entirely.

The reason why he needs it is because we need to feed the output of the generator into the input of a handwound electromagnet at the core of the generator end through sliprings - literally a round piece of metal that another conductor can ride on, to which the magnet is connected. If the input isn't rectified, then the north and south poles of the magnet switch places every cycle and your alternator doesn't alternate very well.

If you can do that, the next problem is controlling the feedback loop. When trying to start the generator, everything is great - you have a tiny little current flow gradually bolstering the field of the ferromagnetic core. Once the field comes up, you have a problem - you are passing a non-zero voltage through a very low resistance. This is will do one of two things - most likely, it will prevent your generator from being able to develop a workable output, because it is shorted. But if you work real hard, and have steady hands, and built a pretty good generator head, the feedback loop will spike the voltage on the generator head and utterly destroy all your work. Note that this isn't free power - the equation sacrifices current flow for voltage and you arc through your insulation. This is almost the exact reason why permanent magnets are used so often - since they are constant, you don't have to have as robust a regulator.

I'll skip over some of the more technical stuff by saying that it is much more likely that the brave adventurer would be forced to build a DC machine. This is actually harder to make, but, it allows the use of a special mechanical rectifier called a commutator. The commutator is a multi layered ring of conductive sections separated by a mineral insulator in thin strips, on which a conductor can slip while it is rolling. Each set of sections on opposite sides of each other is connected to a separate conductive loop, and no two sets are connected. Since the current goes into and out of the commutator at fixed angles, the corresponding magnets can be placed at a certain angle with respect to the commutator entry and exit conductors - the rotor spins but the relationship between the magnetic fields doesn't reverse. (Yes, this is about as non-technical as I can make this - sorry guys.)

This still didn't solve our regulator issue, but if your time traveller knows how everything above works, there's a reasonable chance that he knows what a magnetic amplifier is, and that it's way easier than building a vacuum tube from scratch. This is basically a fancy transformer, designed intentionally to saturate at a specific point and then have a much reduced response. Without getting too into it, if he can do the math, and has copper and an insulator, he can do this and the electromagnet should self start AND not burn up.

So, thousands of hours of grueling material round up, refinement and tool manufacture later, if he didn't starve to death or have a surprise party with some velociraptors, he now has what is actually probably a fairly decent generator head.

The engine should only take about ten times as long. Build a bicycle, maybe.

Edit: I assume this took a downvote because the question tries to avoid the rectification issue. For clarification, in the case where permanent magnets aren't available, avoiding rectification is not an option. A permanent magnet keeps the same polarity as it goes around the armature, but if you attempt that with the raw, unrectified output of a handmade generator, the polarity of the electromagnet flips as it makes it's way around and you get no output.

  • $\begingroup$ The engine is (in comparison) easy, as long as you don't need constant, stable power output (that's still a possibility, but is more work to engineer). Waterwheels, windmills, treadmills (human hamster wheel or dog wheel), and, as you say, the possibility of a crank mechanism driven by human legs. Sadly, I don't quit follow all of your explanation :P Got lost on the rectifier. Also, going to add a note to the question. $\endgroup$
    – user3082
    Commented Sep 27, 2015 at 8:48
  • $\begingroup$ You can bootstrap the generator with a battery, for a fairly stable magnetic field, no? $\endgroup$
    – MichaelK
    Commented Jul 2, 2016 at 16:46
  • $\begingroup$ @MichaelKarnefors, absolutely, but you still run into the problem of regulating field excitation current after the field flash. If you can't do that, you can't regulate the output enough to make it usable anyway. $\endgroup$
    – user8827
    Commented Jul 2, 2016 at 19:13
  • 2
    $\begingroup$ I understood every word of this answer, and all of it checks correct. I do think, everyone is focusing too much on getting the conductors (copper, etc) and aren't thinking enough about how to make the insulation materials. You are going to need to make a varnish for the windings. Depending how far he goes back, he might be able to get it, else, maybe he can try boiling the water out of tree resins and applying them before they cool. it wont be high quality, and it probably wont last long (the heat of operating the generator will degrade it), but it might work just enough to get the job done. $\endgroup$
    – root1657
    Commented Mar 11, 2021 at 8:15


A: Very hard! But not impossible...

You can use a magnet to make electricity, and you can use electricity to make a magnet, but you always need one to make the other! Obviously, an electromagnet is out of the question, since electricity is the resource you are trying to obtain. And we agreed that naturally occurring magnets are very difficult to come by. So you are left trying to create a permanent magnet from scratch.


If you have basic metallurgy available, you can heat an iron rod and expose it to a magnetic field. You might as well use earth's magnetic field, since it's "free", but it also helps if you have any other magnet at all, including a weak lodestone (or any magnets in your laptop! like the one in Mac power plugs ;). With sufficient practice, it should be possible to make stronger magnet than you started with (the heat frees the magnetic domains to be re-oriented, and any induced magnetic field will orient them in the same direction, statistically speaking). You can also use impact force from a hammer to free the domains, but this is probably much less effective than direct heating.

Cool the heated rod, and the new domains will be "frozen" in place. You now have a new permanent magnet. Get another rod, do the same thing, but now use your more powerful newly created magnet to induce a stronger magnetic field in the new rod, and you should be able to make a magnet as powerful as this technology allows. The limit would be how many magnetic domains you can liberate in the heating process, and how many you can orient using the induced magnetic field. The more magnets you have, the stronger a field you can induce in any newly created magnet.


Even with no permanent magnet at all, you can still create one with just iron and a fire hot enough to liberate the magnetic domains within. This is because we live on top of a giant dynamo which conveniently provides a life-saving magnetic field. Without it, you would be reduced to bootstrapping the process with an electromagnet, which might be possible using static electricity, but would surely be very painfully tedious to construct.

  • $\begingroup$ Hmm, I'm assuming there has to be a saturation limit (physics question time), to how effective/energy-dense a pure iron magnet can be - which would be why we're using rare-earth magnets, instead of just creating iron magnets inside MRI units, or something. And, if this is indeed the case (cool while in magnetic field), why isn't every manufactured piece of ferrous metal a magnet (from being cooled while in the Earth's magnetic field)? Special type of cooling required? $\endgroup$
    – user3082
    Commented Sep 26, 2015 at 22:07
  • $\begingroup$ Wouldn't bootstrapping the process using a lemon battery work, too? $\endgroup$
    – celtschk
    Commented Sep 27, 2015 at 15:16
  • $\begingroup$ There is nothing wrong with using field coils in a generator! You seem to think that is not possible. The normal ambient earth's field is enough to bootstrap it when you start cranking. $\endgroup$
    – JDługosz
    Commented Jul 2, 2016 at 15:46

A simple generator is relatively easy to build.
The lower the voltage output, the easier it could be made.

The very first generator, the Faraday disc generator, was created 1831 (by Michael Faraday) using motion similar to that of a spinning-wheel and even used some of the generated current to power the magnet.

Faraday disc generator

The simplicity made it inefficient, but it was enough to engage a young Nikola Tesla, who worked on it's design before developing a much more capable, drum-based generator.

In order to start generating power, you'd either need fairly strong magnets (which would be impossible to locate), or enough electricity to initially power an electromagnet. Interestingly enough, it could easily be possible to extract enough DC current from lightly boiled potatoes, in order to get things started.

So, as long as your traveler landed in a time period where tooling and copper craftsmen were common, then It might be possible. The early Egyptians were one of the youngest civilizations known to make copper jewelry, so that would probably be the earliest, plausible time period.

The real question is ...
What need would there be for crafting an inefficient generator, when simply using potatoes could potentially be more efficient?

Potato batteries

  • $\begingroup$ i hate when people bring up the "potato" battery.. The power does NOT come from the potato! It comes from the Zinc anode being consumed. Remaking the zinc anode consumes a lot more power than it released in the "potato battery" $\endgroup$
    – PcMan
    Commented Mar 8, 2021 at 19:27

You can build as generator as long as you can get or make insulated metal wire (preferably copper), so as you might imagine, you can go pretty far back! The problem, however, is, that generators produce AC power, while devices such as your cellphone require DC power. AC-DC transformers require at the very least capacitors and diodes (and voltage regulators if you want a stable voltage).

I guess you could make capacitors and diodes if you had the required knowledge, but I suspect they'd be unreliable, inefficient and bulky. You'd be better off taking the parts from technology you brought along. A cellphone charger, for example, contains an AC-DC transformer (apart from the AC-AC step down transformer).

  • $\begingroup$ In case this "robust laptop plug" wasn't clear, he can handle AC of a fairly reasonable output, just fine. Please clarify "you can build a generator if you have wire", as I'd like to know how to do it without good magnets. $\endgroup$
    – user3082
    Commented Sep 26, 2015 at 20:38
  • $\begingroup$ You can compensate for good magnets using gigantic magnets! :) As for requiring electricity to build strong magnets, you could always use 1-time batteries (For example, a massive field of row after row of potato batteries). $\endgroup$ Commented Sep 26, 2015 at 21:18
  • $\begingroup$ Cool, but that's still a subsidiary question; given some type of power, how do you make a magnet? Electromagnets work when you power them, but you can't then use them inside a power-generator (or, you've just created free-energy!!) $\endgroup$
    – user3082
    Commented Sep 26, 2015 at 21:58
  • $\begingroup$ Magnetic materials can be found in nature, Magnetite for example (en.wikipedia.org/wiki/Magnetite). While these are extremely weak, they could be used. You could also further magnetize them using temporary electromagnets powered by basic batteries (Potatos and lemons can be used to make those). ________ PS: Sorry for the 6 month delay. I recently logged into the site and saw your comment. $\endgroup$ Commented May 13, 2016 at 10:57
  • $\begingroup$ If you used electromagnets rather than perminant magnets, and wired jt right, you would get the same polarity out, not full AC. With enough separate coils you can get lumpy DC. $\endgroup$
    – JDługosz
    Commented Jul 2, 2016 at 15:42

Real high power generators don't use just magnets. The really powerful ones (like the ones at power stations) actually use electromagnets instead, because its much easier to get a strong magnetic field out of an electromagnet than to get enough rare earth magnets to do the trick.

Of course, this is a chicken and egg moment. The real generators actually do have smaller magnets in them, which can generate just enough current to bootstrap the more powerful electromagnets into operation. From then on, the magnets do help a little, but most of the magnetic field is generated by the electromagnets.

  • $\begingroup$ Well, we don't need really high power generators to run a laptop, now do we? ;) However, it might be interesting to learn more about how the big ones do it, maybe we could make one with electromagnets that's bootstrapped off of lodestone/low-quality/very-small magnets? $\endgroup$
    – user3082
    Commented Sep 27, 2015 at 0:06

It is possible to build the generator if it is possible to make the somewhat insulated wire of any kind, and is possible to make its mechanical part (rotating rotor).

A magnetic field is required around the rotor. We could use electromagnets for it, but at least small permanent magnets would make the generator easier (or even just possible) to start.

The generator does not need the rectifier because a simple mechanical rectifier can be mounted directly on the axis of the generator.

Finally, to get rid of pulsations, it should be possible to build more than one generator and connect them in parallel. The averaged output should be smoother.


If you are time-travelling into the past and want a power source to charge your apparatus, simply bring a dynamo with you (half a dozen for precaution sake). You can rotate its head and it will give you a steady flow of dc electricity at the terminals.

Don't get into the mess of ac generator building in the past. It is going to prove extremely difficult, if not plain impossible. Crude tech would be available with metallurgy and all, but the precision of parts required for a reliable ac generator is something you can hardly ever expect to build.

  • $\begingroup$ It's that "extremely difficult" that I'm interested in investigating. The dude mentioned built one out of wood, but had magnets and wire; the precision wasn't all that great. Flywheel or something to regulate input power of your waterwheel/wind turbine, or it only works at the proper speed. $\endgroup$
    – user3082
    Commented Sep 26, 2015 at 20:41
  • $\begingroup$ If dynamo creation (and consistent DC voltage) is easier than making a dirty AC generator, then please expand your answer to include that - as he could probably bypass his AC/DC laptop plug. $\endgroup$
    – user3082
    Commented Sep 26, 2015 at 20:54
  • $\begingroup$ I said "take it with you", not "invent it there" :d Since dynamos are truly small and easier to carry ... $\endgroup$ Commented Sep 26, 2015 at 21:39
  • $\begingroup$ Got it, so equally as useless as an answer - as that's precluded. But, for curiosity's sake, are they better than a solar charger? $\endgroup$
    – user3082
    Commented Sep 26, 2015 at 21:51
  • $\begingroup$ Yes. Longer life. Virtually eternal. Can be used with mechanical force anywhere. Can be repaired manually too. You cant repair a malfunctional solar panel. $\endgroup$ Commented Sep 26, 2015 at 22:23

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