Check the video: doornails were typically bent 90 degrees during installation. You could certainly collect them after burning the structure down, but you wouldn’t be able to reuse them.
You’ve never unbent nails with a ninety degree bend?
When I lived on the farm we reused nails all the time. They always get bent when you remove them, but it’s trivial to straighten them with a couole of whacks on an anvil, or even with a heavy pair of pliers.
I’ve never worked out just how this would work out, but I’ve wondered some: what would happen if I were able to find Ben Franklin in a public house somewhere and offer to buy the next few rounds in return for his patience.
"Mr. Franklin, I’ve done some studying and come to realize that while our ordinary Base 10 numbering system is well suited for most number reckoning, that other numbering systems might have their own specialized uses.
"In particular, good sir, the simplest number system turns out to be Base 2. As you may easily surmise, it uses only the two digits 0 and 1. It is, mind you, a legitimate number system, for all its simplicity. I’ve tested it, and it can be used for all mathematical operations where we are accustomed to using Base 10. Its main disadvantage is its awkward length: it takes roughly 7 digits to express a value we would ordinarily express with 3 digits. This disadvantage might, however, work to advantage in special circumstances. A random pair of eyes seeing such a seemingly random series of zeros and ones would be hard pressed to remember them accurately for any length of time, even if they were so blatently expressed as zeros and ones.
"I trust, good sir, that you can imagine special circumstances of which I speak. I further trust that you can imagine that the two digits are so simple as to be expressed in any medium imaginable. In effect, there is only one digit, with the other digit consisting of a blank space – or anything else – where the one digit would otherwise be expected. Say, the one digit might consist of a hole punched in a piece of paper, and the zero digit would consist of, well, no hole punched where there might otherwise be one.
“With that, I see the hour is late. May I request to be excused? If you wish to discuss this further, I’ve taken a room at the __ boarding house. I would enjoy speaking with you once again.”
Note to self: check the math first. It takes 10 binary digits to express the value 999.
Ben Franklin: Interesting, to be sure. 
I believe that Monsieur Bouchon created a loom where the pattern is controlled by holes in a long sheet of paper some forty or fifty years ago. However, your system seems to be decidedly useless for recording numbers.
(You seem to be thinking that binary led to the development of computers, but it was the other way round. The development of computers led to the use of binary.)
How about a globe? And a map of the solar system, while you’re at it.
Okay. I suppose my line of thought was to wonder if some interest in improved, say, military ciphers would lead to earlier greater interest in the binary system, which might in turn lead to earlier development of machines (even electrical devices) to use the binary system.
Well, heck, there goes that idea for rewriting history.
I can but not with the twirl in my hands, I use a fire bow [stick, bootlace] a palm stone [small rounded stone with enough of a dish in it to keep the top of the stick in it under pressure] and a flattish hunk of wood with a notch chopped into it, a handful of tinder [fluff from milkweed pod, pine straw, well dried brown and crunchy leaves, paper, corn chips] and a selection of wood ranging from small twigs up to wrist diameter hunks of well dried squaw wood. I can also use flint and steel, but I am more inclined to have shoelace available when out hobbling through the woods. In a pinch, I can disassemble my watchband [paracord, and a plastic buckle, that also oddly enough has a hidden handcuff key as part of the buckle] to get around 3 meters of paracord. I can also take my glasses off on a nice sunny day and do the focused sunlight trick.
combination of Boy Scouts and Army SERE training =)
Like I said I’m no expert in primitive fire making…if there is some fancy technique that literally can convert a few twigs in the hand into a roaring flame in less than a minute I’d very interested in seeing it. Now techniques like the fire bow I am familiar with (never used it myself, and it’s always looked difficult) and have seen used, but it certainly takes more than a minute just to setup and get the appropriate pieces together etc.
The focusing lens method is a good mention and I have even started fires that way myself in my misspent youth, but the original claim of a couple twigs and less than a minute is the headscratcher to me.
I was sort of hoping for a exchange of fantasies-- I mean, I’m thinking that if we could take it as a given that I’d invented time travel, we could at least allow for the possibility that I can re-engineer solar panels either not to require semi-conductors, or figure out a way to make a facsimile from available materials in 1881-- the first grid was established in NYC in 1882, nearly a century after interchangeable parts, and coal-generated power had been around only a few years, albeit, coal as a fuel for heat was centuries old (the expression “carry coals” is already an adage in Shakespeare’s time, and appears in Romeo & Juliet, so I have no illusions about eliminating the use of any fossil fuel, ever).
But it’s OK-- I have found this discussion fascinating. I love thread drift, and I’m glad a mod didn’t step in to try to get this “back on track.”
Keep going. If this thread is about salsa recipes by Sunday, my heart will sing.
I thought ‘a modern invention’ meant existing modern technology, but if we can take fantasy technology back in time, I vote for a Star Trek replicator. 
It could be used to create salsa*, which I’m sure would be a great hit at the court of Henry VIII. He would then launch expeditions to Mexico, and would marry a Mexican princess as his fifth wife. And then what would happen?
*But what would be the best recipe?
Well, I kinda meant “existing technology, with adaptations, so it could work in the time to which it was taken,” not something that didn’t exist at all; I was sort of thinking along the lines of trying to avoid a current problem, or solve an historical one.
Hey, if salsa can save Katherine Howard’s life, I guess that works, though.
Like I said, I don’t really care where the thread ends up (not entirely true-- I don’t want a liberal/conservative stand-off over abortion-- we already have too many).
Mumble mumble Bessemer Method mumble mumble…
Not really serious about the Bessemer Method specifically, I’d do a little research and take back technology or ideas and knowledge related to the smelting of ores. Using Bessemer as an example, I’d have to figure out if it’s even possible to use it in the iron age first, using what is or might be avaliable during that time and in that location etc.
There’s evidence of the use of surprisingly modern iron and even steel production in Africa, using pretty primitive technology
The situation may be different for nails that have been:
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made with 14th-century metallurgy (around the time when “dead as a doornail” was believed to have come into use), and
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been through a long, hot fire (to burn them out of whatever structure they were used in).
I’m speculating, but these seem like things that could potentially render the metal prone to breakage when attempting to straighten them.
But I’d think that being through a long, hot fire would actually make the nails easier to bend. Nails that were hammered in and bent would be expected to be work-hardened by the experience, with the creation of many breaks and defects (try bending a paper clip back and forth a few times to feel the effect. You can get it to easily break at the bend).
But nails that have been extensively heated for a long time will have been annealed, removing the effect of work-hardening, and making easier to bend without breaking. In fact, you might have the opposite result, with the nails becoming too soft to keep a sharp point, and making them more likely to bend when hammered. But you can treat the nails to recover their strength.
Reminds me of a short story (in Analog magazine in 1990, apparently), called “Ben Frankin’s Laser”, in which the time traveler needed to speed up technological advancement, so he taught Ben Franklin a whole bunch of modern physics, with a particular emphasis on Maxwell’s equations, and showed a few simple experiments to show that the physics were valid.
Then it was just matter of letting one of the smartest people in history get on with it.
This would probably be the best way to effect change. Lots of math and physical theories were very hard to discover, but are relatively easy to teach, and are also relatively east to show are true. I remember covering the work that Albert Einstein won a Nobel Prize for in second year of my undergrad degree. You could save decades or centuries of development if you just gave lessons to some key people in history.
That’s asking for trouble. Read L. Sprague de Camp’s story Aristotle and the Gun, which was published in Astounding (which later became Analog) in February 1958
Yeah, but ‘solar panels that don’t use semiconductors’ aren’t a thing. It’s kind of like saying you’d take back the transistor, but make it out of wood so they could manufacture their own.
A solar panel IS a semiconductor. It works similarly to a diode in that one layer is ‘doped’ with a chemical like phosphorous that has extra electrons floating around thensilicon lattice, and a bottom layer that is doped with a chemical like boron that has fewer electrons. When sunlight hits thr top layer the energy frees the electrons to flow from the top layer to the bottom, creating an electric current.
This tech is far beyond anything that could be made before the 20th century. For example, you need high purity silicon, and you need to dope it using very purified chemicals and high temperatures (600-800 degrees). It all has to be controlled very precisely.
If you really want solar power in the past, you’d be better off with solar thermal. Use a bunch of concentrators to heat a vessel full of water to generate steam, then use the mechanical energy to do work. But even if you gave that to people of the past, the only part they’d likely care about would be the steam turbine. It would be much easier to just create the steam by burning coal.
My new candidate for 'technology to to,take to the past would be a handful of books:
Those books would probably be far more useful in advancing tech in the past.