Hi all
Back in college I did an experiment where I needed a temperature gradient in a moving stream of air. My first attempt was to use Ni-chrome wire (the type used in some space heaters) and non-galvanized steal threaded rods. The rods were placed in roughly a V shape (though the bottom was not connected and more like \ / )and the wire was wound around them in a way that they formed horizontal lines of Ni-chrome wire spanning across the V. This involved wrapping the wire around the rods several times before having the wire go to the other rod, wrapping it again on the other rod several times before having it go back again. Maybe making about 4- 5 loops or so around each rod before moving it to the other side. The end product looked something like this \=/. but with many more strands across the V (then the 2 shown here) and also varying length.
In the end this is how it looked (ignore the periods and the 1’s)):
(+) \======/(-)
…1.\====/
…1…\==/
The rods were energized, I don’t recall if it was ac or dc, but was on a variable power switch. The idea is that the shorter wires would heat more than the longer ones in what I assumes was a constant voltage across each rod and the moving air would have a temperature gradient as it moved through it in a wind tunnel. Power was applied to the steal rods, not the Ni-Ch wire via leads and clips.
What happened which is still a head scratcher to me, is that every time I applied power the entire array of horizontal Ni-Chrome wires disintegrated almost instantly. Perhaps I could apply minimal power, I think I could if I remember correctly but that produced no usable heat, but stepping it up caused it to ‘pop’ all at once. I tried a single horizontal wire and it worked fine, glowed red hot as it should and transferred heat to the air stream. But try as I might I could not get the \=/ to work and it always popped.
I ended up doing this experiment differently, but to this day i don’t have an explanation as to why that didn’t work and reaching out to the SD knowledge base for answers. The only thing I can come up with is the windings around each rod acted as an inductor and electromagnet but even then I can’t wrap my head around the mechanism that would pop the array.
What was the gauge of the nichrome wire? Did you tighten them on the rods? None of that helps explain what you experienced though. Nichrome expands as it heats so usually a wire holder has to maintain spring tension on the wire to keep it taut, doesn’t seem necessary in your case unless the wires stretched sufficiently to touch each other. What kind of power did you provide? I have used nichrome foam cutters that worked fine with an old style light dimmer varying voltage from an ordinary AC line. Possibly by the time you could get the longest wire which was first in the chain to glow you were providing too much voltage to the other wires. In general, the type of power supply used would be significant, but it was likely just a variable AC transformer connected to 120V AC.
He’s using the steel rods as conductors. They were probably heating up also.
He’s using more than just 3 wires, so creating a huge resistive load. Maybe the steel rods got hot enough to deform, extending in opposite directions, and snapping the wires.
Quite the opposite, surely? If they were in parallel as his description seems to indicate - the total resistance would be at or under the smallest resistance (the shortest wire length). I suspect it was the total current that resulted that had the bad effect, but I’m not messing about with Ohm’s Law calcs at suppertime…
Melting point of Nichrome is about 2500F. Since the resistance at that temperature becomes non-linear, lets assume we want to get to 2000F .
So if we have a 20 gauge (fairly thick) Nichrome wire and the lowest run of the wire (the bottomost run of the V ) is 1 inch from one rod to the other, then you need around 1 V to get to 2000F (the current in this case is around 17.5 amps). 110V will melt the rung.
If you insist on not coiling the nichrome wire, then you will need the lowest (and shortest) rung on your V to be about 10 ft long. So a 10ft long 20 gauge nichrome wire would accept 110V on its ends and heat up to round about 2000F (not melt). Under these conditions, it will consume about 2 kW and 18 amps.
BTW : I did the same mistake when I was in 6th grade and learnt about electromagnets. I just took a wire and wrapped it around a rod and connected it to the main lines (we had 220V back in India). Had good fireworks - and a neighbor taught me how to fix fuses (we didn’t have breakers back then). The neighbor also taught me how to make a good electromagnet by taking me to a shop that did motor coil rewinding. Made lots of fun electromagnets / induction heaters.
You are correct that the measured resistance across the rods should be minimal, so it’s not a huge resistive load but a huge current draw. I am not at all sure what the current draw is in that circumstance but it would be high. I think more likely to blow a fuse than cause wires to snap.
Exactly. The wires are in parallel, but different lengths. Therefore, much more current will go through the shortest leg. If you put in enough current to heat the longest wire, the short one will get too much and burn out. Then even more current will flow through the other legs, and they would burn out in sequence. Total current would decrease as each wire fails, but the current through the remaining wires would increase.
I’ll bet if the ‘explosion’ had been recorded with a high speed camera, it would have shown a cascading failure from botton to top.
If the nichrone wire is thin enough, it IS the fuse.
Just saw your post, yes, they are short wires and your calculations make sense based on my experience with cutting wire voltages. So if he provides enough voltage to make the longest wire glow it will burn out all the shorter ones. Good thinking.
Kanicbird was microns away from creating a perpetual motion device of the second order! Clearly, the Technocracy sabotage the invention before he knew what he had. Sigh and you call yourselves scientists.
Note- The above is shtick. I waited til a serious answer was posted and don’t think it wasn’t rough on me.
Not sure, though I may still have it somewhere. It was thin, maybe 20-24 (edit more like 36 now that I look at some other 24 gauge wire, it was thin)
Not particularly, just enough to hold them in place.
Don’t recall, it may have been an old style light dimmer, it’s very possible. but it was on some adjustable device
Also just to restate it worked fine if there was only 1 wire across and it happily glowed
Wouldn’t the shortest one glow first? Since there is equal voltage across every wire the shortest one would have the least length to dissipate heat and absorb that voltage.