“You don't have a voltmeter. Do you have a potato?” | Hacker News Reader

“You don't have a voltmeter. Do you have a potato?”

diy.stackexchange.com

464 pointsjasonhansel96 comments

Great potato story. Loosely related story here. My late grandfather in Belarus (the land of potatoes) loved potatoes just like everyone around. Well, he was an electrician at the concrete/brick making factory for entirety of his career, from right after the war until he retired in the late 80ies. The amount of electricity used to fire the kilns for bricks is well known to be huge. He said that they'd see the rats scutter along the power lines on the wall and every once in a while they'd touch the rails wrong and disappear in the scintillating disintegration. Oh, and he let me weld random junk together with his own cobbled together electrical welding kit when I was like 8, oh I loved that so much. I miss him.

Touch with their teeth I suppose?

I was working at some other factory where rats would "cut" data cables occasionally.

If they were uninsulated cables (like overhead powerlines) they might have just bridged two of them.

> he let me weld random junk together with his own cobbled together electrical welding kit when I was like 8, oh I loved that so much. I miss him.

I think a lot of us have similar experiences with European (grand)parents born around the era of the two wars. Honestly think that them being born in horrible times yet living during (comparatively) great times helped form a generation of people that stood out.

I miss them.

The land of potatoes is Peru.

Not sure why the downvotes. A bit pedantic and distracting from a good story, perhaps, but accurate.

> The potato was originally believed to have been domesticated by Native Americans independently in multiple locations, but later genetic studies traced a single origin, in the area of present-day southern Peru and extreme northwestern Bolivia.

Yes and it arrived in Europe by way of the conquistadors.

However, it has evolved a bit since. I could be wrong, but much like with bananas the OG version would surprise you in terms of taste/looks.

As far as I understand it, Peru already had a lot of highly cultivated potato crops. They have over 4000 different kinds and had sophisticated methods for preservation[0].

[0] https://en.wikipedia.org/wiki/Chu%C3%B1o

Ah I thought this would be a new take on that apocryphal bayesian story

>An engineer draws a random sample of electron tubes and measures their voltage. The measurements range from 75 to 99 volts. A statistician computes the sample mean and a confidence interval for the true mean. Later the statistician discovers that the voltmeter reads only as far as 100, so the population appears to be 'censored'. This necessitates a new analysis, if the statistician is orthodox. However, the engineer says he has another meter reading to 1000 volts, which he would have used if any voltage had been over 100. This is a relief to the statistician, because it means the population was effectively uncensored after all. But, the next day the engineer informs the statistician that this second meter was not working at the time of the measuring. The statistician ascertains that the engineer would not have held up the measurements until the meter was fixed, and informs him that new measurements are required. The engineer is astounded. "Next you'll be asking about my oscilloscope".

Can you explain the story? I understand an oscilloscope is for looking at signals, but i dont understand the significance

The context can be found here:

> https://en.wikipedia.org/wiki/Likelihood_principle#The_voltm...

Specifically if you read the "Example 2" section above that blurb, and then read the "throwback" bit at the end after the voltmeter story, it makes more sense.

What is a good book for someone comfortable with maths (but very out of practice) to get up to speed on the topic. Not being scared of not understanding stuff and needing to learn prerequisites (so doesn’t need to be too easy). Because the wikipedia article isn’t really for learning but it does give a flavour. And it sounds fascinating, sort of like a monte hall problem to me.

Oscilloscopes can also be used to measure voltages, often in excess of 100V, so in the spirit of those extremely unlikely "explanations" that show up in the explainxkcd tables, I offer this.

PS this story doesn't make sense because you can tell when a voltmeter is out of range.

> PS this story doesn't make sense because you can tell when a voltmeter is out of range.

It's also a bit sloppy with the bounds. To me, "reads only as far as 100" includes 100V, whereas the measured voltages only go as high as 99V.

While we're complaining, you can't measure the "voltage" of an electron tube out of circuit.

Sure you can, just buy a SMU (source measurement unit, few multimeters cobbled together with 4 quadrant power supply)

I think the point is that the sample readings were in the range of voltmeter, but if some sample crossed 100V it would show as 100V in the voltmeter. So even though no sample crossed 100V, the data is clipped still.

That's not good statistics - just because the precision of measurements falls off sharply at 100V doesn't mean that out-of-range readings aren't valid measurements.

Assume we got one out of range measurement out of 100. We can't say anything about variance of reading if we don't have any prior on variance ie using bayesian statistics. That's the joke that traditional statistician can't figure out the obvious thing.

Why would the population be censored if the voltmeter never read anything above 100 volts?

Sometimes low-tech works best can litterly save your life. This kind of reminds me how you can start a fire with a watter bottle [1].

Good job MTA @ diy.stackexchange.com for recommening this and jasonhansel @ HN for posting it.

[1] https://www.youtube.com/watch?v=QwQJ-3pZfwc

Unbelievable!

That is absolutely amazing, and I will be trying that next barbecue day.

For those who don't know, on a slightly related nugget, you can also use any sort of fat as a lighter fluid as well. Cook up some bacon (for instance), collect the fat/oil from the pan, and next time you're grilling - a few napkins (or other sort of kindling) rubbed around in the bacon fat is more than enough to heat your wood/coal to burning.

Now to set those napkins on fire with a water bottle!

Lighting fire using the sun and a lens is a neat trick. You can also form a lens using ice. However, in most conditions where you need a fire to survive you don't have sufficient sunlight or sufficiently dry fuel for the method, so I'd recommend learning other methods for lighting fire, in particular the method of always carrying a lighter with you when you leave civilization.

You don't even need the bottle, just make sure the water is in the correct phase: https://www.youtube.com/watch?v=thbSSuo1Z00

> litterly

> watter

I remember this from the old 'potato clock' science kits from the 80s. They had a copper cathode and zinc anode that when inserted into a potato (most fruit/vegetables or acidic liquids also worked) generated enough power to run a small LCD clock. In a potato, the copper cathode always left a green spot.

Potato? Who needs a potato? A glass of salt water will suffice.

Just put the wires at some distance in the salt water and look. The wire emitting the blueish fog (chlorine) is the positive, and the one emitting bubbles (hydrogen) is the negative.

I always feel like I could have more chlorine gas in my life

That's how I accidentally learned how to produce chlorine gas as a kid!

Well, only if chlorine is added to the water. It should also emit oxygen.

> salt water

Meaning there's Sodium chloride (NaCl) in solution which not only makes the solution (more) conductive but also in combination with the application of electric current should electrolyze the solution into gaseous O2, H2 and Cl2.

For sure, I missed that.

So Factorio was right, you do get chlorine from electrolyzing brine...

> blueish fog (chlorine) is the positive

Would that not be copper oxide?

Source: did that as a teenager and did not die (trying to synthesize hydrogen as any normally constituted teenager would do), and the copper wires just ended up being entirely corroded until the immerged part disappeared. No hydrogen as I did not seal the apparatus properly. But I had a nice blueish precipitate instead. I absolutely had no idea it could have generated chlorine. I did sleep in the same room as the experiment for a full week. So I guess either my bedroom was well ventilated or it did not produce enough chlorine, or that it was just copper oxide.

My understanding is that usually the chlorine just reacts with the anode (cathode? I can never remember), unless you use an anode that it can't react with (graphite for instance).

Even if it did make chlorine, as long as your power wasn't too high it might not hurt you (much) due to ventilation. It is also very reactive, so it tends to get used up reacting with whatever random crap is near it.

> blueish fog (chlorine)

Chlorine is more yellow/green, so that's unlikely to be chlorine, I'd think.

"Do not allow the bare wires to touch each other from this point on."

What happens if they touch? Note that this is while the adapter is unplugged, before sticking the wires into the potato, so shorting them isn't a concern. I can't really imagine that some kind of contamination is a problem since both wires are (probably) copper, nor would I expect any kind of reaction as long as it is unplugged.

At some point during the following steps you're turning on the power, If they were touching at that time you'd get some real nice sparks and a tripped breaker or blown fuse. To simplify instructions the poster just said don't let em touch after this step as opposed to something like "When power is applied the wires can't touch"

Nah, none of that. Just your charger gets bricked so not knowing which one of the wires are positive and negative going to be the least of your worries from that point on.

That's the theory. I have seen smoke coming out of a charger when the 5v DC side got short circuited due to wire damage. Quickly unplugged obviously, but I assume there was a real possibility of the AC side shorting if things melt inside the charger.

Of course, that's always a possibility.

Well any DC converter worth its salt ought to have short protection, so likely nothing. But power would be turned off for the duration so the experiment wouldn't work.

This reminds me of the old trick to detect the polarity of the ignition coil on an engine, using a pencil.

Do tell.

First example vid I found using those keywords: https://m.youtube.com/watch?v=kXaOP8O4HpI

Which seems to show that you use the pencil graphite as an electrode and spark it to ground. The 'fatness' of the spark determines the polarity.

Written description: https://navyaviation.tpub.com/14329/Figure-6-52-One-Method-O...

...although I haven't found any explanation online why it works. This method has probably been known for over a century, so I suspect it may be in some old physics books.

Can anyone explain what's really happening here? Why green? Why potato?

Copper salts are green. The copper will only give up an electron and make itself available to form salts with the ions in the potato at the positive electrode. This should also work with anything else conductive, you'd probably be able to tell doing this in a glass of salt water provided that the water is still enough.

So does this only work with copper wires then?

Yes.

There are likely other things you could observe by electrolyzing some convenient substance with just about any wire. But be careful — some electrolysis products are pretty nasty. Producing chlorine gas, for example, is straightforward.

If you have a salted potato (rescued from a salad perhaps), I would not do the experiment indoors, nor would I stick my face close to it.

Sheesh. You are not going to make enough of anything this way to hurt a termite, much less a human.

There are ways to kill someone with potatoes, but they don't involve electrolysis.

“There are ways to kill someone with potatoes” with no further explanation than a counter example is probably my favorite thing I’ve read today.

The preferred method seems to be to fry the potatoes in hot grease, then salt the hell out of them. Unsuspecting humans readily gobble these up. With repeated application, death by cardiovascular disease follows within decades.

Apparently potato salad is an almost perfect bacterial growth medium - a few hours can lead to rapid proliferation.

All one needs is two tea bags and some wax paper!

Did someone said about The Great Potato Cannon?

You can easily injure yourself with a 12V power supply, some wire, and some salt water. You can chlorinate an entire swimming pool with a power supply smaller than the average workstation’s and an appropriate electrolytic cell.

You can easily injure yourself with a 12V power supply, some wire, and some salt water

How, without breaking the skin?

I suppose you could burn yourself if you deliberately wrapped the wires around an appendage and immersed it in the salt water. It'd take a lot of current, and whether that would do more harm than just drinking the salt water is questionable.

Think chemical injury, not electrical injury. If you stick a pair of wires in salt water and supply a few volts (12V is more than enough — it’s enough overpotential to get lots of reactions going quickly), you will see:

The cathode reduces H+ to H2. Hydrogen gas is mostly harmless if breathed, but if you collect it, it’s explosive.

Water minus that H+ that turned into hydrogen gas leaves OH- behind. Concentrate that enough and you have a base that will dissolve your skin.

The anode can oxidize water to oxygen, which can be a fire or explosion hazard but is otherwise fine. (A stoichiometric mixture of hydrogen and oxygen is Bad News.) But this is not the major reaction because…

The anode can also oxidize Cl- (the chloride in your salt) to Cl2: chlorine gas. In the absence of considerable care (which you don’t have in this hypothetical potato salad or glass of saltwater), that chlorine gas will bubble right out of your little reaction, where it will end up in your eyes or your airways. And it will dissolve into your tears or the fluids in your airways and chlorinate you. You do not want do this. NIOSH says that chlorine gas is immediately dangerous to life and heath at 10 ppm, and you can easily achieve that in a cup with a little power supply. (Like I said, a hundred watts or so can chlorinate an entire swimming pool. Chlorinating your cornea is trivial in comparison.)

This is a major industrial process, which you should not do in your house:

https://en.m.wikipedia.org/wiki/Chloralkali_process

(Shrug) Unless you deliberately set up an industrial- or large-lab scale electrolysis rig, you will get more chlorine in your face by opening a bottle of bleach. Try it.

Potato

For other metals you can enjoy watching how one wire erodes away and other one stays nice and bright. Does not need to be potato either — any water based conductive medium will do. Even a cup of hard tap water is conductive enough. Distilled/RO/DI water will not conduct reasonably well. You can just add a drop of salt/vinegar/juice/etc to make it more conductive.

Copper salts are a start, but I think I'd prefer mine with copper pepper as well.

In a solution (or potato in this case) running electricity between two points will cause one point to lose material.

So one wire will lose material into the potato.

Assuming the wire is copper (much wire is) it will appear green in similar vein to how the statue of liberty is green.

I too was intrigued by this and did a quick google search.

I came upon this which I think is a similar (if not the same) reaction as is occurring in OP.

https://thehappyscientist.com/content/simple-electrolysis

And in the final paragraph states that, and I'm loosely plagiarizing here, the negative ions created in an electrolysis reaction are attracted to the positive/live terminal*, and in the presence of chlorine (as in salts found in the potato) the copper (wiring) react to form green copper chloride.

Edit: not battery, but positive/live terminal

The way to remember it is that the negative ions of Chlorine Cl- move towards the positive wire electrode.

At the copper wire of the positive electrode the chlorine reacts with the copper to make green hydrated copper(II) chloride - CuCl2·2H2O - https://en.m.wikipedia.org/wiki/Copper(II)_chloride

Others have covered the copper angle. But it doesn't have to be a potato. A bit of paper towel moistened with salty water or saliva would also work, all you need is for current to flow initially (that's the purpose of the salt), and for something to carry the copper ions (the water).

The level of cleverness and resourcefulness is impressive. To the point that it seems magic or makes me suspicious. I guess this is how the layman feels about instructions to delete System32 to add more RAM.

There should be a step somewhere there that says "don't hold potato with hands while it is plugged"

How dangerous is this? Just thinking about working with live wires makes me nervous.

It is a low voltage DC adapter, you should be safe as long you do not put it in your mouth.

The potato or the wires?

Mostly the potato, I'd say... as it will probably contain potentially toxic copper salts after the operation. If the adapter generates 9-12V, it will be pretty unpleasant to have that voltage in your mouth, but probably not dangerous (like licking a 9V battery to check if it's still charged...)

Dry skin is a reasonably effective insulator against the kinds of voltages typically produced by wall warts, i.e. less than about 24V.

But wet skin, especially if there's salt involved, is considerably more conductive. As a kid I had applied a mere 12V to some slightly salty water, when some small object fell into the soup. Thinking "it's just 12V" and thinking it harmless, I went to fish the offending object out with my fingers. This was a rather unforgettably unpleasant experience, I will attest.

So, cautionary tale there - even with low voltage, watch out if your hands are wet!

it's unplugged

Seems to be like the answer is making fun of the question which is pretty easy to answer with very simple and cheap tools.

There was a well known cheap tool retailer that had a voltmeter adequate for this kind of measurement for less than the price of a good coffee. I think they even had a coupon for it, making it practically free for a while.

The best touch in that story is how the poster works around the lack of a voltmeter by assuming the OP possesses a soldering iron and the skills to use it.

I've had a cheap soldering iron in the closet since I moved in and only bought a voltmeter like last year, because I've done my soldering mostly at the local hacker space for the last 10 years, so I find that part pretty relatable ;)

Don't forget to cook and eat that potato!

It might contain an unhealthy amount of copper salts if you're unlucky, I guess.

You don't need to use an entire potato, a small 1 inch/3 cm cube will be fine.

what do you do with the rest of the potato then?

Cook and eat it.

Another neat trick in the same ballpark (but not involving a potato :-p) is to create a 220v to 110v adapter with a "resistance bridge".

Plugging two (hopefully pretty equal resistance) incandescent lights in series, each lamp should have a 110v potential difference... say:

A ----(60W lamp)-- B --(60W lamp)----- C

By plugging A and C "cables" to a 220V electrical outlet, one should be able to grab 110V from either A-B or B-C :-) (assuming both lamps have about the same resistance). Most electronic devices work with a wide range of input voltage any way, so it doesn't have to be extremely precise.

Should work but don't plug the 110v equipment until the lights are totally on. The initial rush current will cause higher voltages for a split second that may damage it.

If the light that is in parallel with the circuit you are trying to power up malfunctions, you going to be in for a nasty surprise. This method, ignoring the fact that you're wasting at least 120W in plus, is also quite unreliable.

You want to be very careful about what kind of equipment you power like this. If it has resistance that's substantial compared to the resistance of the lamps, the voltage divider won't be a neat 50:50 anymore and you'll get more than 110 V.

That's why you use two potato halves instead of light bulbs. You can calibrate the wire distances to create the exact relative resistance for the divider to function to spec with the load.

Oh god please fucking dont

This is a bad idea on multiple levels. Do not do this.

Among the many reasons this is a bad idea: the "hot" resistance of an incandescent bulb is many times the "cold" resistance. For example, [0] finds that a 110V 40W bulb is 24Ω when cold, but 360Ω when hot.

Why does this matter? Let's put two of these bulbs in series, one of them hot and the other cold, and run 220V through them. The total resistance will be 384Ω, so the current will be V/R, or 0.57A. The hot bulb will have IR volts across it, or 206V; the cold bulb will have 14V. Bingo: your voltage divider is way, way off. Even better, putting 200+ volts across a 110V bulb will blow it (power would be IV, or 117W), and you've got either nothing or 220V.

Now, you won't be starting with one bulb hot and the other cold, but the two bulbs won't be exactly equal. The one with the higher resistance will be hotter than the other one; can you be sure that the imbalance won't slowly get larger and larger?

[0]: https://electronics.stackexchange.com/questions/571836/why-d...

Haha so controversial :-p I did not say it was a good idea, just a neat trick.

I actually _had_ to do this once, when I forgot a 220v to 110v adapter for a wireless microphone, for a wedding, in an hard to access location [1], and there was just no time to go get a transformer from anywhere else :-) Luckily, the whole "setup" did not turn into fire for the 30 minutes or so that the mic was needed and the wedding speech could actually happen without anyone needing to scream :-)

1: https://en.wikipedia.org/wiki/Tigre,_Buenos_Aires

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