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

330 points9
danielodievich7 hours ago

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.

fredrikholm4 hours ago

> 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.

dunno74563 hours ago

The land of potatoes is Peru.

h0l0cube2 hours ago

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.

recuter36 minutes ago

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.

jve4 hours ago

Touch with their teeth I suppose?

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

krackers8 hours ago

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".

unlikelymordant7 hours ago

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

fenomas7 hours ago

The context can be found here:


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.

quickthrower26 hours ago

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.

whatshisface6 hours ago

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.

aix16 hours ago

> 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.

whatshisface5 hours ago
charcircuit3 hours ago

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

tux8 hours ago

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 @ for recommening this and jasonhansel @ HN for posting it.


adrianN3 hours ago

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.

aequitas3 hours ago

You don't even need the bottle, just make sure the water is in the correct phase:

somenameforme4 hours ago

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!

keepquestioning8 hours ago


briHass8 hours ago

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.

squarefoot2 hours ago

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.

joe__f46 minutes ago

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

ilyt33 minutes ago

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

moring4 hours ago

"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.

Doxin4 hours ago

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"

unnouinceput3 hours ago

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.

jusssi57 minutes ago

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.

userbinator8 hours ago

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

BenFrantzDale8 hours ago

Do tell.

robocat7 hours ago

First example vid I found using those keywords:

emmanueloga_4 hours ago

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.

ilyt32 minutes ago

Oh god please fucking dont

Denvercoder93 hours ago

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.

kimburgess2 hours ago

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.

cfn4 hours ago

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.

unnouinceput3 hours ago

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.

altilunium8 hours ago

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

DeIonizedPlasma8 hours ago

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.

eloff8 hours ago

So does this only work with copper wires then?

IndrekR4 hours ago

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.

NotYourLawyer8 hours ago


amluto8 hours ago
pipeline_peak7 hours ago


Cyberdog5 hours ago

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

myself2487 hours ago

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).

nmeta8 hours ago

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.

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

robocat6 hours ago

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 -

_carbyau_8 hours ago

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.

2-718-281-8282 hours ago

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

WheelsAtLarge4 hours ago

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

kuon4 hours ago

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

Razengan4 hours ago

The potato or the wires?

ale422 hours ago

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...)

Eleison233 hours ago

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.

wink2 hours ago

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 ;)

SicSemperUranus8 hours ago

Don't forget to cook and eat that potato!

etaioinshrdlu8 hours ago

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

ars7 hours ago

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

k_roy7 hours ago

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

wnoise6 hours ago

Cook and eat it.