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  • This is a classic demonstration which involves what I would call a pickled

  • gherkin, many of you might just call a pickle. The demonstration involves taking

  • two electrically conducting pieces of metal, say forks and sticking them into

  • each end of the gherkin and then applying a high voltage across it.

  • Now, I must say immediately this is potentially an extremely dangerous experiment and

  • one you shouldn't try because if you touch the metal you will get a huge shock.

  • Neil had a historic piece of apparatus which we used. Unfortunately the plastic

  • safety cover had been lost, but because we were using it under controlled

  • conditions, it was safe. I found the piece of equipment about to be thrown out and

  • I wrote a note to Neil saying: 'save it for periodic videos'. Probably we should go

  • straight to watching what happens.

  • The first thing to realize is that when you pickle something you put it into a

  • mixture of salt and vinegar. Salt contains ions, sodium ions, chloride ions

  • and salt solution is electrically conducting but it's not very

  • electrically conducting compared to metal. So what you've got, our two

  • electrical conductors with a resistance in between and when you pass

  • a current through it, the resistor will heat up.

  • What you have here and you have to understand that a gherkin is a biological specimen. And biological

  • specimens are different, so it's not like doing a chemical experiment where the

  • chemical should be exactly reproducible as you can see this gherkin is

  • completely different from the other ones, not even as long. So you can't expect

  • everything to be totally reproducible.

  • What happens is that when you switch on the current, to begin with,

  • you see nothing and then at one of the ends and it's important it's at the end

  • because the maximum resistance appears to be where the fork goes into the

  • Gherkin, because there's less opportunity for the electricity to be conducted.

  • That heats up and it heats up enough for the sodium ions to give out light, in the

  • same way that you may have seen from sodium street lights, now what is

  • interesting is because mains electricity in most countries is so-called

  • alternating current, that it switches from positive to negative, positive to

  • negative. So the polarity switches around going through zero in the UK it's at 50

  • times a second in the States it's at 60 times a second. And if you watched the

  • gherkin under high-speed you can see that the light flashes on and off as the

  • current changes so if you think about the voltage this site is the so-called

  • neutral which stays at zero volts and this side goes up and down to + 200

  • volts down minus 200 volts, up and down, and the first time we saw it

  • there was a strong glow from the neutral end.

  • And we spent quite a lot of time thinking, why should it be at the neutral

  • end, rather than where the voltage is changing. So in the end we thought we'd

  • just try it again. And the next gherkin it went at the other end, so it's clearly chance.

  • If you look at the poor gherkin afterwards, it's a bit shriveled.

  • And the end where the light came out is charred.

  • We've actually got hot enough to burn the gherkin. You may know what temperature

  • gherkins burn, but we didn't.

  • So we decided to use a thermal-imaging camera.

  • The first thing that I noticed is you can see the steam coming out of the

  • gherkin, because the steam is hot and therefore is picked up by the camera,

  • whereas our camera barely noticed it in the dark. You can see that it really is

  • heating up and you can see that the end where the light is coming from gets much

  • hotter than the other end. We then thought that it would be interesting to

  • try and increase the resistance of the middle of the gherkin, so Neil cut out a

  • piece from the middle of the Gherkin like this. Our hypothesis was that if we made

  • the Gherkin narrow at that point, that part of the Gherkin should get hotter.

  • Because it's thinner it will conduct the electricity less well, so it has

  • higher resistance and the heating effect is related to both the current and the

  • resistance. And the current going through the whole gherkin must be the same, but

  • the current density will be higher in this small point and it'll get hotter.

  • And in fact what we found when we looked with the thermal-imaging camera

  • indeed it did get very hot there. Interestingly even where Neil had

  • cut it it didn't get hot enough to give any emission from the sodium, but you may

  • remember we got quite excited why it went at one end rather than the other, and

  • quite by chance in this experiment what happened was that one end lit up and

  • then after a bit the other end lit up. For a few moments both were glowing and

  • then it went to the other end. And you can see really well with the

  • thermal-imaging camera that the forks themselves, the prongs of the fork, get

  • very hot. Because they're conducting away the heat from this hot zone.

  • I have no idea why it changed from one end to the other.

  • I'm not an expert on gherkin structure and anyway it's probably pure chance.

  • But you'd have to do lots and lots more gherkins to be sure, when you look at

  • these experiments and you can see there are videos of this on youtube, you should

  • always think what is the science behind it.

  • They're great demonstrations but so often they're wasted, people just take it

  • as a joke and don't think about the physics or the chemistry behind it.

  • [laughs]

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This is a classic demonstration which involves what I would call a pickled


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B1 中級

電化ピクルス - 動画の周期律表 (Electrified Pickle - Periodic Table of Videos)

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    林宜悉 に公開 2021 年 01 月 14 日