The Differential



One of those annoying-to-those-who-know-better shortcuts that movies and TV shows sometimes take is the visual trope of throwing a piece of wood (or a rock) at an “electrified fence” and producing an exciting shower of sparks. Typically, one character is just about to touch the fence, only to be pulled back just in time by another character who throws something at the fence to show the first character how they almost bought it.

It looks good — everyone loves a good sparking. In fact, you may have noticed how many action scenes take place in factories that seem mainly to manufacture sparks and steam. You may have noticed how often welders seem to be creating showers of sparks in the background of every action movie.

But this isn’t about our love of sparks.


Apparently, welding goes on in all factories — even abandoned ones — all the time! (This may be some sort of union rule.)

It’s sort of about electricity, but that’s only to get things going. It’s really about differences between things and how that’s where all the fun is. Sameness is dull and boring — nothing ever happens. That’s kind of the definition of sameness (it’s also the definition of boring).

It can be comforting for things to always be the same, and we all need some level of comfort in our lives (especially some times). But ultimately it’s boring when nothing ever, ever changes.

Let me start at the beginning. Back to the electric fence.

Here’s an important thing about electricity: It flows. When it just sits there, nothing happens. In particular, sparks don’t happen. Even throwing a stick at electricity doesn’t make sparks happen unless very specific conditions are met. (And those conditions are almost never met in TV shows and movies.)

Getting electricity to flow is like getting water to flow: it takes pressure. In the former case, electrical pressure; in the latter, water (or physical) pressure.

With water the pressure can be generated by water pumps or it can come from a difference in altitude. In the second case, pressure comes from the combination of gravity and the (considerable) weight of water. Water always flows downhill if it can.

water tower hot cold

You never realized it worked this way, did you! (Mostly because it doesn’t really! Which fits in with today’s general theme.)

A water tower works by using a pump to push the water uphill into the tower. Gravity creates pressure that lets the water flow downhill into the pipe system that distributes it to your tap.

Water sitting in a lake does not flow unless there’s an outlet leading downhill. Likewise water in the water tower doesn’t flow until someone opens a tap providing an outlet.

Electricity acts in analogous fashion. Electrical generators act like pumps; they create electrical pressure that pushes electricity. Batteries act like water towers, they store electricity that is “under pressure” and waiting for someone to “open a tap” allowing it to flow.

[Vocabulary: Electrical pressure is voltage (“volts”); the water equivalent might be pounds per square inch. Flowing electricity is current or amperage (“amps”); the water equivalent might be gallons per minute.]

Here’s the key point. Electricity requires two things to flow: pressure (from a generator or battery) and a path. Water also requires a path, but water flows readily through the air; electricity does not.


Think of these wires as “pipes” for electricity, which flows through the copper. The plastic coating acts as the pipe wall.

Electricity requires a conductor — which is not the leader of a band or trolley car, but some material through which electrons can flow. Most metals make good conductors (especially silver, gold, and copper). Glass and rubber (and dry wood!) are non-conductors. Electricity resists flowing through them.

You can think of electrical conductors as “water pipes” for electrons. They provide a path for current to flow (assuming there is pressure to push it). For electricity to do anything, let alone create sexy sparks, it needs to flow.

Which brings us back to the electrified fence.

You can think of the fence as somewhat like a big water pipe containing water under pressure. Throwing a stick at the fence is like throwing a stick at a water pipe. The same thing happens in both cases: Nothing!

Firstly, the stick doesn’t conduct electricity, so it doesn’t provide a path for the electrons any more than a stick provides a path for water to flow through it.

fence sparks

Remember this? Utter bunk! One: the sparks are nowhere near his hands. Two: no path to ground. Three: anything making sparks that big is likely instantly fatal. In fact, it’s a typical Spielbergism!

More importantly, even if the stick did conduct, there is nowhere for the electricity to go. The stick is touching only the fence! It would be like adding a small capped pipe to a water pipe. A tiny bit of water initially flows into the new pipe, but it has nowhere to go (due to the cap), so it does not flow through that new piece.

The deal with electrified fences is that the electricity has to flow through what touches it — usually to the ground. The ground, generally speaking, has zero electrical pressure, so anything with electrical pressure — if given a conducting path to ground — will discharge through that conductor to the ground.

Think of that as opening a tap in a water system. When a cow brushes against an electric fence, the current flows from the fence, through the cow’s body and legs, to the ground. If cows — and bodies in general — didn’t conduct electricity, electric fences wouldn’t work (if cows wore rubber boots, that would insulate them from the ground).

[Some farm boys have had the, um, shocking experience of taking a piss and accidentally having their stream touch an electric fence. The salts in urine make it a fairly good conductor (pure water does not). The experience is apparently memorable, but not fatal. We can assume the sorts of fences used by movie villains have considerably more juice.]

Will Rogers

I’m thinking… nuf sed!

Something else about electric fences often ignored in movies: They’re built on the ground. Which means they need to be protected from the ground, otherwise they would simply short to ground. Electricity would flow right out of the fence and into the ground.

So electrified fences are immediately identifiable from the insulators (non-conductors) used to protect them from the ground (or to protect the electrified part from the rest of the fence).

A wet, or very green, stick that touches both the ground and the fence simultaneously could allow current to flow. If we assume a villainous fence with lots of juice, maybe, just maybe, the stick heats up and explodes or bursts into flame (which would make that a very serious fence).

We would not expect — at any point — to see a shower of metallic sparks. That’s pure Hollywood. That requires an arc between two bits of metal. And a fair amount of juice. And a complete path for flow.

fence insulators

See the insulators? Dead giveaway it is electrified!

But enough about fences and sparks. The bigger point is that flowing electricity — and flowing water — requires a difference in pressure. For that matter, wind (flowing air) also requires a difference in (air) pressure to occur. And temperature differences do all sort of neat things, such as cook our pizzas and chill our beers.

The real point here is that interesting things happen with differences. This is part of the point of Yin and Yang. Life happens in the tension between opposites (or, at least, different things).

If you are still, or moving at a constant speed, you have no sense of motion. But if you slow down or speed up (or change direction), you do. Again, it’s the differential that makes motion interesting.

Differences are so interesting that, among the science-minded, a Greek letter is dedicated to the idea: Delta (Δ). Delta means difference. In some science fiction stories, acceleration (speeding up) and deceleration (slowing down) are both referred to as “delta-v” (the ‘v’ is for velocity).


A world of Infinite Diversity in Infinite Combination!

Often the best ideas come from working through different ideas, taking the best each has to offer and synthesizing a new idea. The political two-party system is based on that premise. It’s so effective that it may explain, in part, why third parties play such a minor role in politics.

The best work collaborations I’ve experienced — the ones that produced outstanding results — often involved partnerships with people who didn’t always see eye-to-eye with me. In working out the differences, we produced great stuff.

Of course, this requires compromise and working together — a skill our government seems to have discarded.

I’ll leave you with the Mandelbrot Fractal, which I’ve mentioned (more than once) in this blog. The thing about the Mandelbrot is that it’s a contiguous shape — you could straighten it out into a disc (or rectangle or hexagon). All the incredible beauty of that fractal lies at the boundary, between the inside of the shape and its outside.

Indeed, almost all the interesting things in life come from differences. Aren’t “different” people the really interesting ones? They may not always be the ones you like most, but you can’t deny they are interesting. Likewise movies, TV shows, food, music and much, much else in life.


A small portion of Mr. Mandel’s Brot. The black you see is the actual Mandelbrot set. All the interesting bits are near the boundary.

So! Go forth. And be dare to be different!

About Wyrd Smythe

The canonical fool on the hill watching the sunset and the rotation of the planet and thinking what he imagines are large thoughts. View all posts by Wyrd Smythe

10 responses to “The Differential

  • Hariod Brawn

    Thanks for the information Wyrd, and I see you’ve posted this in ‘Basics’. Oh dear, that makes me feel even worse. I think I must be one of those types that Will Rogers says has to pee on the electric fence for themselves. Though actually, I’ve always said:

    There are only two types of people in this world:

    1) Those who can extrapolate from incomplete data.

    • Wyrd Smythe

      I’m sorry! I wasn’t trying to make you feel bad! I don’t quip many quotable quips, but one time inspiration (rather than perspiration) struck, and this came out: “One man’s ‘Duh!’ is another man’s ‘Huh?'” (FWIW, I filed it under Basics more for lack of knowing where it really belonged. One of these days I really need to go over and redo some of these categories.)

      I love your two types! It reminds me of one of mine:

      There are two secrets to success:
      1. Don’t tell people everything you know.

  • Blues Fairy

    When I first saw the topic, I immediately thought of the Jurassic Park scene. I have zilch interest in physics, but boy was this interesting! I think if I had you as a physics teacher in school, things would turn out differently ( my physics teacher made watching grass grow entertaining ! :/ )

    • Wyrd Smythe

      Thanks! I think one key to teaching any subject is to love and know the subject. If nothing else, your enthusiasm can be infectious. And if you know the material well enough, it’s a lot easier trying to explain it. A famous quote (that it turns out no one ever actually said) goes, “If you can’t explain it to your [grandmother | aunt | uncle | four-year-old child] then you don’t really understand it.”

      • Blues Fairy

        It’s usually attributed to Einstein. right? – another sin of regurgitating common knowledge. Perpetuates misinformation dunnit.

      • Wyrd Smythe

        Yep. Old Al. Also commonly attributed to physicist Richard Feynman. Neither of whom said it. No one did, but various people have said things similar. Feynman, for example, once did say that he didn’t understand a topic well enough to lecture on it.

        One of the downsides of the interweb is the Q&A sites where people ask questions and well-meaning people guess at answers (often incorrectly). This misinformation become searchable and turns up in results of people looking for the answer. It’s especially bad when it comes to quotes. Trying to find out who said something — and if they said it at all — becomes a detective’s task.

      • Blues Fairy

        Oh man do I know the drudgery of doing the detective’s work. I recently came across a story on tumblr about some African tribe with extraordinary traditions. i wanted to reblog it but decided to confirm the story as I always do. Turns out it’s as true as the cinderella tale. I was enraged! I ended up reblogging the story (that had been reblogged over 200 000 times at that point) Along with the rebuttal of the story and an explanation. It’s imperative to be inquisitive in our days of digital regurgitation

      • Wyrd Smythe

        As my mom would say: Exactly! So much chaff as to be dismaying and off-putting, but the wheat is so tasty when you can find it that it makes it worthwhile!

  • SelfAwarePatterns

    TV and fiction tropes are an interesting phenomenon. A lot of times we know they’re false, but they’re often so common that we just accept them and lazy script writers capitalize on it. The other day, I was watching an old Six Million Dollar Man episode from the 70’s, and it had one of the characters fall into that old time trope: quicksand, which you almost never see used anymore.

    • Wyrd Smythe

      Good point; it’s been a while since anyone was sinking in quicksand! 🙂

      There is something to be said for the iconography of visual storytelling. One can read icons (such as sinking in quicksand or, for that matter, dealing with electrified fences) as a kind of short-hand for moving the story along. When those icons become so unreal or silly that they detract, then they stop working very well.

      Computer hacking has become a kind of icon. People ascribe a kind of magic to hacking; any hacker good enough (or having good enough gear) can hack into any machine and make it do any thing. That one has always been a problem for me, and as computers and networks become standard story fodder, it’s beginning to really grate.

      The use of computers in general is starting to reflect severe laziness on the part of writers. They’ve become kind of the modern “magical spells.”

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