---
title: 'The Liquid Hammer Toy You Can''t Buy'
source: 'https://youtube.com/watch?v=RWbCC6f_k1c'
video_id: 'RWbCC6f_k1c'
date: 2026-07-01
duration_sec: 899
---

# The Liquid Hammer Toy You Can't Buy

> Source: [The Liquid Hammer Toy You Can't Buy](https://youtube.com/watch?v=RWbCC6f_k1c)

## Summary

This video explores the physics behind a Victorian curiosity called a water hammer, a sealed glass tube containing only water and a near-vacuum, which produces a distinct 'clack' sound and a faint flash of light when shaken. The creator investigates the roles of cavitation, incompressibility, and vacuum in generating these effects.

### Key Points

- **The Water Hammer Toy** [00:00] — A water hammer is a sealed glass tube containing only water and a near-vacuum. Shaking it produces a unique 'clack' sound and a flash of light at the bottom, unlike a similar tube containing air which sloshes.
- **Physics of the Clack Sound** [01:54] — When the tube is slammed down, water keeps moving due to momentum. Without air to slow or break it up, it slams into the bottom unimpeded, creating a clap-like sound. Slow-motion reveals cavitation bubbles and rebounds.
- **Comparison with Rain** [02:42] — The slosh tube containing air breaks water into smaller droplets, similar to how air limits raindrop size (max ~8mm) by breaking up larger drops. The slosh sound resembles rain, while the vacuum tube sounds like clapping.
- **Water Hammer in Plumbing** [03:43] — Water hammer in domestic plumbing occurs when a tap closes, forcing incompressible water to stop suddenly, causing a bang. The video links to Practical Engineering's coverage of this phenomenon.
- **DIY Water Hammer Experiment** [04:20] — A bottle half-filled with water struck with a mallet can also create a cavitation bubble and clack sound. However, the combined weight of water and atmospheric pressure can break the bottle, unlike the sealed tube.
- **Failed Jet Cutter Attempt** [05:31] — An attempt to create a powerful jet from a hole in the bottle's bottom failed because the bottle broke before any jet could form. The creator notes cavitation bubbles are not useful for cutting.
- **Ultrasonic Mist Makers** [06:13] — Ultrasonic mist makers use high-frequency vibrations (piezoelectric speaker) to create tiny ripples on water, breaking off into droplets. Cavitation bubbles collapse at the speaker surface, focusing energy to launch the mist.
- **Brighter Flash with Phosphoric Acid** [08:40] — A friend's tube uses phosphoric acid (lower vapor pressure) and a constriction to accelerate the liquid. This yields a more energetic impact and a visible flash due to sonoluminescence.
- **Capturing the Flash with Full Spectrum Camera** [10:16] — The flash is mostly ultraviolet (UV), invisible to humans and standard cameras. A full spectrum camera (without UV-blocking filter) captures a small extra UV range, making the flash visible.
- **House Pipe Sounds and KiwiCo Sponsorship** [12:53] — A separate sound in house pipes (whistling) is caused by vortex shedding. The video includes a segment promoting KiwiCo subscription boxes for STEM projects.

### Conclusion

The water hammer toy demonstrates fascinating physics of vacuum, cavitation, and sonoluminescence. The flash, mostly UV, can be captured with a full spectrum camera, revealing a hidden world of light.

## Transcript

Interesting. Interestingly, you can make that blink sound on your own with a single glass and nothing inside but pure water. And all you have to do is shake it like this.
Like this. The glass needs to be a special shape. Though if you do get the shape right, not only do you get that clack sound, you also get a flash of light at the bottom there.
The camera isn't picking it up and actually I can't see it with my eyes either. But I believe it is possible to film that flash using this weird camera that makes me look a bit like an alien.
I've been trying to get hold of one of these things for so long. It's an old Victorian curiosity called a water hammer. There's a company in America that sells them but they don't ship to the UK so I've been looking for someone to make one for me. Eventually I found Terry Adams
and actually I got her to make two. The reason I got her to make two is, well, listen to this one. That makes the standard sloshing sound that you would expect. But this one makes that weird, smacking sound. And interestingly, they both contain only water. So what's the
difference? Well, this one only contains water in the casual sense. Like this glass doesn't contain any sugar. It doesn't contain any flavourings. It only contains water. But if you
wanted to be pedantic, you could say, well, it also contains air. And that's true for this one. It contains water and it contains air. Whereas this only contains water in the
strict sense. You've got water here and here it's a vacuum. It's not a perfect vacuum because some of the water will evaporate so you've got water vapour in there. It's a tiny amount.
It's very close to a vacuum. What it means is, when you slam the tube down and suddenly stop, the glass stops moving but the water keeps going because it has momentum. And crucially there's no air in the way to slow it down or break it up. So it races towards the end
of the glass fire and slams into the bottom unimpeded, making a clap sound. A bit like clapping your hands together. But actually there's a bit more going on which I didn't realise until I filmed it in slow motion. The water seems to bounce off the glass a little bit. So like
there's a rebound. And so a second gap appears briefly after the first impact. That's called a cavitation bubble. You even get a second bounce which is cool. And look here, on the rebounds, any cavities that happen to be floating around, expand and collapse. So strange. Interestingly,
that slosh sound you get with the vial that also has water in it is related to why raindrops have a maximum size with the slosh tube as I'm calling it. The air seems to push the water
to one side and so it sort of sloshes into the bottom of the vial. But in general, as everything's moving around in there, the air breaks up larger bodies of water into smaller droplets. And a similar thing happens when water falls out of the sky. Droplets grow in size when they collide and merge
but to see what happens when they fall, I've tried to recreate a kind of terminal velocity scenario here with a pipette and a hairdryer. See how this oversized drop is eventually broken up by the air
pushing into it from below. But below a certain size, surface tension is enough to hold the droplet together against the force from the air. And it turns out the maximum size of a raindrop is about 8mm. But anyway, that's why this sounds more like rain and less like someone clapping.
Interestingly enough, the water hammer toy got its name before that water hammer sound you get in domestic plumbing. Though they are somewhat related. This hammer sound depends on water being
incompressible. If it was squishy, you wouldn't get that sharp sound when it hit the glass. Similarly, when you close a tap or a faucet in your house, all the incompressible water in your pies is forced
to come to a sudden stop and so you get a bang. Practical engineering already did a great video on that kind of water hammer, so I'll leave a link to that in the description. Interestingly, you don't
need a sealed vial to experience this water hammer effect for yourself. All you need is a bottle half of water and a mallet. And if you can hit the top of the bottle hard enough and if the glass moves
downwards fast enough, it actually leaves the water behind, so you end up with what's called a cavitation bubble at the bottom. Wow, yeah. And just like with the water hammer tube, when the water comes down again and hits the bottom of the bottle, it makes that clack sound. Don't actually do it
because if the water hits the bottom of the bottle with too much force, it can actually break the bottle. The reason the water bottle breaks, but this vial doesn't, is because well, all we have here is
the weight of the water pushing back down on the bottom of the vial. But in the case of the bottle, you also have all the atmospheric pressure pushing down on that water as well. So it slams into the
bottom of the bottle, not just with the weight of the water, but with the weight of all the atmosphere above our heads. I didn't know you had a high speed camera. Can I ever go?
A bit of a tangent, but something I always wanted to try. What if when the water was slamming back down onto the bottom of the bottle, there was a tiny hole in the bottom of the bottle? Would you get like an incredibly powerful jet coming out of that hole? Of course, if there was a hole in the bottom of
the bottle from the beginning, then you wouldn't get a vacuum cavitation in the first place because air would just rush in through the hole. So what I've got here is a bit of rubber gently stuck over the hole like a little valve. Let's see what happens. Unfortunately, the bottle still breaks, which is a shame
because I was hoping that maybe we could turn it into some kind of powerful jet cutter or something like that, but I guess cavitation bubbles just aren't useful. What about this? What about those
broken areas? What about mismakers? Actually, ultrasonic mismakers are really cool. What happens in an ultrasonic mismaker is there's a piezoelectric speaker essentially that vibrates millions of times
per second. It's ultrasonic. Those vibrations travel up through the water until they get to the surface. When they reach the surface, they create these ripples. In the same way that this speaker is
creating ripples on the surface of the water here. If you increase the amplitude of the vibrations, then the ripples get taller until they're so tall that they break off into droplets. It's the same principle behind the Chinese spouting bowl. Link in the description for that video.
And you can do a similar thing with a cup of tea and a polystyrene cup. Crucially though, the size of the ripples depend on the frequency of the sound. So lower tones that have longer wavelengths
produce wider ripples. Whereas high-frequency sounds like ultrasound have teeny tiny wavelengths. And so they produce teeny tiny ripples. And so when those teeny tiny ripples break up into drops, the drops are teeny tiny. And so you get a fine mist. So far that has nothing to do with
cavitation. But it turns out you do need cavitation bubbles collapsing at the surface of that piezoelectric speaker if you want to get any mist at all. And that's because the cavitation
bubbles in a roundabout way focus all that sound energy into a small enough point that you can actually launch these tiny droplets. So you can imagine this piezoelectric disc moving up
and down millions of times per second. And that's far too fast for the water to be able to keep up. So when the disc moves up, it pushes the water up. But when it moves back down again, it leaves
the water behind. It creates these vacuum cavitation bubbles. And crucially, these bubbles are being pressed from all sides. So when they collapse, they draw water in from above. But they also draw water
in from the sides. Which means you have this net movement of water upwards. The water is being pushed upwards and then water is being drawn in from the sides to replace it. That creates a mini
fountain above the piezoelectric speaker. This kind of rounded cone shape. And that cone that focuses the wave energy just to the very tip. And only when it's focused in that way, do you have enough
amplitude for the droplets to snap off from those ripples? I mentioned earlier that the flash of light you get at the bottom of this vial is too dim to see. But my friend Andrea Sella has
a slightly different version of this thing. This is the little plank tube that I made. You made that. Yeah. Yeah. You put some good skills. No, I used to have better skills than I do. It has two unique
features that might make the flash visible. And what is got is phosphoric acid. Yeah. And if you look at it, it's clearly not water. It's more viscous than that. Acid has a much lower vapor pressure.
Well, that means there's much less vapor in this part of the tube. It's much closer to a vacuum. Well, that means there's even less impeding the motion of the liquid. So when it hits the glass, it hits harder. The second thing it has is a constriction. Why is there a constriction? Why is it
getting narrower? So the fluid comes in when it comes to momentum, right? Yeah. The tube narrowing sort of accelerates. It's actually the principle behind this pool toy that you used to be able to buy. You can't get them anymore, but look, I 3D printed this miniature version. I mean, you push it into the
water, the water jets out of the top. But anyway, the result is that this tube has a much faster liquid and a much better vacuum. So the impact is a lot more energetic. And the more energetic, the impact
the brighter the flash, which is why it's possible, even with the naked eye, to see this flash. Oh, a face. Oh, yeah. Loads. That's cool. This is the best we managed to get on camera with
the ISO jack right up and the aperture open all the way. It's still very faint, but then I had an idea. I need a full spectrum camera stat. Okay, before I show you the results that we got when we switched out
to this weird sounding camera, where does this flash come from anyway? Well, in Andreas' tube, there's a tiny bit of Zenon gas. So when the cavity collapses, there's a little bit of Zenon being crushed together. And this is happening at supersonic speed. So there's no time for thermal energy to escape.
The Zenon gets so hot that it turns into a plasma. In other words, electrons are torn from the outer shells. So electrons and Zenon ions are whizzing around like crazy. And when an electron flies
near a Zenon ion, it slows down a bit. And when a charged particle like an electron loses energy in this way, it's released as a photon of light. This is called Bremstrahlung radiation and it's the leading
theory for why you get a flash. Okay, but here's the cool part. This is the spectrum of Bremstrahlung radiation. And this is what gave me the idea for how we might be able to get a much better shot of that flash. See how most of the spectrum is in the ultraviolet. Human vision can only see this part
here. So most of it, which is in the ultraviolet, we can't see. And because digital cameras want to mimic human eyes, digital cameras can only see this part too. But here's the thing. The silicon of a
digital camera sensor is actually naturally sensitive to all this spectrum. So camera manufacturers have to add filters that cut out this part. And this part so that things end up looking the same as how
our eyes see things. But if you remove that filter, what you're left with is known as a full spectrum camera. Unfortunately, the glass of the tube actually blocks everything below this wavelength. By switching
to a full spectrum camera, we gain this little bit of spectrum here that we wouldn't have otherwise. It is only a small range of wavelengths that we're adding, but it accounts for a huge amount
of light because the spectrum ramps up into the ultraviolet. And look, there it is. How cool is that? Wow.
That is so good. That is so good. I did not know. You had a full spectrum camera. Can I ever go?
The pipes in our house also make a whistling sound sometimes. Only when we run the hot tap though. My son thought that maybe there was one of those farting valve things in the pipes. If you're wondering what a farting valve is, it's one of these. And if you're wondering why my son is coming up with
funny names for valves, it's because, well, a couple of these turned up in the most recent Kiwi co-create. Kiwi co-crease is a subscription box where your kids get an exciting STEM project in the post every month.
It's playtested like crazy, so there's never any glitches or roadblocks like having to go to the shops for supplies. And the instructions are really clear and the kids just love it. One of the best things about being a parent is watching your kids develop skills, like watching them slowly get into
reading or whatever. And one of the cool things about Kiwi co-crease is we've been watching our kids develop engineering skills like tool use and mechanical intuition, which is amazing to see. And it's something that they really want to do. Like I often find myself saying, sorry kids, you
can't do Kiwi co right now because daddy has to film this one. This is Kiwi co-crease take on the bottle rocket, by the way, and it's cool because it's not just about pressure, it's chemistry in there as well. But after we discovered that valves can fart, we spent hours trying to track down
the source of the sound in our own pipes. In the end, we discovered that the whining sound was caused by vortex shedding. You know, it's like this every month. You think it's just going to be a STEM project in a box, and the next thing you know, you're in the attic with your ear up against the pipe. And I
love it. Go to kiwico.com forward slash Steve Mold and use code Steve Mold to check out to get 50% off your first monthly craves. The link is also in the description. So check out Kiwi co-today. I hope you enjoyed this video. If you did, don't forget to hit subscribe. And the algorithm thinks
you'll enjoy this video next.