How to Make a Cartesian Diver

Making a Diver

Grab a clean, empty 2 liter (0.5 US gal) soda bottle. This will be the "ocean" your diver dives in. It doesn't matter what is on the outside, so long as the inside is clean.

Fill the bottle completely to the top, so that it is almost overflowing. You want as much water as you can get in.

Use an eyedropper for you diver, or create your own with household objects. An eyedropper is a natural Cartesian diver. All you need to do is fill it up roughly 1 4 {\displaystyle {\frac {1}{4}}} {\frac {1}{4}} of the way with water. However, in a pinch, you have other options for your diver that don't need to be filled with water: Find a pen cap, one that has no holes in it. Place a pea-sized bit of modeling clay around the bottom part, the stick coming off the cap. Leave the opening at the bottom of the cap uninhibited. Cut a bendy-straw in half 2-3 inches below the bend. Roll some clay between your hands to make a snake about the width of your straw. Things on the clay in one end of the straw to stop it up, then wrap the rest around the other side of the straw so it stays in a U-shape, with the bend on top. Packets of ketchup or soy sauce work as well, as do unopened mini-candies, like Milk Ways. To check if the foods will work, first place them in a bowl of water. They should barely float.

Place your diver in the bottle and seal the top tightly. Remember, you want the water level to be near the very tip so that there is not a lot of air in the bottle.

Gently squeeze the bottle to make the diver drop. If you squeeze the sides of the bottle, the diver should plummet to the bottom. Try to find the pressure needed to make it stay in one place. You could also try and trick your friends by squeezing as gently as possible, so it looks like you're doing it by magic.

Troubleshoot the diver if it is not working. Place the diver into a normal bowl of water. It should just barely float, with the air bubble (the top of the dropper, the bend in the straw, etc.) just above the water. Make sure that the bottom is sealed for homemade divers, and add a little more or less water to the eyedropper to get the right balance.

Understanding the Diver

Think about density to explain why the "magic" diver floats at first. The Cartesian diver was invented by Renee Descartes to explain properties of gasses. But it only makes sense when you remember how density works. In short, objects only float in water if they are less dense than water. Air is much less dense, so your airy diver starts at the top. Density is basically how heavy something is per square inch. A box filled with feathers, for example, is much lighter than the same box filled with metal. Since they are the same sizes, but different weights, you'd say the metal box is much more "dense."

Increase density when you squeeze the bottle. When you squeeze the bottle, none of the water escapes. But the area it is in gets smaller, so it pushes together against the air bubbles nearby, compressing them into smaller packets of air. But this is like squeezing a sleeping bag together -- it gets smaller, but it still weighs the same amount. The air, thus, gets denser.

Watch as the denser air inside your diver causes it to sink. Squeeze the bottle enough and, eventually, the size of the air inside your diver gets so small that is it denser than the water surrounding it. This causes the diver to sink because it has too much density to keep floating.

Think about practical applications of the diver. The obvious parallel is to scuba divers, who wear heavy metal belts to help them sink. But they also have "buoyancy compensators," small bags they can fill with low-density oxygen from the tank to help them surface again. Submarines have ballast tanks that are filled with water to help them dive. When they need to go up, air is pumped in to push the water out, bringing the sub back to the surface.

Pushing the Experiment Further

Use salt water and predict if there is going to be a change. Do you need more water in your eye-dropper? Does it take more pressure to make the diver drop? Predict what will happen in advance. Then make two bottles and compare the two. Hint: Salt water is more dense than fresh water.

Don't fill the bottle all the way and test again. Leave some air in the top of the bottle and then give it another shot. How much can you take out and still keep the diver working? Any? Half? None? Make a prediction first using your knowledge of the divers. Hint: When you squeeze the bottle, you're compressing everything, not just the air in the diver.

Try a different sized bottle. Use a small 1-litre personal soda, then try it with a gallon jug. Do you have to squeeze any harder to make the diver work? Hint: Is it easier or harder to compress a big metal spring compared to a small one?

Try changing the temperature of the water. What happens if you use cold water? What about hot? Make sure it is not so hot it melts the plastic, but sticking it in the fridge for an hour or microwaving for a minute or so should do the trick. Hint: Warmer water has a lower density than cooler water.