Make a Volta's Hailstorm

Like many demonstrations with static electricity, Volta's hailstorm relies on the fact that opposite charges attract, while like charges repel; and some materials allow charge to flow relatively easily (conductors), while others don't (insulators).

In a Volta's hailstorm, two conducting plates are separated by an insulating air gap, and surrounded by a clear insulating material. In between are lightweight conducting bits of material that are free to move around. By giving the conducting bits a charge and putting them in a strong electric field (both of which are accomplished by charging the plates to a high voltage), they feel an electric force strong enough to overcome gravity, and jump around. The dancing particles are a lot of fun to hear and watch.

For the hailstorm shown below, I chose to reuse a ring cut from a sparkling water bottle, the metal lids of two cookie tins, and some styrofoam microbeads (coated with calligraphy ink).

There are very many options when choosing materials for your Volta's hailstorm. The only restrictions are to have two conducting surfaces, clear insulating walls, and lightweight conducting particles. But the ideas of "insulating" and "conducting" can be very surprising at high voltage.

For something to be "insulating" at the voltages we're using, its resistance should be billions or trillions of Ohms. Things can have a resistance lower than that if even a little water is adsorbed on the surface (from the humidity in the air).[1] This can work to your advantage, or cause problems, depending on whether you want that thing to be conducting. [2]

So, unless it's extremely dry, you can use cardboard, paper, or wood (without shellac or varnish) as your "conducting" plates. But I prefer to use metals, since they're more obviously conducting. It also helps to use things that are fairly smooth, without sharp corners: at these voltages, electric fields near corners and points can be strong enough to split air molecules into ions, making the air conduct the charge away.

But some things that might seem like good conductors have enough of an insulating coating to prevent charges from getting through. I've had good luck with aluminum pie pans, aluminum foil, and lids that look like bare metal. But the metal ends of Pringles® cans, or tuna fish cans, or lids from jars of pickles or spaghetti sauce were less effective. If you find that particles stick to the top plate rather than bouncing off, you have an insulating coating thick enough to weaken the effects you're looking for.

For the lightweight conducting particles, you want something with a low density. If they're damp enough, you can use loosely balled-up bits of paper towels or tissues; cereals like crispy rice, puffed wheat, or small spheres (like Kix^®); or vermiculite.

But I prefer things that conduct well enough in any conditions. Hollow balls of aluminum foil work well and make a nice sound against other pieces of metal, but can be tricky to make and easy to accidentally squash; and if they're solid balls, they're too heavy. Instead, coating low-density small objects with calligraphy ink works well. Calligraphy ink's high carbon content allows it to conduct well enough for these electrostatics applications, even though it isn't conductive enough to use for many circuits. I tried three different types of calligraphy ink; all worked equally well.

The clear walls need to insulate against charges flowing; and at these voltages, some things conduct that you may expect to be insulating. For example, most glass jars and glassware are made of soda-lime glass [3], which strongly attracts water, and doesn't work for some electrostatic applications. Instead, most (but not all) plastics work well. (The borosilicate glass used in chemistry labs, and some types of "crystal" glassware are more expensive insulating options.)

If you want to be sure something insulates or conducts at the voltages you're interested in, my favorite method is to make a sensitive Volta's hailstorm (built as described below), then put the material you're interested in between your hand and the "ground" plate of the hailstorm. If the particles jump, then it conducts; if not, it insulates.[4]

Before you glue together your first hailstorm, you really need to test that your materials and thicknesses will work. So, make the jumping particles as described below, cut a few rings of different heights from a plastic bottle, and use bare metals, before trying to finalize your design. Using the inside of an aluminum pie pan as your bottom plate lets you keep the jumping particles contained as you make adjustments, and gives you something big to clip onto or touch.

[1] Extremely pure water is a good electrical insulator. But most materials get dissolved by water (at least a little bit) producing ions that can flow and form a reasonable conductor. [back]

[2] Even a good insulating material may not work well if it's very thin: sparks can pass through it. [back]

[3] The "soda" is sodium oxide, which, if that were the only additive, would make the glass water-soluble. That's counteracted by the "lime" - calcium oxide often coming from limestone. Source: Wikipedia. [back]

[4] Of course, that doesn't help if you want to make your first one. So, stick with plastic if you want to avoid trial and error. [back]

To jump easily up and down, you want something lightweight and electrically conducting. My favorite is styrofoam - a staple of static electricity demonstrations. If you want it to be as sensitive as possible, the styrofoam should be as small as possible. A great source: microbeads. I got plenty from a pillow, but you can also buy it from craft stores. Larger pieces around a half inch in diameter (cut from styrofoam packing peanuts, or bean bag fill, etc.) also work very well, and make a more dramatic noise.

Of course, styrofoam works so well for so many electrostatics demonstrations because it's a good electrical insulator that doesn't readily absorb moisture. How to make it conducting? Some people coat it with graphite, but I've had very good luck with black calligraphy ink. [5]

I used:

• A disposable cup (I used a fruit cup - cleaned out, of course.)
• Calligraphy ink
• Styrofoam microbeads
• Toothpicks
• (Optional) Tweezers. Choose tweezers you don't mind getting coated with ink or can clean well.
• Aluminum foil

Steps:

• Drip a few drops of calligraphy ink into the cup.
• Drop several styrofoam microbeads into the ink.
• Use a toothpick to roll them around, to ensure they're completely coated with ink.
• Use a toothpick to roll a bead out of the ink (to let most of the excess ink drain off). Use the tweezers or a pair of toothpicks to remove it from the cup.
• Place the bead on the aluminum foil to dry. Roll it around so the ink doesn't pool at the base.
• (Optional) While drying, roll the bead every 30 seconds or so. This helps prevent the dried bead having any exposed sytrofoam (without an ink coating) on the base - which tends to cause the beads to stick to the walls of the chamber.
• Repeat until you have as many beads as you plan to use.

If using larger pieces of styrofoam, the process is very similar. You may get a more even coating of ink by using a paintbrush to get ink into voids or bubbles in the styrofoam, and by "spearing" it with a toothpick and drying it in the air, instead of drying on aluminum foil.

[5] I tried "Winsor & Newton^TM Black Calligaphy Ink for Fountain & Dip Pens", "Dr. Ph. Martin's® Black Star® Waterproof India Ink (Hicarb)", and "Handy Art® Black Velvet Waterproof India Ink". I did not notice any difference between them, for this application. [back]

I used:

• Two metal lids from cookie tins
• A (clean) bottle of Sparkling Ice® brand sparkling water
• A dry-erase marker
• A sharp knife
• Scissors
• Blocks/books/etc to use as spacers
• Optional: orange oil-based cleaner

First make the plastic ring to use as the side walls:

• Decide how far the plates should be separated. Closer together will be more sensitive to a wide range of voltages, but is perhaps less dramatic. And the space should not be too small: otherwise, you risk the particles "shorting out" the plates. At least 3 times the diameter of the jumping particles is usually OK. I chose about 2 inches, since I'd tried a variety of distances (before gluing) and found that this worked and was easy to see. You'll probably have better luck with a smaller gap for your first attempt.
• Remove the label from the bottle, and clean the sides if necessary. Cleaners based on orange oil usually do a good job of removing adhesives.
• Use books (or other flat objects) to give a consistent space between your table and the dry-erase marker. The bottom book should be thick enough to allow you to draw on the smooth part of the bottle (above the bottom rounded section). The top book should be just as thick as your planned thickness for the Hailstorm. Mark the bottle at both levels.
• Use the knife to cut slots into the bottle, then use scissors to gradually trim the plastic away until you match the lines.[6]

Again, many other types of plastic jars and plastic bottles work well; it's best to try it out before committing to gluing it all together. The assembly looks a little cleaner if the plastic is around the same diameter as the metal plates; but your hailstorm will work just as well if the plastic is smaller than the plates.

Then get the metal plates:

• Pop the lids off of the tins.
• Clean them.

If your lids work well, that's all there is to it. As noted above, you could use many other things for the plates. (For example: cut and sand wooden disks, then paint them with calligraphy ink; wrap anything in well-smoothed aluminum foil; etc.) But (as described above) some metals have an insulating coating that reduces the response of the jumping particles to voltages. So, try it out (with a quick non-glued test) before moving on.

[6] You could use the knife directly against the bottle, without ever drawing lines. That gives cuts that are very clean, and there's no wasted plastic, but it takes more time and is a little tricky. If you do, hold the knife as described for the marker, and run the bottle against the knife. Gently press the bottle against the knife, and rotate the bottle. Cut a groove and continue turning the bottle until it is cut completely through. (It's much easier to get a straight and smooth cut by allowing the blade to make several passes over the bottle, rather than trying to make a single cut.) When making the second cut, it helps a lot to stuff the bottle with paper or a piece of a pool noodle, so it doesn't collapse under the pressure from the knife. [back]

You will need:

• All the parts made above
• Glue. Many types will work; I chose hot glue since I had some on hand, and it solidifies quickly.[7]

Steps:

• Use a small amount of glue at 2-4 points to "pin" the plastic ring to one of the lids.
• Run a bead of hot glue around the edge to seal it.
• Wait for the glue to cool.
• Pour the jumping particles into the "cup" you just made.
• "Pin" the remaining lid to the pastic ring with a few more dabs of hot glue.
• Run another bead of hot glue around the edge. Try to avoid having any of the beads touch any glue that seeps through.
• Once the glue cools, your hailstorm is ready to use.

[7] Do not use cyanoacrylate (super) glue: it deposits a white coating on the inside of the clear plastic, and possibly also on the metal caps - especially since it's sealed so the vapors can't escape. [back]

If you use your Volta's Hailstorm in low-humidity conditions, there are a lot of options, as long as both plates have a path for different charges to flow. Some of them you probably already have at home. For example: rest the bottom plate on a conducting table (or a sheet of aluminum foil that you touch with one hand). Then charge the top plate with a comb that you've rubbed through your hair, or a brush that you've run through cat fur, or a balloon that you've rubbed against your hair or a wool sweater, or a piece of PVC pipe that you've rubbed against wool, or clothes that you've just taken out of the dryer (that have a lot of static cling, which tends to happen if you have very different materials tumbling together, like wool and synthetics).

For a more dramatic demonstration, "classic" forms of electrostatic generators, such as a Van de Graaf generator, a Wimshurst machine, or (the easiest to make at home) an electrophorus (shown in the second picture above) also work well - but are less likely for you to have already. (But you can buy some rather inexpensively, such as the Fun Fly Stick™.)

Finally, if you want to make sure it will work in any weather conditions, you can connect the plates to the terminals of a negative ion generator. That's what I did when recording the videos above (since I did it in fairly humid conditions).[8] My method: put a block of styrofoam on a table, followed by an aluminum pie pan. Clip the pie pan to the negative terminal of the negative ion generator - that will be the charged plate. Then, touch the top plate with my finger, or another conducting object to ground the top plate.

[8] I purchased mine (shown in the first figure above) at The Electronic Goldmine. I have no affiliation with them; I just like some of the things they sell. You could also extract the parts you need from an ionizing air purifier, or a racket-style bug zapper (or just run wires to its terminals). [back]

You may have noticed these instructions describing a lot of things that work, and others that don't. How do I know? I tried it myself (and often ran into problems). So don't expect everything to be perfect on your first attempt. You should take advantage of the problems I ran into so you don't repeat them. But you may run into problems that I didn't run into.

If it's not working for you, the most likely issue is a failure to get enough of an electric charge to test. You generally need enough charge that you can hear and/or feel a spark - so this tends to work best in cool, dry months. If you buy a negative ion generator and connect the proper two wires to the two plates, that guarantees you'll have something that works, no matter the weather.

If you can't, make sure the bottom plate is grounded (e.g. resting on a metal pie pan that's touching something big and conducting). Then put wool and synthetic fabrics together in a clothes dryer, and let it tumble in warm air for several minutes. You should notice the wool and synthetic clinging together very strongly when it's done. While they're stuck together, bring them into the same room as the Hailstorm. When you peel them apart, you should hear some powerful shocks. Pass one of the fabrics along the top plate (making sure it can't touch anything connected to the bottom plate). If that fails, you may have another issue.

Another possible issue is using ink that doesn't conduct. In that case, you should see the styrofoam moving around a little in response to the charge - but they'll tend to stick to the charged plates and the walls. If that happens, try a different type of ink, or little hollow balls of aluminum foil. (I prefer to make foil balls by wrapping the thin foil from Hershey's Kisses around the tip of a Q-tip, cutting it to size, sliding it off, and gently rounding it off.)

The last possible issue is using side walls that don't insulate well enough. Here's a way to test, but unfortunately it isn't perfect, and works best in dry conditions: try rubbing the plastic jar, can, or bottle against different types of things. (Tissue paper and wool are the first two things I would try.) If you can get it to "charge up" and attract thin strips of paper, then it should be a good enough insulator. If it doesn't, that still doesn't guarantee it's a bad material - but it's easy enough to try another material just in case.

Why do the particles jump?

I'll describe this for the way I set up my hailstorm with the negative ion generator: the bottom plate being given a negative charge, and the top plate being grounded. The explanation is very similar for any other arrangement.

As mentioned earlier, opposite sign charges attract, and like sign charges repel. The negative ion generator produces negative charges at one terminal. These push against each other, getting as far away from each other as possible. But when they are surrounded by good insulating materials (such as air), they have nowhere to go.

But if a conducting path is made, the charges will flow easily along it, covering the surface of the conductor. In my case, the negative charges flowed down a wire and spread across the pie pan and bottom hailstorm plate. And since the styrofoam beads (with the conducting ink coating) were touching that plate, the negative charge spread across them as well.

So, thanks to negative charges repelling each other, there is an electric force F_E up on each of the beads. But that may not be strong enough to lift any of them up: there's also a gravitational force F_g pulling down. That's a large part of the reason why you don't see the particles flying around when you first give one plate a charge. (Even if it is strong enough to lift some up, the first ones to be lifted up will then push down on the rest making it more difficult for the rest to move up.) When the gravitational force F_g down is stronger than the electric force F_E up, the only reason the beads don't fall downward is the push up from the contact with the bottom plate.[9]

But when you touch a conductor from the top plate to "ground" (anything that is very easy to draw charges out of, or push charges into), you notice the particles madly jumping up and down. Why?

When the top plate is connected to ground, positive charges from ground are attracted, getting as close as they can to the bottom plate; while negative charges in the top plate are repelled, getting as far from the bottom plate as they can. This leaves the top plate with a positive charge - and roughly doubles the electric force (the beads being repelled from the bottom plate, and attracted to the top plate).

Once a negatively-charged bead jumps up to the top plate, its negative charges "see" a path to get much farther away from the bottom plate - leaving that bead positive. At that point, it's repelled down by the top plate, attracted down by the bottom plate, gets a push down from the contact with the top plate [10], and still has gravity pulling down. So, it flies down rapidly until it hits the bottom plate - where the process is similar to when it first lifted off, except it now gets an upward force from contact with the bottom plate during the bounce. [11]

Does this mean the plate that you initially charge has to be on the bottom, and the grounded plate on the top? No. If the situation were reversed, with the top plate being given a negative charge and the bottom plate being grounded, then positive charges from ground would flow to the bottom plate, while negative charges would flow out of that bottom plate. And so the beads (with their conducting ink coating) would get a positive charge - and just as strong an upward force as when you apply the charge to the bottom plate. In fact, every time I try this, it works the same with either plate getting the charge, as long as the other plate is grounded.

Why are smaller particles more sensitive?

"Charges want to spread out. So having more area to spread over should cause bigger jumping particles to have more charge than smaller ones. More charge should give more force, right?"

That thinking as a lot of pieces correct, but it's missing a big part of the story. When considering electric forces on particles, you use the equation F_E = qE where F_E is the electric force on the particle, q is the charge of the particle, and E is the electric field the particle is in. And the electric field between parallel plates is [12] found from V = Ed or E = V/d where V is the voltage across the plates and d is the distance separating the plates. Since we're applying a fixed voltage V across a fixed distance d, the electric field isn't different when using different-sized jumping particles (as long as we use the same separation between the plates). So it's true that, in this case, the electric force F_E goes exactly as the charge q.

And it's also true that the charge q is directly proportional to the area, as long as the shape remains the same - in this case a sphere. And the surface area of a sphere is 4πr².

So, doubling the radius of the sphere should give you an electric force four times as strong. But that's only half the story.

To lift upwards, you need to overcome gravity. And the gravitational force F_g = m g where m is the mass of the particle, and g is the gravitational field (which is close to 9.8 m/s² on most of the earth).

g doesn't change for different size particles, but m certainly does. How? If the density stays the same, then we can use the equation for density to decide: ρ=m/V or m = ρV where ρ is the density, m is the mass, and V is the volume - which for a sphere is 4/3 πr³.

So, if the density stays the same, then doubling the size does give you 2² = four times the electric force upward - but it also gives you 2³ = eight times the gravitational force downward. And so, to get the particles to jump, you'd either need to apply double the voltage, or have the plates separated by half the distance, when using a particle twice as big.[13]

Galileo in his Two New Sciences (published in 1638) had some interesting things to say about similar scaling arguments - for example, why structures that work well on a small scale collapse under their own weight when made bigger, all things being scaled up in proportion. (Compare an Eiffel tower made of toothpicks, vs. full-size made entirely of wood.)

Note: the animation above is heavily inspired by this page by William Beaty, who has some of the clearest explanations of electrical things I've seen online. In my case, I used "red" for "positively charged" and "blue" for "negatively charged" so that "purple" is "electrically neutral."

[9] Called a "Normal force" in physics classes - from the Latin norma for "carpenter's square" which carpenters use to make sure things are perpendicular. So, it's the "perpendicular-to-the-surface force." Never call it the "natural force" or "usual force" - it has a totally different meaning. I teach my students to build an autocorrect function into their brains: "normal" → "perpendicular". [back]

[10] Another Normal Force, this time pointed down. [back]

[11] If the top plate weren't grounded but some beads were still able to lift off the bottom plate, each bead would deposit some negative charge on the top plate. So with each additional bead hitting the top plate, there would be more and more downward force from the top plate onto each bead until the upward force from the bottom plate couldn't overcome the combination of that force and gravity. [back]

[12] To a very good approximation - this equation is only perfect for plates that are infinitely big, and with no other materials around. [back]

[13] This is not perfect for the styrofoam I used: the larger packing peanuts were less dense than the microbeads, mainly because the bigger pieces had larger air bubbles. So, though the smaller particles were cleary more sensitive, it wasn't twice as much. [back]

Volta's Hailstorm is named after Alessandro Volta (1745-1827). Some science demonstrations are named after famous scientists or mathematicians, even if they had no role in their development. I haven't verified this in any of Volta's writings, but I found one fairly early source (Elementary treatise on physics, published 1875) that does attribute it to Volta, saying:

   "Fig. 560 represents an apparatus originally devised by Volta for the purpose of illustrating what he supposed to be the motion of hail between two clouds oppositely electrified. It consists of a tubulated glass shade, with a metal base, on which are some pith balls. The tubulure has a metal cap, through which passes a brass rod, provided with a metallic disc or sphere at the lower end, and at the upper end with a knob, which touches the prime conductor.

   "When the machine is worked, the sphere becoming positively electrified, attracts the light pith balls, which are then immediately repelled, and, having lost their charge of positive electricity, are again attracted, again repelled, and so on, as long as the machine continues to be worked. An amusing modification of this experiment is frequently made by placing between the two plates small pith figures, somewhat loaded at the base. When the machine is worked, the figures execute a regular dance."

Volta certainly did electrostatics experiments, and his theory of hail involved changes in both heat and electrical capacity as a droplet freezes - so it's very plausible that he used this demonstration in his discussions of hail.

In this figure (reproduced above), the upper electrode is a sphere - which works well, although the response is somewhat weaker (requiring a higher voltage) than parallel plates. And unlike most modern Hailstorms, it uses a bell-jar shape - which seems common in several (but not  all ) other  older  examples. This suggests other interesting things that can be done with plastic soda bottles, including one with a variable gap between the plates (another one I built, shown in the second figure).

Some old references describe this demonstration as "dancing pith balls," "the electrical dance," "electric hail," etc. These descriptions are just as good, and may help you find additional references.

Have fun with the wonders of static electricity! And if you found this helpful, let me know!