My first show outside the Bay Area. I'll be at the USASEF all weekend. I have a 50-minute show on the Carver Stage at 2:00 pm on Saturday and a 20 minute show on the Einstein Stage at 11:30 am on Sunday. See you there
For the last several years Ranjit Bhatnagar has made a different musical instrument each day in February. His Optical Siren from 2012's batch caught me attention.
Inspired by Ranjit's siren, I decided to make a larger one for my show. It ended up being fairly straight forward.
The thing in the back is a cordless drill. I cut a circle, using aviation snips, out of a piece of perforated aluminum plate that I bought from Online Metals -- Aluminum 3003-H14 Perforated Sheet Round Hole 0.063" Thick (0.125" dia. holes) 0.1875" stagger. The edges were a bit ragged, even after sanding, so I covered the edges in a couple of layers of duct tape to cushion.
I had to drill out the center a little to put in a screw.
The detector is a optical diode from DigiKey. I got mine from Radio Shack but they don't sell them anymore. I connected it in series to 1/8 inch mini plug (headphone plug) and a 9 V battery. I plugged headphone plug into a small Radio Shack amplifier. You can see this construction as it is similar to the set up for the Communicating with Light arrangement from the Exploratorium. Remember the photodiode is a diode, and so it will work much better one way than the other. You may have to swap it.
If you have a fluorescent light in the room, you will hear a buzzing right away. The photodiode works as a switch. The more light that hits it, the lower the resistance, letting more current flow from the battery. Fluorescent lights blink on and off hundreds of times a second, so the amplifier is getting hundreds of pulses of electricity a second. These pulses cause the speaker cone to move out and in, making a compression and an expansion in the air. In a chain reaction or domino way, these compressions and expansions move through the air until they get to your ear where it interprets them as sound.
More compressions per secnod are higher pitches and fewer compressions are lower pitches. Bigger compressiosn or expansions are louder sounds.
The photodiode is so sensitive that even the fluctuation in the intensity of an incandescent light bulb will make a tone.
The main idea though is to create the flashes of light using the spinning disk. Start the drill and place the diode near the spinning disk. When the photodiode is near the center, the pitch is lower. When it is near the edge, it is higher. Why?
This is a video that I made with the team at the National Science Foundation.
Has something to say about color theory as well.
I have three upcoming shows as part of the Bay Area Science Festival. Friday is a small intimate show at the Atlas Cafe from 6-7 http://www.bayareascience.org/11/04/science-pub-crawl/. On Sunday (tentatively at 2 pm), I'll be at AT&T Park for Discovery Days http://www.bayareascience.org/11/06/dd-at-att-park/ but first, I'll be at Mad Scientist Nightlife at the California Academy http://www.bayareascience.org/11/03/mad-science-nightlife/.
A few companies sell a magnetic clamp for creating strong magnetic fields. Although the clamps are sturdy and versitile, they are expensive. Often very expensive. I decided to make my own.
I used a Jorgensen wood clamp. The clamps are very strong, and the threaded screw rods allow for very precise adjustment. The magnets are from K&J Magnetics and are 1" x 2" x 0.5" N42 rare earth magnet with (and this is the good thing) counter sunk screws holes. Excellent. Mine took a #10 screw and I used non-magnetic stainless steel, though magnetic steel would have probably been okay. (They are out of stock on this particular magnet, but they expect more and others would work.)
The key is to drill holes in the wooden handle first, then drive in the screws.
The idea was to make a pendulum that would show eddy currents. The bar is made of aluminum an 1/8 inch thick and the plates are made from 6 x 12 x 1/8 inch aluminum as well. I cut the slots on a miter saw but I don't recommend that as it was very dangerous, although it works with a carbide blade. I tried a bunch of ways of attaching the plate to the blade, but opted for the clamp which can be changed out the quickest. All the metal was obtained from Online Metals. They cut to order and for cheap.
I built a copy of David Kardelis's glass breaker. <http://www.personal.psu.edu/ref7/apparatus/2006%20competition/kardelis.htm> It breaks pieces of window glass rather than wine glasses. It has some cool advantages over the traditional wine glass breaking with sound.
- The neatest thing is that it debunks the idea that only crystal wine glasses can be shattered with sound.
- You can see from a distance that the glass is moving and the mode of vibration is really obvious.
- The breaking frequency is about 34 Hz, nearly outside the range of human hearing. Much more comfortable.
The glass breaker has three holes. Two supports are added and a strip of window glass is placed on top them. Sound waves come out of the holes, with the two outside holes being totally out of phase with the center hole. The sound is generated by two speakers that face each other inside of the box.
I broke 3 inch by 24 inch strips of window glass at 33.5 Hz.
I've made a few changes to make glass breaking more reliable.
First, used window is full of micro-fine cracks almost invisible in the glass. These make the glass easy to break. New window glass is much, much harder to break and when it breaks it makes many small pieces. So, when I use new glass, I score it with a glass cutter around the center to help control the breaking.
Second, I bought a good subwoofer amplifier that cost about $100. Mine is a 12 V car amplifier that I run with a computer power supply. You could buy a plate amplifier too. The advantage over a normal receiver is that the output current and voltage have much lower distortion. Subwoofer amps also dissipate heat better. I have burned out a receiver running it for a long time.
Third, after some excellent advice from fellow physics demonstrators on the mailing list TAP-L, I bought an HP 204C frequency generator. It makes beautiful sine waves. Other frequency generators, especially digital ones, often produce sine waves with jaggeties that seem to interfere with motion. Many of the them make obvious high frequency sound. I bought mine on Ebay for a good price of about $50.
With these changes, it breaks glass more reliably than my wine glass breaker without blowing my eardrums out.
Optical illusions sometimes seem like artifacts of the graphics and drawing, but sometimes normal objects can seem strange. Look at the bicycle gears and hub below.
(Click to enlarge.) Weirdly, they don't look like they both will fit on the same hub. To my eyes, the center hole of the left gear set looks smaller than the center hole of the right gear.
They are of course the same size.
And they both fit on the hub.
This is a physical example of a classic optical illusion where the size of the outside affects your opinion of the inside. The Exploratorium has a good Adobe Shockwave implementation of this effect. <http://www.exploratorium.edu/exhibits/changingill/> The third illusion is pretty much the same as this one.
What suprised me was that the illusion was just as strong in person with real objects as with a graphic on a screen.
My school has window with our school name on it in vinyl letters.
In the morning on a sunny day, they cast a shadow on the wall.
The picture is actually in focus, but the shadow is blurry. Notice something interesting about the shadow. The places where there are holes, like the center of the "o" in community or the "B" in bay are actually darker than the shadows of the letters. Weird.
If you put a piece of posterboard closer to the glass, the centers are brighter like you'd expect.
As the board gets farther from the window the holes close up and then get darker.
Microwaves are invisible, so you can't see them inside microwave oven, but their presence can be detected with neon lamps. The changing electromagnetic field from the microwaves will make charged particles move, and so the electrons in the metal legs will move creating current. This current makes the lamps glow. I drilled a grid in a piece of 1/4 inch acrylic and slipped the lamps in. I bought the lamps here, but Tom Senior found a better price here. As the platter turns, the lamps light up showing where the microwaves are the strongest.
My grid is based on another group's work.
Steve Spangler Science http://www.stevespanglerscience.com/experiment/00000076 gives a pretty good description on how to make a Vortex Launcher.
Just after Halloween is the time to buy a fog machine. They get very hard to find by spring and you will need to pay a fair bit more to buy one.
Smaller vortex launchers like the Zero Launcher are pretty cool too. http://www.amazon.com/Zero-Toys-Launcher-BLUE/dp/B000FK5JR4
I've had pretty good luck making glass cups out of bottles through this method. While the break is rarely perfectly straight, it is usually good enough to make a glass out of it.
The comments from the Corning Glass Museum point out that a fine flame works better, and I've used both kinds of torches. The fine flame does seem a bit better, but a plumbers torch works too.
If the bottle has lots of bumps in it and looks irregular, it will probably cut weirdly.
Make your scribe mark straight. The crack seems to follow its directions, so if you scribe at an angle, cut will be at an angle.
I haven't used Dan Rojas's technique since I haven't bought the equipment. His solar oven materials are excellent, so maybe this is a better method too.
There are lots of designs of this, especially ones that use paper plates and suspensions. They all seem much more complicated and finicky for barely any more sound. This http://www.exo.net/~pauld/activities/magnetism/speaker.html from Paul Doherty of the Exploratorium seems to work the best, especially for the novice.
The easist solution is to buy the full thing from Pasco, but at $319 it is expensive. A kit that includes everything but the board and air supply is cheaper at $139 but is still costly but not outrageous, especially if you already own a cordless leaf blower. I really recommend this kit.
Building your own is pretty cool too. This set of plans from Bill Beatty (my hero) explains what to do really well and will set you back less than $50 plus the cordless leaf blower.
However, you can combine both of these sets to get a great design for a bit less money. Make the skirt in Bill's plans by using ballistic nylon like Pasco uses. Buy a yard and a half of 330 or 500 denier cordura nylon, like this stuff. Cut it into a circle a couple of inches larger than your hovercraft disk. Hem a hollow edge all the way around the outside of the circle. Follow Bill's advice for cutting circles out of the center. Thread a piece of stiff wire around the seam. By pulling the wire tight, you can keep the skirt attached to the board.
You will need to seal the skirt along the edge. Staples and silicone sealant will probably work, but heavy staples and heavy tape work well too. A rubber bumper like the one Pasco sells used to be available online, but the company where I saw it has gone out of business. (Thanks Steve for telling me!) I'm looking for a replacement.
Connect an LED to the output of an audio source in such a way that the LED flashes with the source. Then attach an applified speaker to a solar cell and presto you are communicating via light.
The Exploratorium has a series of books called "Snacks" which explain how to make cool science experiments. Unlike some other books, though, the experiments actually work, and also unlike other some other books their books come with correct explanations of the science behind them. Check out a selection at
You can find the books at most big public libraries. See a list at Amazon here.
Oh, full disclosure, I work/volunteer with the Exploratorium, but I've never worked on a Snack book.