A client saw this YouTube video of Pythagoras theory demonstrated using liquid and asked if I could make a version for them
Seemed simple enough. The client has a laser cutter and something we could reproduce would be best so after making a test one they could knock them out cheaply, one per classroom or even learning table.
I suggested we use 25mm and 50mm polystyrene balls in flat trays so you could demonstrate the theory with two sizes
I started by trying to work out certain basics like;
1: how does it work? Well the liquid runs out of two holes at the corner of the big square linking to the smaller squares. This is disguised by that big stick on yellow triangle so you cant see the holes which have to be big enough for the liquid to drain fairly quickly under gravity
2: What's the liquid? Well I guessed it was distilled water with an added chemical to stop it going off (I have seen similar tricks used in tabletop water features) and some blue dye
3: How do you keep the liquid in? That's when it got tricky. What you see here is a demo, not something you put in a classroom and expect to get knocked about in a storage cupboard. Plus there's got to be at least a kilo of water sloshing around in there so you have some forces to contend with. My best guess is that it is made from 10 mm acrylic, cut with a laser cutter and then joined by capillary action using a Tensol type cement, run into the gaps using a metal needle and a syringe. The bolts we can see hold it on the backboard that rotates, and spreads the load of the water so it doesn't strain the corners and cause leaks. there are similar ones in science museums but they are also fitted to a static board so can't be pulled around too much by users
4: How do you get the liquid in? Again its a guess but I reckon its from a small plug under that yellow triangle accessed from the back
5: Can I make one like this? No. At least not with the time and materials budget I have available. I could make one that would last a month or so but rough handling would quickly cause leaks. I could seal all the edges with 1mm neoprene sheet, sandwiched between the acrylic layers but experience tells me this would still leak eventually as water is 'sneaky' stuff
So I started looking for an alternative way and started with what I could replace the liquid with. Sand seems ideal but must not scratch the inside of the surfaces. Playground sand uses more rounded grains than the standard building 'sharp' sand so would be a reasonable choice but would probably still scratch a little. I also looked at Millefiore glass beads used in enamelling but these are pricey for the amount needed, and too big at 2 mm. I then looked at seeds, most will fall apart or crush when in use, meaning there wouldn't be enough to fill the spaces after a while but Black onion seeds (Nigella) looked promising till I checked on Wikipedia and discovered they are poisonous if eaten in large quantities!
I then looked at sand alternatives and considered salt, too likely to powder down, and Hama beads, too big as they are 5mm and the gap between the acrylic sheets is going to be 5mm. You can buy 2mm beads in bulk but any slight irregularity and they wouldn't fill the spaces effectively. Could increase the gap to 10mm with two layers but that would need an economically unviable amount of 2mm beads. I briefly researched magic sand whose hydrophobic coating would prevent sticking together under moisture, then discovered they stick together MORE when damp than normal sand!
After a while I thought about shot blasting and quickly discovered that glass beads for shot blasting is a safe, inert and CHEAP material. Given the amount of handling these will get, I do not think there will be a problem with the glass powdering as these beads are specifically made to be reused over and over as , impacting on hard surfaces. If stored in a slightly damp cupboard it shouldn't stick together and should be suitable for smaller versions of the Pythagoras model as well
So now I have the design drawn out in CAD, three layers of 5mm acrylic, laser cut, bolted together with M3 pancake flat head bolts and secured with washers and Nylock M3 nuts so students cannot easily open it.
A special section of the centre layer will be removable by undoing a fixing bolt to allow for filling and is cut a little bit bigger to take account of the laser beam width. There are two flats on the circular rim so it can be stood up while filling.
NEXT - Make one! more details later
