## Sunday, November 16, 2014

### The scale of the Universe, in log-polar coordinates

Have you enjoyed the awe-inspiring interactive flash animation by, Cary and Michael Huang The Scale of the Universe? If not, then do it before reading. I enjoyed it a lot, and when few days ago I've seen it again in the G+ stream, an idea appeared in my mind: make a log-polar image of it.

Logarithmic polar coordinate system is a natural way to show things of different scale on the same diagram. Previously I've already used it to make a photographic image of the Earth: Logarithmic zoom at the Palace square, Saint Petersburg, and played with Google Maps: Log-polar system applied to Google Maps, so I already had some python scripts to do the thing. So I took the Python, and after a night of scripting and Gimp'ing got this! (Linked image is 800x18000 pixels)

In case you would like better resolution, here is the full 2000x48000 version.

## Monday, November 10, 2014

### Crystal growing: Rochelle salt

Few more nice crystals grew today, this time it is Rochelle salt, or sodium potassium tartrate tetrahydrate: $$NaK C_4H_6O_6 \cdot 4H_2O.$$

This compound is very well known among crystal growers because of its availability and ability to form big and clear crystals easily, so I won't say much about the growing process. The usual method with slow evaporation was used, with seed crystal suspended on a thin thread. The crystals on the photo grew only in 3 weeks.

### Preparation

Rochelle salt can be prepared from the cream of tartar (unfortunately, it is not sold in my country, so I ordered some on eBay), and baking soda. Some instructions on preparation can be found on the Dom's crystal growing page (UPD: the site is down, but web archive have it), though I've simplified that recipe by using baking soda directly, without the heating step. Simply stir some cream of tartar powder in hot water and add small portions of baking soda until it stops fizzing. The reaction is: $$K H C_4H_6O_6 + NaHCO_3 \rightarrow K Na C_4H_6O_6 + H_2O + CO_2 \uparrow.$$

### Safety

This compound is not toxic and, in fact, edible (but not tasty at all). I would not recommend consuming big quantities though.

### More photos

More photos of these crystals are in the gallery Crystal growing: Rochelle salt.

### Update: piezoelectric effect

Rochelle salt is well known for its property to generate electricity, when mechanical force is applied to the crystal: piezoeffect. Actually, the voltage is so big that it can make flash a light emitting diode, directly connected to the crystal.

I took 2 pieces of aluminum foil and tightly attached them to the crystal sides with thread and adhesive tape. Wires are connected to these foil pieces with another piece of the same tape, and a pair of LEDs in opposite direction is connected to wires.

Then, in the dark room, I can see dim but visible red flashes, when the crystal is hit along the vertical axis (blue arrow on the schematic). To protect crystal from smashing, I put a small wooden piece between the crystal and the "hammer" (actually teaspoon).

So cool. Not that I ever doubted the existence of this effect, but it is really pleasure to observe such effects on a crystals, grown "from scratch", at the corner of the table.