This is Day 5 of my collection of printable equations. To see the whole collection, go to:http://www.thingiverse.com/thing:227210
For today's equation, I'm revisiting the Archimede's spiral (r = atheta) of day #3 (http://www.thingiverse.com/thing:228425). A variant is Fermat's spiral, r = +/-atheta^0.5 (http://en.wikipedia.org/wiki/Fermat%27s_spiral). In contrast to Archimede's, Fermat's spiral exhibits a flat region near theta = 0 and a continuously decreasing distance between turns. It also, to the best of my knowledge, has no practical application. But it looks cool.
As always, I'm going to continue to post one equation per day until get bored or people lose interest. If there's a particular equation you'd like to see, leave a comment and I'll give it a shot.