I have one of these, given to me by a publisher’s rep. And I was startled one day to see it start dipping on its own. I eventually figured out it was due to the fan exhaust from my laptop. But that led to a little more research, and the discovery of a lovely little 2003 paper from American Journal of Physics.
http://nautilus.fis.uc.pt/personal/mfiolhais/artigosdid/did14.pdf
An even older one that you’ll have ti visit the library for (remember those?)
http://scitation.aip.org/content/aapt/journal/ajp/29/6/10.1119/1.1937801
And a beautiful design for one you can build yourself out of soda straws that is NOT a heat engine. It runs on chemical potential.
http://mpalffy.lci.kent.edu/PAPERS/DBPublished.pdf
I thereafter gave it as a final exam problem in a thermal physics class. It has a very peculiar PV diagram.
