The trammel is an invention by Archimedes to draw or create ellipses. Solitaire software has created an app called Virtual Trammel which shows several different trammels and how to use them.
There are two trammels and a trammel clock shown in the app. Unfortunately, only two allow you to play with the parts to see how they actually work. This app talks about the Archimedes Trammel which is one you can adjust lengths and move about. Then there are two inverted Trammels, one without the any indication of the shape and one with. These are the same thing, just different pages in the five page instruction.
The fourth page is the Super Trammel which uses three markers to form a triangle. This one can move and is fun to play with. Finally is the trammel clock which is a clock and moves every second. It is interesting to see hand moving in an elliptical pathway.
This last one could be used if you do a short section on Mathematical clocks, this would fit in perfectly and lead into a unit on conic sections. I love finding odd things like this to incorporate history and allow an introduction to a new topic.
This particular app is one I would classify as more of one for unique information. I think I would use it more on my iPad as an introduction rather than putting it on student iPads. Its ok but not the greatest because I'd love to play with the inverted trammel so experience the differences.