This ia a part of the machine built by Metrovicks for Professor Hartree of Manchester University in 1935, and which is now on display at the Museum of Science and Industry in Manchester.
This demonstration solves the differential equations:
dx/dt = y dy/dt = -x which has the obvious solution x = sin(t) y = -cos(t) and if you plot x against y, you will get a circle
To start the movie, use the econtext menu (right click) and set (or unset) "play". Select "rewind" to return to the beginning.
Here is the plot produced in the above demonstration, plus a second run round the circle to illustrate its reproducibility.
It is probably not a good idea to have this movie running at the same time as the other one.