Foucault's Pendulum is named after French Physicist Leon Foucault. It is a simple device conceived as an experiment to demonstrate the rotation of the Earth.
The first thing you need is as much head room as you can get. Although in theory the pendulum could be any size a longer string mean a slow swing which mean less air resistance. Next key is a heavy bob for the pendulum minimizing the effects of air currents. And if you want do the math you will need to know your latitude. The formula for figuring out how many degrees the pendulum will turn in a day is N = 360 sin (Your Latitude). It requires some patience and you will need to make some sort of markings on the floor to provide reference for how much the pendulum moves. As you watch you'll notice the pendulum seem to shift without any force acting on it.
We used an old croquet ball for our bob and Boo's papa was willing to bring in a tall ladder to make use of the vaulted ceilings to maximize head room.