This plastic compass can be used to draw large circles on glass, plastic, or any other smooth and flat surface. It attaches to the surface with a suction pad, and can be fixed at a certain radius, so that the circle is perfect. It is possible to draw circles with a diameter of over a metre.
Please note that this compass is designed for use either with chalk or with standard (i.e. not thick) marker pens. This is because the mechanism used to attach the drawing instrument to the compass will not accept pens whose barrels are larger than that of a standard Sharpie. The barrel of the drawing instrument must also be thicker than a pencil.