?? Who cares about ?? Circumference divided by *diameter*? A circle is defined by its *radius*, not its diameter! Behold ? (tau), at least TWICE as good as ?: http://tauday.com/
Uh... it can have as many magnets as you want... but I won't be standing under any windows that thing is cleaning... (specially when the owner is placing it on the window or taking it away...)
Behold ? (tau), at least TWICE as good as ?: http://tauday.com/