# Maths on a Mug #15

Posted on#mathsonamug #maths #newpoundcoin pic.twitter.com/lMQm5uOUyl

— Sean Elvidge (@seanelvidge) November 4, 2016

Keeping with the theme of yesterdays blog post about the new pound coin and shapes of constant diameter here is a nice little result known as Barbier’s theorem. Simply, it says that for any curve of constant diameter (width; \(w\)) the perimeter of that shape is \(\pi\times w\). A nice result. The proof follows from Crofton’s formula.

Interestingly though the result doesn’t hold for higher dimensional shapes – e.g. the surface area for solids of constant width depends on more than the width alone.