[math-fun] dirac scissors trick + balinese cup trick

(This is the same thing that says you can hold a full cup of coffee in one hand, and without letting go or spilling, turn it around twice and get back in the same position. However, if you turn it around just once, your arm will be in a different, probably awkward, position.)

I read that Dirac illustrated this fact, that a 360 degree rotation isn't the same as a 720 degree one, using strings tied to a pair of scissors, right? My question: where/when did he do it? Thane Plambeck 650 321 4884 office 650 323 4928 fax http://www.plambeck.org