Well, somebunny has to do it.
Paul Krugman today
Today’s Financial Times bears a banner headline on p.1: “US election hangs on a knife edge”. Aside from everything else, surely this gets the cliche wrong: you rest on a knife edge, don’t you? If you try to hang on one, I think you just cut off your fingers.Being an old Rabett, Eli pointed out that
the knife's edge refers to an old fashioned weighing balance where the two pans are supported by a beam and the "knife's edge" is the fulcrum point.If you look at pictures of old balances the knife edge is in the middle.
the knife edge is the little triangular thing supporting the beam at the top of the central pillar or you could dip into the tool chest for some threaded rod and single blade razors. You can find the fancier double pan balances in antique stores or professors' offices (Eli has two).
An interesting version is the Mettler subtractive balance (you can still find some in labs), where the weights were rings that could be lifted off the beam by an arrangement of gears and levers (11). The counterweight (3) was equal to the sum of the weights of all the rings that hung from the beam. The knife edges are shown in blue, one supporting the whole beam and the other the pan and rings (near 7)
So, what do we use today? Electronic scales (this is like telling a gunny sergeant that a rifle is a gun, but sad to say it is true) measure the change in resistance of a strain gauge load cell, accurate, inexpensive (relatively) but not elegant