Airy Points of a Meter Bar

If a meter bar of length l and of rectangular cross-section is to be compared with a line standard of typically H cross-section, then it is necessary to support the rectangular bar in such a way that the distance between the engravings will not be altered by deflections due to the weight of the bar. This is achieved by supporting the bar on two half-circular cylinders located at the Airy Points which are, actually, two parallel, symmetrically spaced lines separated by the distance l/3. This problem is both interesting in itself and suitable for use in an undergraduate course in mechanics. It is presented here for its pedagogical value since it is apparently unknown to most professors of physics of the present generation.