Improve finish and tolerance on a long thin rod?

[zondar]

RDM stands for :Rollie's Dad's Method and is a technique for measuring alignment on a spindle (lathe or mill) that compensates for a non-ideal test rod. When I said "in the spirit of", I just meant that there might be some way to perform your determination of "g" in such a way that nonuniformities in the invar rod would cancel out. Perhaps by turning the rod 180 degrees and repeating the experiment, then taking the average (for example).

Ah! Well, you are on the right track! Pretty much every primary gravity pendulum is reversible by design. They have two sets of knives (used as pivots), one on each end. By reversing the pendulum and adjusting the balance between the two sides such that the period is the same each way up, a few difficult to determine parameters (mainly: where is the center of oscillation?) falls out of the equations, and then the main remaining things you need to find are the period (hopefully the same each way) and the distance between the knives. Minor irregularities in the rod are best suppressed like all irregularities, but within reason they fall out in that process too.