NavList:

Randall is too generous when he attributes the design of the grinding jig to me. When a beam is symmetrically stressed as Randal describes it can be shown (but not by me!) that it forms a circular arc, important when the level is used to measure small angles, but not when it is used simply to set things level. Whatever method was used in Britain to grind vials was in the early 1950's not working well and the National Physical Laboratory was asked to help. The Laboratory devised the jig and a small-angle measuring device to help in their investigation. I learned of the method from someone who had spent a lifetime there working as a technician. Randal is to be congratulated in the way that he has removed all complications to produce something that can be made in (nearly) every back yard. He does not stress the point but the reader will gather that the outer ring bolts must be tightened symmetrically. If the vial is too sensitive on testing, the bolts can be tightened some more - symmetrically.

Using hexagonal end caps are a sensible way to avoid a machining operation. Following assembly, I would suggest that when the pair of flats that give the best centring of the bubble has been found, that they be lapped flat on a piece of fine emery paper resting on a piece of plate glass or a surface plate, Indeed, to get perfect centring, one or the other flat can be lapped to make small adjustments, bearing in mind that on reversal end for end, the bubble must stay central.

Bill Morris
Pukenui
New Zealand
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