Message text written by INTERNET:sundial@rrz.uni-koeln.de >I'm helping to restore a Richard Melville (worked 1832-1871) slate dial with multiple gnomons. The gnomons are held in with 2BA brass screws using square nuts. The gnomons themselves are nominally 1/4 inch thick Can anyone tell me when the BA (British Association) screwthread standard started? I think it points to the gnomons being (old) replacements.<
John if you go to http://www.bhi.co.uk/hints/bathrd.htm There's a history which starts: "Way back in the 1890's, the British Association for Advancement of Science (BA) realised there were no English screw threads standards for small electrical and scientific equipment, so they proposed the BA system loosely based on the Thury threads already in use in Europe (hence the metric sizes). They quickly caught on and rapidly became the standard in the UK. They are good news - thread angle of 47-1/2 degrees (good for holding in thin work like clock wheels); rounded tips and crests (less power needed to cut, less risk of tearing threads, less wear on the tools); enormous range of sizes (0 - 26BA)." I think you are right - the slate Melville dials I have seen all have their gnomons set in lead. Though it's possible the centre (main) ones might have some further support underneath. Patrick -
