John Carmichael wrote: >Subject: Wall Declination Measurement
>Hopefully, somebody from the sundial list will help us. This is my own preferred method of determining wall declination but, before originally adopting it, I needed to be sure that a) the 'nail' was truly pependicular to the measuring board and b) the plumbline/pencil line ran exactly through the axis of the nail. Without this certainty the results might be questionable so to that end I developed my 'precision declinometer' which I know has been adopted in various forms by other diallists. It uses a tapered 'gnomon' spike sliding in a machined vee groove*. The novel idea was to include a needle point in the butt end of the spike which punctures the recording paper exactly on its axis. For anyone who missed the previous offer via this list some years ago I can supply a gif and jpeg showing how it is made and used. Like John Davis I also prefer to take a number of readings over an extended period then take an average. * An accurately drill hole is almost as good but 'drilling is the least precise process in engineering' and unless a good sliding fit is obtained then the tip and needle point can wobble off centre whereas a vee groove constrains the spike precisely in two perpendicular planes. Tony Moss -
