Silas Higgon wrote: > Hello dialists, > > I am a newcomer to the internet and have been following your discussions > with interest. I am a dial maker and am sometimes asked to make a > wall-dial for another country. If I am sent a compass bearing for the wall > I am left with the problem of magnetic variation and have to ask the > customer to take sun readings so that I can calculate the wall direction. > It would be much simpler if I could find the magnetic variation of a place > by some means. Can anyone please help? >
Silas I did not know until recently, when I found I have more than 30 compasses: I am a compass collector. Just for the fun of it, I started comparing different makers, models, styles. At first it came as a suprise to find that different compasses will show completely different readings. 2, 3, 5 or even 10 degrees east or west are quite common. Also, using the same compass you can get quite different readings as a consequence of temperature (I guess), pressure (altitude) and, especially, how you hold it in your hand. That is, if it is completely level or not, inclined to one or the other direction, so on and so forth. Temperature and pressure will affect it especially if it has air bubles in the liquid it is supposed to be suspended. I think the temperature will also affect the way the needle pivots around it axle because of the friction. Also, I've found that sometimes, when you turn your compass 180 degrees, it will not show 180 degrees but some degress more or some degrees less. The bottom-line is: there are too many factors influencing the reading. As if that wasn't enough, a layperson can not use it properly, can not even take parallax into acount, can not take into account local forces like iron in the floor or in the wall. Here in Brazil we say that if counsels were good we would not give them for free, we would charge for them. :-) but here my free advice goes: don't ever ask your client to read a compass. 999 times in 1000 they will give you completely wrong readings. In the 1 case left reading will be within 2 or 3 degress off. - fernando -- Fernando Cabral Padrao iX Sistemas Abertos mailto:[EMAIL PROTECTED] http://www.pix.com.br mailto:[EMAIL PROTECTED] Fone: +55 61 321-2433 Fax: +55 61 225-3082 15º 45' 04.9" S 47º 49' 58.6" W 19º 37' 57.0" S 45º 17' 13.6" W
