Declination

Declination is the deviation of Magnetic North from true North in a particular location. Declination varies with time - wildly during infrequent periods of pole reversal.

Lava flows that solidified at Steen's Mountain during a 4500 year reversal process show that the magnetic poles wandered across the equator three times. The strength of the field was reduced to about 20% of maximum. There is no evidence that it fell to zero.

A calculator on the web gives the present declination in Bangkok as less than one degree, not the ~10° suggested by a casual measurement in the lab. True north can be found by casting the shadow of a plumb line on a day that the Sun passes overhead in Bangkok (twice a year). Magnetic field measurement should be made outside to eliminate as much as possible the influence of magnetic materials in the immediate vicinity.

Calculator

http://www.ngdc.noaa.gov/seg/geomag/

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