Magnetic Declination
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-310
2025-11-29
7
At most places the magnetic needle does not point to the true north, but instead a little to the east or west of north. The chief reason for this is that the north pole of the earth and the magnetic north pole do not coincide. The magnetic north pole is about latitude 70° N. and longitude 96° W.
By reference to Fig. 1, it is readily seen that in the Eastern states, as in Maine, for example, the needle points to the west of the true north, while in California it points to the east of the true north. The angle between the direction of the needle at any place and the true meridian at that place is the declination. In some places the needle points to the true north; a line drawn through such places is called an agonic line or line of no declination. Figure 1 shows not only the agonic line in the United States, but also a line drawn through places where the declination is 1° west, another where the declination is 1° east, and so on.
Fig. 1
form, and which is called the annual variation. In surveying land it is very important that the amount of this variation should be known, and records of this change are made and preserved. By comparing these records, it is seen that the agonic line is moving westward, i.e., the declination of places east of the line is increasing, and that of places west of the line is diminishing. The annual change at Philadelphia is about 4 minutes.
الاكثر قراءة في المغناطيسية
اخر الاخبار
اخبار العتبة العباسية المقدسة
