New trends in physics teaching, v.4; The ... - unesdoc - Unesco

New Trends in Physics Teaching IV
Then
or
H,M sin 8 = H, M cos 8
tan 8 = H,/He
The horizontal component of the earth's magnetic field varies with geomagnetic latitude. It is
approximately 0.2 gauss on average (in air, 1 gauss = tesla), but a map for this component
is given in figure 27. There is also a slow secular variation of this field. Using these values, it is
possible to calculate the field strength produced by the bar magnet a distance d from its centre.
The edge of the circle has been graduated in units of 0.2 tan 8, so that you can read the values,
of a field H, directly off the circle, for a place where the earth's field is 0.2 gauss.
-
120' 150. 180° 150' 120. SOo 60° 30° Oo 30° 60' SOo 120°
Figure 27. Map showing lines of equal geomagnetic horizontal intensity H (in lo-'
Office.)
tesla) for 1975. (US. NavaZOceunographic
Far along the axis of the bar magnet, the field is given by M/r3, where Y is the distance from
the centre of the magnet to the paper clip. It is simple to check on this relationship, by plotting
the measured field strength against l/r3. The most accurate readings of H, are obtained when
8 is approximately 45", so it is a good idea to start by placing the magnet at a distance giving
this deflection. With the magnet placed at position B, rather than A, the field at the clip is again
perpendicular to that of the earth, and is given by 2A4/r3, which can also be checked. This experiment
makes the point that we can get a reasonable quantitative value for the magnetic moment
of a magnet using the earth's field for calibration. Most experiments do the inverse to use a
calibrated magnet to obtain an accurate value for the earth's field.
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