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

String and tape experiments
So far, the experiments have been somewhat qualitative. However, we can construct a tangent
magnetometer which wil measure the strength of our bar magnet by comparison with the earth's
field.
The magnetized paper clip is hung by a long thread from a table or chair, as shown in figure 26,
to swing just above the floor. Make sure the thread is unwound so that it provides the minimum
torque, and that the piece of tape attached to the clip acts as an air damper. Place the graduated
circle of figure 26 beneath the clip so that 0" is orientated north.
W
S
Figure 26.
Bring up the magnet with its axis along the 90" or E. W. line, as shown. Two couples act on the
clip. One arises from the horizontal component of the earth's magnetic field He, and the other
the field at the clip produced by the magnet H,. When the paper clip is sitting at an angle 8 to
the north, as shown, the two couples are in equilibrium: that due to the earth's field is mH,L sin 0,
where m is the pole strength of the paper clip, and L the separation of the poles. The magnetic
moment, M is equal to mL, and the couple due to the magnet is
H, M cos e
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