The flux density of magnetic fields may be measured by various methods, three of which are described below. The first and third methods are only suitable for constant magnetic fields while the second may be used for either steady or varying fields.
This is simply a slice of
semiconducting material with a small current passing through it. When it is placed in the
magnetic field a p.d. that is directly proportional to the magnetic flux density is produced across
the slice at right angles to the current direction.
The instrument must first be calibrated
by placing it in a magnetic field of known flux density, such as the centre of a long solenoid. The
sensitivity of the probe in millivolts per tesla can then be found. The probe is then placed in the
unknown field and its flux density found.
This
method can be used to measure both constant and varying fields and we will consider first its
use for varying fields.
(i) Measuring varying magnetic
fields
The search coil is a small flat coil of fine insulated wire with a large number of
turns. Those in use in schools usually have between 500 and 2000 turns with an average
diameter of 0.5 cm. It is mounted on an insulated handle as shown in Figure 1.
When the coil is placed in a varying magnetic field an e.m.f is induced in it which is directly proportional to the flux density of the field. If this e.m.f. is measured the strength of the field may be found.
In the measurement of steady magnetic fields the search coil is connected to a
ballistic galvanometer ( a sensitive meter where the maximum deflection is proportional to the
charge flowing in a short time).
The search coil is placed in the field and held still, the
galvanometer reading will be zero. If the coil is now removed quickly to a large distance from the
field a current will flow in the coil and it can be shown that the maximum deflection shown by the
galvanometer is proportional to the field strength. (Figure 3)
This instrument, as its name suggests, is used primarily for the
measurement of the flux density of the magnetic field of the Earth. The principle is the same as
that described for the search coil, but since the flux density of the Earth's field is low a coil of
many turns and large area is needed because the induced voltages will be small.
The
coil is placed in the position shown in Figure 4(a) and rotated suddenly through 180o; the
resulting kick on the galvanometer is proportional to the charge passed which is proportional to
the flux cut. Therefore if the experiment is repeated starting with the coil in position shown in
Figure 4(b) the ratio of the two deflections gives the ratio of the vertical and horizontal
components of the Earth's field, that is, the tangent of the angle of
dip.
