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Deflection magnetometre, tangent galvanometer.
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Magnetic flux density at the axial position
Adjecent figure shows a point P near a long conductor XY carrying a current I. MN is a short current carrying conductor, kept at the point P, parallel to the conductor XY.
(i) What is the direction of magnetic flux density ‘B’ at the point P due to the current flowing through XY?
(ii) What is the direction of the force experienced by the conductor MN due to the current flowing through XY?
(i) Direction of the magnetic flux density is perpendicular to the surface and inwardly directed to the plane of the paper.
(ii) Direction of force experienced by the conductor MN is towards conductor XY.
is kept in two uniform and perpendicular magnetic fields 
as shown below:
(i) What is the effect of each of the magnetic fields
on the needle?
(ii) When the needle is in equilibrium, obtain an expression for an angle θ made by the needle with 
in terms 
of only.
(i)
The needle is under the action of two perpendicular and uniform magnetic field BF and Bh.
This will set a couple of force on the needle as BF out in a direction opposite to BH.
This will tend to rotate the needle.
(ii)
When the needle is in equilibrium position then 
will make an angle θ with the direction of BH.
Let the pole strength of the N and S be the m.
Then moment of couple due to field BF = m x BH x SA
Moment of couple due to field BH = m x BF x NA
In the equilibrium position the moments of two couples are equal.
M x BH x SA = m x BF x NA
SA / NA x BH = BF
BH = BF
BF = BH tan θ
Hence proved.
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Mock Test Series
