‹‹ Previous | Page 1 | Next ››
A magnetic needle is suspended so that two mutually
perpendicular magnetic fields act on it; one due to a current carrying
circular coil, and the other due to the earth's magnetic field (horizontal
component BH). The needle settles in a position given by tangent
law: B = BH tan q
A T.G. consists of:
instrument is levelled so that the magnetic needle is exactly horizontal
(2) The coil is arranged in magnetic meridian.
Two Magnetic Fields Acting on the Needle
B due to current in the coil acts in East-West direction.
(2) Field BH, which is the horizontal component of earth's
magnetic field; acts in North-South direction.
Magnetic field produced at the centre of the current careying
by the coil is given by:
m0 n I
where, B = magnetic field at the centre of the circular coil
current in the coil
n = number of
turns of the coil
m0 = permeability
of free space.
B = BH tan q … by tangent law
= BH tan q
\ I =
2 r BH
is known as reduction
\ I = K tan q
\ Sensitivity increases
Relative error in measurement of current I =
Taking I = K tan q and differentiating,
2 d q
sin 2 q
Accuracy is maximum when
error is minimum.
i.e., when sin 2q is maximum
Putting maximum value of sin 2q = 1
sin 2q = 1
2q = 90°
q = 45°
Accuracy is maximum at q = 45°