Galvanometer
It is an electrical device used to measure the very small current in milli-ampere.
Principle
When coil is placed in a magnatice field, torque produce which tends to rotate the coil.
Construction
It consist of a coil. The coil is made by wounding the copper wire on a non-magnatic material.The coil is suspended in a u- shaped magnet by a wire. The one end of the coil is attached with the mirror and the second end is attached with helical spring. Both ends of the wire also acts as terminalas shown in fig.
In fig N and S are north and south poles of concave shaped magnet resceptively. D is the iron cylinder. The concave shape and iron cylinder placed inside the coil makes magnatic field radial and strong. The plane of the coil always remains parallal to the magnatic field.
Working
Now when current passed through the coil,torque produces, which deflect the coil. Hence
Deflecting torque = NIABcos α
As α is the angle between plane of the coil and Magnatic field B. This angle may always be zero.
So τd = NIAB cos 0
. τd = NIAB
Restoring torque
The spring attached with one end of the coil will re-twist the coil towards its orignal position. The restoring torque is directly proportional to the twist angle θ.
Restoring torque θ
. τr θ
. τr = Cθ
Where C is the constant of proportionality and called twist constant.
(www.pak-physicsnotes.blogspot.com)
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