|
|
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| Deflecting
magnetic torque = BINA Cosa |
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| where | ||||
|
B = Strength of the magnetic
field. I = Current in the coil. A = Area of the coil. N = Number of turns in the coil. a = The angle of deflection of the coil. |
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| |
The restoring elastic torque is proportional to the angle of twist of the suspension strip | |||
| According to Hook’s Law | ||||
| Restoring
torque = C q |
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| Where | ||||
| q
= Angle of twist. C = torque per unit twist. |
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| Under equilibrium condition : | ||||
tm
= tc |
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|
Putting
the values of torque BINA Cos a = Cq I = Cq /BNA Cosa --->(i) |
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| If the magnetic field were uniform (as with flat pole pieces) a would continuously increase with q and Cosa factor would not be constant. Then the current "I" would not be proportional to "q" and the scale of the galvanometer not linear. However, due to the radial magnetic field the plane of the coil is always parallel to the field irrespective of the position of the coil rotates. So "a" the angle between the plane of the coil and direction of the field is always zero. Hence Cos a = I. The equation (i) therefore, reduces to: | ||||
| I
= Cq/BAN |
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| Since C, B, A and N are constant for a galvanometer, therefore, | ||||
|
I a q |
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| Thus current through the coil is directly proportional to the angle of twist of the suspension (or deflection) q. | ||||
|
SENSITIVITY
OF GALVANOMETER |
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| Sensitivity of galvanometer is defined as the current in micro ampere required to consume one millimeter deflection on a scale placed 1m away from the mirror. | ||||
|
TO
INCREASE SENSITIVITY OF GALVANOMETER |
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| Now we know that : | ||||
| I = Cq/BNA | ||||
| Sensitivity
will be large if for small value of current (I) q
in large. To increase "q", we
should decrease C/BNA. So, (i) "C" should be small: For this long wire of small area of cross section should be used. But "C" constant is too small. (ii) Area of coil should is large: But this makes size of galvanometer large. (iii) "N" Number of turns of coil should be increased: But this increases the weight of coil. (iv) "B" should be large: This is achieved by: (a) Using powerful magnet. (b) Introducing iron core in coil. |
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| Contact
us: info@citycollegiate.com |
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