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Basics of meter & Measurements

  • 29 Mar 2018
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Moving coil galvanometer (MCG)

The moving coil galvanometer was first invented by lord kelvin and later modified by D’arsonval. The MCG is used for measurement and detection of current and electric charges. There are two types

1.     Suspended type MCG

2.     Pivoted type

The suspended type MCG consist of a rectangular coil of fine insulated wire of large number of turns. It is suspended between the poles of strong permanent circular magnet by means of a phosphor – bronze fibre. A soft iron cylinder is fixed to frame of the galvanometer at the centre of the coil, without touching the coil. A mirror strip attached to the fibre reflects a beam of light. The reflected light spot is obtained on a horizontal scale at a small distance. The current enters through the suspension fibre and leaves through a flexible spring at the bottom of the coil. The upper end of the phosphor – bronze fibre and the lower end of spring are connected to two terminals. The arrangement is enclosed in a case provide with a glass window in front and levelling screws at the bottom.

Conversion of a galvanometer into an ammeter

A galvanometer can be converted into an ammeter by connecting a suitable low resistance parallel to it. Then ammeter can be used for measuring large current.

Note: Connection a shut resistance parallel to any ammeter and can be used for measuring large current. The front dial can be remarked as required.

Note2. Ammeter connect serious to the circuit to measure it. Means the whole current pass through shunt and the meter actually measure voltage across the small shunt resistance.

Calculations

Let Ig be the maximum current that can be passed through the galvanometer without damaging it. (At the time Galvanometer deflect to max reading).  Let main current is ‘I’ and ‘Rg’ is the resistance of galvanometer Rs is the Resistance of Shunt, Is is the current through the Shunt.

Ig X Rg  = Is X Rs    


 So  Shunt Rs  = Ig x Rg / Is



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