Electrical Measuring Instruments and its Electrical types


for the detection or measurement of electric current potential difference, and resistance certain instruments have been devised viz the galvanometer for the detection of small currents or measurements of small currents of the order of micro amperes or millionaires the voltmeter of potentiate for the measurements of potential difference (or voltage) between two points of a cur-cit or the emf of a source the ammeter for the measurements of large currents the Wheatstone bridge the meter bridge the post office box and the ohmmeter for the measurements of resistance.

The moving coll galvanometer. :-

The moving coll galvanometer is a basic electrical instrument it is used for the detection or measurement of small currents.

Its underlying principle is the face that when a current flows in a rectangular coll placed in a magnetic field it experience a magnetic torque, If it is free to rotate under a controlling torque.It rotates through an angle proportional to the current flowing through it.The Although the galvanometer deflects full scale by a small current a few micro-amperes or millionaires nevertheless it can measure such small currents if the deflection is properly calibrated.

The primitive form of the moving col galvanometer was developed by the french scientist D arson val Edward Weston improved upon D arson val original version and gave us the modern moving coll galvanometer.

The flat rectangular coll of then enamel insulated wire of suitable number of turns wound on a light nonmetallic (or aluminium) frame is suspended between the cylindrical concave pole pieces of the permanent.u-shaped magnet by a thin phosphor bronze strip.One end of the wire of the coil is soldered to the strip.

The others end of the strip is fixed to the frame of the galvanometer and connected to an external terminal.It serves as one current let through which the current enters or leaves the coil.The others end of the wire of the coil is soldered to a loose and soft spiral of wire connected to another external terminal the soft spiral of wire serves as the other pieces is placed where the coil moves freely the sot iron cylinder makes the magnetic field stronger and radial such that into whatever position the coil rotates the magnetic field is always parallel to its plan.

When a current passes through the galvanometer coil it experiences a magnetic deflecting torque which tends to rotate it from its rest positions as the coil rotates it produces a twist in the suspension strip.The coil rotates until the elastics restoring torque due to the strip does not equal and cancel the deflecting magnetic torque and then it attains equilibrium and stops rotating any further.

In the previous chapter the deflecting magnetic torque was derived as:

Deflecting magnetic torque = BIAN cos a where B = strength of the magnetic field.

I = Current in the coil.

A = Area of the coil.

N = Number of tuns in the coil.

and a = the angle of deflection of the coil.

The restoring elastic torque is proportional to the angle of twist of the suspension strip provided it obeys Hooke’s law. Thus restoring elastic torque = c 0. where 0 is the angle of twist of the suspension strip.( 0 is different from but proportional to a) and c is the torque per unit twist of the suspension strip for equilibrium.

Deflecting magnetic torque = restoring elastic torque.

or BIAN cos ac = c 0

or I = __________c_____________ 0 —————– ( 15.1)

BIAM cps CC.


If the magnetic field were uniform ( as with flat cos a factor a would continuously increase with 0 and cos a factor would not be constant.

Then the current I no-meter not linear however due to the radial magnetic field the plane of the coil is always parallel to the field irrespective of the position the coil rotates so a the angle between the plane of the coil and the direction of the field is always zero hence cos a – 1 i.e. constant as are B.A and N


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