Absolute instruments are those which give the value of the quantity being measured in terms of the deflection of the instrument and the constant of the instrument.Tangent galvanometer and Rayleigh's current balance are the two examples. The tangent galvanometer gives the value of current(i) in terms of the tangent of the deflection produced (tan θ) by the current,the radius (r), the number turns(N) of the galvanometer and the horizontal components of earth's field(H).
i=(Hr)tanθ/2πN
These instruments do not need any calibration.Calibration is the operation of making, adjusting or making a scale so that the readings of an instrument conform to an accepted standard.Absolute instruments are used only in laboratories as standardizing instruments.
Secondary instruments are those in which the value of the electrical quantity to be measured can be determined from the deflection of the pointer.The scale of such instruments is calibrated in terms of the units of the quantity being measured.These instruments are in most common use in laboratories, industries power station etc.
i=(Hr)tanθ/2πN
These instruments do not need any calibration.Calibration is the operation of making, adjusting or making a scale so that the readings of an instrument conform to an accepted standard.Absolute instruments are used only in laboratories as standardizing instruments.
Secondary instruments are those in which the value of the electrical quantity to be measured can be determined from the deflection of the pointer.The scale of such instruments is calibrated in terms of the units of the quantity being measured.These instruments are in most common use in laboratories, industries power station etc.
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