The instruments of this type, the deflection of the instrument provides a basis for determining the quantity under measurement. The measured quantity. produces some physical effect with deflects or produces a mechanical displacement of the moving system of the instrument. An opposing effect is built in the instrument which tries to oppose the deflection or the mechanical displacement of the moving system.
The opposing effect is closely related to the deflection or mechanical displacement which can be directly observed. The opposing effect is so designed that its magnitude increases with the increase of deflection or mechanical displacement of the moving system caused by the quantity under measurement.
The balance is achieved when opposing effect equals to cause producing the deflection or mechanical movement. The value of the measured quantity can then be inferred from the deflection or mechanical displacement at the point of balance.
For example, in a permanent magnet moving coil (PMMC) ammeter, the deflection of the moving coil is proportional to the current I, the quantity under measurement.
The torque Td acting on the moving coil (See Fig) is proportional to current I or Td = GI where G is a constant, which is dependent upon flux density, number of turns and area of moving coil. The opposing effect is produced by a spring whose torque Tc is proportional to deflection, θ or Tc = Kθ where K is the spring constant whose value depends upon the material and the dimensions of the spring. Under conditions of balance, Tc = Td or deflection θ = (G/K) I or current I = (K/G) θ.
The value of measured quantity, current I, in this case depends upon the value of deflection θ, and the constants of the meter, G and K. Thus, in order to make the instrument direct reading, it reads the value of current I directly in terms of the deflection θ, it will have of G and K.
Thus, we conclude, in a deflection type instrument, the value of measured quantity depends upon the calibration of the instrument.