AMMETER
An ammeter is an instrument used to measure currents in electric circuits directly in ampere (A). The instrument measuring currents of the order of milliampere (mA) is called milliammeter.An ideal ammeter has zero resistance.Ammeter is essentially a galvanometer which is inserted in the circuit in series so that whole of the current in the circuit passes through it. The deflection produced in the ammeter is a measure of the current.Since, however, the coil of the ammeter has some resistance, so on inserting it in the series of the circuit, the resistance of the circuit increases and the current in the circuit somewhat decreases. Therefore the current read by the ammeter is less than the actual current to be measured.
Hence it is necessary that the resistance of the ammeter be very small compared to other resistance in the circuit.
CONVERSION OF GALVANOMETER TO AMMETER
A galvanometer as such cannot be used as an ammeter because it has appreciable resistance and it can measure only a limited current corresponding to the maximum deflection on its scale.
An ammeter is made by connecting a low resistance S in parallel with a pivoted-type moving-coil galvanometer G. S is knows as "shunt". Its value depends upon the range of the required ammeter and can be calculated as follows:
Let G be the resistance of the coil of the galvanometer and ig be the current which, on passing through the galvanometer produces full-scale deflection. if i is maximum current to be measured, then a part ig of the current i should pass through the galvanometr G and the rest (i-ig) through the "shunt" S. Since G and S are parallel, the potential difference across them will be same :
ig * G =(i-ig) * S
ig/i = S/(S+G)
S=(i/(i-ig))*G
Resistance of ammeter:
1/R= 1/G + 1/S
R= GS/(G+S).
done by
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