An ammeter is a low resistance galvanometer and is connected in series in a circuit to read current directly in ‘a’.
The resistance of an ammeter is to be made as low as possible so that it may read current without any appreciable error. Therefore to convert a galvanometer into ammeter a shunt resistance. (i.e., small resistance in parallel) is connected across the coil of galvanometer.
Let G be the resistance of galvanometer and the current required for full scale deflection. Suppose this galvanometer is to converted into ammeter of range I ampere. and the value of shunt required is S. If is current in shunt, then from Fig.
Also potential difference across
Substituting value of from (1), we get
This is the working equation of conversion of galvanometer into ammeter
The resistance of ammeter so formed is given by
If is figure of merit of the galvanometer and is the number of scale divisions, then Out of total main current amperes, only a small permissible value flows through the galvanometer and the rest passes through the shunt.
Remark : An ideal ammeter has zero resistance.
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