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Ammeter is connected in series with a resistance or an element  of a circuit, in which we need to measure the current.  So the resistance of the meter should be as low as possible in order not to increase resistance of the circuit...

Let r be the internal resistance of the ammeter.  A small resistance S is connected in parallel with the ammeter.  Then the effective resistance of the ammeter with the shunt resistance = R =  r S / (r +s) 
   R < S    as well as  R < r
 we choose S so small that     effective resistance is less than that...and is negligible.

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