Suppose a milliammeter is to be converted to an ammeter. Smaller current was supposed to be flowing earlier and now larger current is to be accommodated. The excess current would require another route to travel for which a shunt would be required to be connected across the terminals of the milliammeter. Since a parallel resistor drops the overall resistance, the resistance of the ammeter system would be reduced in comparison to the resistance of the milliammeter system.
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