Given below are two statements. One is labelled as Assertion (A) and the other is labelled as reason (R). 

For an ammeter,

$\mathrm{i}={\mathrm{i}}_{\mathrm{g}}\left(1+\frac{\mathrm{G}}{\mathrm{S}}\right)$

This implies, if S increases, i will decrease, and if S decreases, i will increase.

So, for ammeter of higher range, shunt will be low and for ammeter of lower range, shunt will be high. 

Hence, to increase the range of ammeter, additional shunt needs to be used across it.

And, higher the range, lower is the resistance of ammeter.