Why is ammeter connected in series and voltmeter in parallel in the circuit?

Explanation

In series, `I` is same for all elements, so if an ammeter is
connect in series with a resistance `R`, its reading will be
equal to the current through `R`(as shows in Fig. S6.1).

Similarly, in parallel, `V` is same for all elements. So if a
voltmter is connected in parellel with a resistance `R`, potential
difference across it will be same that across `R`. This is why
an ammeter is always connected in series, while a voltmeter is
connected in parellel with the element to measure `I` (through it ) and `V` (across it).