Question

Why we connect ammeter in series and volt metre in parallel?

Answer 1

In order for a voltmeter to measure a device's voltage, it must be connectedin parallel to that device . This is necessary because objects in parallelexperience the same potential difference. n ammeter measures the electric current in a circuit. ... This is necessary because objects in seriesexperience the same current.

Answer 2

The Voltmeter is a device used to measure the potential difference between two points. 
Recall the mathematical expression from Ohm's Law : V=I⋅RV=I⋅R (V - Voltage, I - Current, R - Resistance)

You know the value of I and R. It's the V you are seeking. Now, if you connect it in series, nothing magnificent would happen. The Voltmeter is a device of significantly high resistance, and it would impede the flow of current. Open circuit, and nothing spectacular achieved.

Now, the Ammeter, is a device of a marginally lower resistance value, since it's designed to measure the value of current in circuit. So, it allows the current to pass through it, so as to obtain a reading. Now, if you connect an Ammeter in the parallel configuration, a large value of current would flow in the branch with the Ammeter (It's all in the facts. Current chooses path of least resistance. Light bends in dense medium so as to maintain it's speed, but that's a conversation for some other time). Eh?

V=I⋅RV=I⋅R

I is inversely proportional to R. Lesser the value of resistance, larger the value of current. With a large amount of current flowing through the branch having the ammeter, even though alternate routes are available, the circuit would be short-circuited, and your Ammeter's wire would burn out. Essentially, it'll be damaged, and would display inaccurate readings.