Question

why ammeter is used only inseries and voltmeter only in parallel?

Answer 1

In order for a voltmeter to measure a device's voltage, it must beconnected in parallel to that device . This is necessary because objects in parallel experience the same potential difference. n ammetermeasures the electric current in a circuit. ... This is necessary because objects in series experience the same current
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Answer 2

Ammeter measures value of current flowing in circuit, so current should flow inside ammeter to give proper result. And it has very low resistance to ensure the correct measurement of current in the circuit.
If it is connected in parallel across any load then all current in circuit will choose lower resistive path (i.e ammeter) to cause it's circuit to be damaged. Hence it is used in series.
A Voltmeter has very high resistance to ensure that it's connection do not alter flow of current in the circuit.Now if it is connected in series then no current will be there in the circuit due to it's high resistance. Hence it is connected in parallel to the load across which potential difference is to be measured.