please explain why ammetar is connected in series and voltmeter in parrarel

An ammeter is used to measure the magnitude of current flowing in the circuit.It should be connected in series, so that maximum current can pass through it, for accurate measurement of current..An ammeter has low resistance so that the resistance in the circuit or the current in the circuit remains unchanged when it is inserted in the circuit.

A voltmeter is used to measure the potential difference between the two points of a circuit.It is connected across the two points between which the potential is to be measured.(i.e in parallel to the flow of current). The potential difference refers to two points. So to measure the potential difference between two points in a circuit, we connect the two terminals of the voltmeter to those points and hence voltmeter is connected in parallel. A volmeter has a very high resistance so that on connecting it in the circuit, there is negligible change in the circuit resistance (and hence does not draw appreciable amount of current from the circuit ). Its of no use to connect it in series as the electrical potential difference between two points at the same line is zero , which implies the readings will always be zero.