Why is a voltmeter is always connected in parallel in a electric circuit?

An ideal voltmeter has infinitely large electrical resistance. Thus, to be introduced in parallel with the element on which to measure the voltage  will not disturb the circuit. If the voltimeter is placed in serie, will interrupt the circuit because its resistance is too large.

A voltmeter is not placed "in a circuit".    It measures the potential difference
between two points in the circuit, so it only touches the two points.  Technically,
that places the voltmeter in parallel with everything else in the circuit that's between
those two points.

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