Why is voltmeter always connected in parallel?
What will happen if it is connected in series?
Voltmeter is used to measure potential difference.It is always connected in parallel across the points where the potential difference is to measured so one end of the voltmeter is connected to the point A of a resistance AB and the other end to the point B,and also it has a high resistance so that it takes a negligible current from the circuit.
Voltmeters have high resistance, so in series, they would greatly decrease the current going to the circuit.
Ammeters are very low R, so in parallel, the current would bypass the circuit, all go through the ammeter, and blow out the ammeter.