Question

When voltmeter is attached in series then ammeter shows zero.why?

Answer 1

The voltmeters have very high impedance across them. They allow a very low amount of current to pass through the meter. Else if the resistance is lower, the current instead of passing through the desired circuit, will pass through the meter. This is also called loading effect. This also result in the less accurate reading. So the voltmeters are made of high impedance.

The connecting of voltmeter in series is equivalent to connecting a very high resistance in series with the circuit. By this only small insignificant amount of current flow through the circuit and nearly results in an open circuit.