Why voltmeter less accurate in measuring potential difference than a potentionmeter?

How is a voltmeter connected in the circuit to measure potential difference between two points?

Name a device used to measure potential difference between two points in a circuit?

Why is it necessary to take accurate measurements ?

What is the difference between a voltmeter and an ammeter?

Two resistances of 400 ohm and 800 ohm are connected in series with 6 volt battery of negligible internal resistance. A voltmeter of resistance 10,000ohm is used to measure the potential difference across 400 ohm. The error in the measurement of potential difference in volt approximately .

What are requirements for accurate measurement ?