Why should voltmeter have high resistance?
Voltmeter is an instrument that is used to measure the potential difference between two points in an electric circuit. The voltage in the circuit will be visible in the analog voltmeters which will move the needle through the scale according to the voltage in the circuit. The digital voltmeters will show the measurement of the voltage in numbers with the help of analog to digital converters. Generally Ammeter is connected in series to the circuit while voltmeters are connected in parallel to the circuit. Ammeter is the instrument that measures the current flowing in the circuit while voltmeter measures the potential difference.
All these meters should not alter the behavior of the circuit. Ammeter which is connected in series to the circuit has to possess less resistance, so that it can measure exactly the current passing through the meter as well as in the circuit. As the voltmeter is parallel to the circuit, it tends to divert the total current that is passing in the circuit. If the current passing in the circuit is diverted by another path, then the exact voltage of the circuit cannot be determined. So, the path in which the voltmeter is connected which is dividing the pathway of the current travel should allow least current to pass through that branch. Voltmeter should have very high resistance in it in order to make the total current pass through the actual circuit. This will make the voltmeter to measure the potential difference between the two points accurately. The voltmeter having high resistance will take as less current through it as possible. The current passing through it is almost negligible due to the presence of very high resistance.
Hence, the voltage of the total current passing in the circuit can be accurately measured by the voltmeter. If the voltmeter takes in more current, then it will make the actual current to drop in the circuit. This will make the instrument to give wrong reading of potential difference. The high resistance of voltmeter is to avoid voltage fluctuations in the circuit when the voltmeter is added and removed to it.