What is loading effect? why we kept High internal resistance in voltmeter?

When a voltmeter is connected across a resistance of, whose voltage is to be measured,then voltmeter draws a current for its own winding, so it acts as a load on the system and due to this there is an error in the reading of voltmeter.This phenomenon known as Loading effect.

Resistance Measured if it is low

R_voltmeter∥R_{load <} R_load

_{so measured value of resistance is less than its actual value. TO ELIMINATE THIS LOADING EFFECT VOLTMETER MUST BE IDEAL THAT IS ITS RESISTANCE MUST BE INFINITE.so higher the resistance and sensitivity of voltmeter,lesser is loading effect.}

Illustrative Example

With no voltmeter connected to the circuit, there should be exactly 12 volts across each 250 MΩ resistor in the series circuit, the two equal-value resistors dividing the total voltage (24 volts) exactly in half. However, if the voltmeter in question has a lead-to-lead resistance of 10 MΩ (a common amount for a modern digital voltmeter), its resistance will create a parallel subcircuit with the lower resistor of the divider when connected

his effectively reduces the lower resistance from 250 MΩ to 9.615 MΩ (250 MΩ and 10 MΩ in parallel), drastically altering voltage drops in the circuit. The lower resistor will now have far less voltage across it than before, and the upper resistor far more.

  voltage divider with resistance values of 250 MΩ and 9.615 MΩ will divide 24 volts into portions of 23.1111 volts and 0.8889 volts, respectively. Since the voltmeter is part of that 9.615 MΩ resistance, that is what it will indicate: 0.8889 volts.

This effect is called loading.there always will be some degree of loading, causing the meter to indicate less than the true voltage with no meter connected. Obviously, the higher the voltmeter resistance, the less loading of the circuit under test, and that is why an ideal voltmeter has infinite internal resistance.

Note: your suggestion are welcoming and invited.