Voltmeters must be connected in parallel with the resistor. It must have a very high resistance so that it does not alter the amount of current through the resistor. But what will happen if its resistance is not that high compared to the resistance of resistor. Will the voltmeter still show same reading? How does a voltmeter measure the voltage?
Suppose two resistance of 1000ohm and 2000ohm are placed in series with 50V mains supply. What wll be the reading on a voltmeter of internal resistance 2000ohm when placed across the 1000ohm resistor? What is the fractional change in voltage when voltmeter is connected?