Question

Is it correct to say that a potentiometer is equivalent to an ideal voltmeter?

Answer 1

A potentionmeter is basically a device with a variable resistance (see e.g. http://en.wikipedia.org/wiki/Potentiometer ). What should the resistance of an ideal voltmeter be? (Hint: it has to do with the current through the voltmeter)

Answer 2

A voltmeter requires some current to flow through itself to be able to show some deflection.
This would require it to have some resistance.This would change the resistance of the circuit and consequently the current . The potential drops across each of the components of the circuit changes and it is here that the error is produced. For perfect results voltmeter should have infinite resistance and connected parallel across a component. But a infinite resistance voltmeter won't even work.
Potentiometer works on null deflection method i.e. when the galvanometer shows zero deflection and since total potential drop in the circuit is zero, we can easily find the potential across a component by knowing the voltage fed(batteries) to the component.