When a voltmeter with an internal resistance of r [Ω] was connected to a constant voltage power supply, the voltmeter showed E1 [V]. Next, when a multiplier with resistance r [Ω] was connected in series with the voltmeter and connected to the same power supply, the voltmeter showed E2 [V]. Find the internal resistance r [Ω] of this voltmeter, explaining step by step by step.