In the circuit (http://i.imgur.com/II38M.jpg),a variable resistor R is connected across the terminals of a battery, and an ideal ammeter and ideal voltmeter are also connected. In the figure, E is the internal emf of the battery, and r is its internal resistance. All the connecting cables have no appreciable resistance. The ammeter reads 3.10 A when the voltmeter reads 5.08 V, and the voltmeter reads 15.0 V when the ammeter reads 0.00 V.
Vab = E - IR
The Attempt at a Solution
I thought since an ideal voltmeter has infinite resistance, so there would be no current through the resistor. I thought it should just be 15V then, but I feel like I am missing something. Any guidance would be greatly appreciated, thanks.