It is possible to devise clever circuit arrangements that behave as ideal meters. The circuit in Figure P19.83 is able to measure the voltage across resistor R₄ without affecting the original circuit. The resistor R₁ is an adjustable resistor whose value can be varied (e.g., by turning a knob). Analyze this circuit and show that for a certain value of the ratio R₂/(R₁ + R₂), this voltmeter circuit has no effect on the current in the original circuit. Find the value of this ratio. Express your answer in terms of ε₁, ε₂, R₃, and R₄.