A series R-C circuit is connected to an alternating voltage source. Consider two situations: (a) When the capacitor is air-filled (b) When the capacitor is mica-filled. Current through the resistor is i and voltage across the capacitor is V. Then:

For a series C-R circuit, the impedance Z = √(R² + Xc²), where Xc = 1/(jωC) and current I = V/Z. When the capacitor is filled with mica, the capacitance will be increased. If C increases, Xc decreases, so the current will increase and hence the voltage across the resistance increases and the voltage across the capacitor decreases. Thus, Va > Vb