In an AC series LCR circuit, the voltages across the components are not added algebraically but vectorially using phasors. The net voltage $V$ is given by $V = \sqrt{V_R^2 + (V_L - V_C)^2}$ . If the circuit is at resonance (or if the voltmeter is measuring the source voltage which appears entirely across the resistor because $V_L$ and $V_C$ cancel each other out), the reading corresponds to $V_R$. Since $V_L$ and $V_C$ are $180^\circ$ out of phase, their effects cancel when their magnitudes are equal. The voltmeter reads the RMS value . Assuming the standard resonant condition where the source voltage is 300 V, the reading across the resistor is 300 V.