Two resistance of $100 \Omega$ and $200 \Omega$ are connected in series with a battery of $4 \mathrm{~V}$ and negligible internal resistance. A voltmeter is used to measure voltage across $100 \Omega$ resistance, which gives reading as $1 \mathrm{~V}$. The resistance of voltmeter must be $\underline{\hspace{2cm}}\Omega$.