Giả sử dòng điện qua Vôn kế có chiều từ trên xuống.
Ta có: \(I_1=I_V+I_2\)
\(\Leftrightarrow\dfrac{U_1}{R_1}=\dfrac{U_V}{R_V}+\dfrac{U-U_1}{R_2}\Leftrightarrow\dfrac{U_1}{2}=\dfrac{U_V}{150}+\dfrac{10-U_1}{9}\left(1\right)\)
Ta cũng có: \(I_4=I_V+I_3\)
\(\Leftrightarrow\dfrac{U_4}{R_4}=\dfrac{U_V}{R_V}+\dfrac{U_3}{R_3}\)
\(\Leftrightarrow\dfrac{U_2-U_V}{7}=\dfrac{U_V}{150}+\dfrac{U_1+U_V}{3}\)
\(\Leftrightarrow\dfrac{U-U_1-U_V}{7}=\dfrac{U_V}{150}+\dfrac{U_1+U_V}{3}\)
\(\Leftrightarrow\dfrac{10-U_1-U_V}{7}=\dfrac{U_V}{150}+\dfrac{U_1+U_V}{3}\left(2\right)\)
Từ \(\left(1\right),\left(2\right)\Rightarrow U_V\approx1,15\left(V\right)\)