B_{1} = \frac{F_{12}}{I_{2} l} = K \frac{I_{1} I_{2}}{r I_{2} l} l = K \frac{I_{1}}{r}

${B_1 = \frac{F_{12}}{I_2 l} = K \frac{I_1 I_2}{r I_2 l} l = K \frac{I_1}{r}}$

Error