  The effect of uniform magnetic field applied along a fixed horizontal
direction in Rayleigh-B\'enard convection in low-Prandtl-number fluids has been
studied using a low dimensional model. The model shows the onset of convection
(primary instability) in the form of two dimensional stationary rolls in the
absence of magnetic field, when the Rayleigh number $R$ is raised above a
critical value $R_c$. The flow becomes three dimensional at slightly higher
values of Rayleigh number via wavy instability. These wavy rolls become chaotic
for slightly higher values of $R$ in low-Prandtl-number ($P_r$) fluids. A
uniform magnetic field along horizontal plane strongly affects all kinds of
convective flows observed at higher values of $R$ in its absence. As the
magnetic field is raised above certain value, it orients the convective rolls
in its own direction. Although the horizontal magnetic field does not change
the threshold for the primary instability, it affects the threshold for
secondary (wavy) instability. It inhibits the onset of wavy instability. The
critical Rayleigh number $R_o (Q,P_r)$ at the onset of wavy instability, which
depends on Chandrasekhar's number $Q$ and $P_r$, increases monotonically with
$Q$ for a fixed value of $P_r$. The dimensionless number $R_o (Q, P_r)/(R_c Q
P_r)$ scales with $Q$ as $Q^{-1}$. A stronger magnetic field suppresses chaos
and makes the flow two dimensional with roll pattern aligned along its
direction.
