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In this study we consider the effect of a horizontal magnetic field on the Rayleigh-Bénard convection in a finite liquid metal layer contained in a cuboid vessel (20020040 mm 3 ) of aspect ratio  = 5. Laboratory experiments are performed for measuring temperature and flow field in the low melting point alloy GaInSn at Prandtl number Pr  0.03 and in a Rayleigh number range 2.310 4  Ra  2.610 5 . The field direction is aligned parallel to one pair of the two side walls. The field strength is varied up to a maximum value of 320 mT (Ha = 2470, Q = 6.1110 6 , definitions of all non-dimensional numbers are given in the text). The magnetic field forces the flow to form two-dimensional rolls whose axes are parallel to the direction of the field lines. The experiments confirm the predictions made by Busse and Clever (J. Mécanique Théorique et Appliquée, 1983 [1]) who showed that the application of the horizontal magnetic field extends the range in which steady two-dimensional roll structures exist ('Busse balloon') towards higher Ra numbers. A transition from the steady to a time-dependent oscillatory flow occurs when Ra exceeds a critical value for a given Chandrasekhar number Q, which is also equivalent to a reduction of the ratio Q/Ra. Our measurements reveal that the first developing oscillations are clearly of two-dimensional nature, in particular a mutual increase and decrease in the size of adjacent convection rolls is observed without the formation of any detectable gradients in the velocity field along the magnetic field direction. At a ratio of Q/Ra  1, the first 3D structures appear, which initially manifest themselves in a slight inclination of the rolls with respect to the magnetic field direction. Immediately in the course of this, there arise also disturbances in the spaces between adjacent convection rolls, which are advected along the rolls due to the secondary flow driven by Ekman pumping. The transition to fully-developed three-dimensional structures and then to a turbulent regime takes place with further lowering Q/Ra. vertical temperature differences, and height of the fluid layer. The thermo-physical properties of the liquid are described by the thermal expansion rate , the thermal diffusivity , and kinematic viscosity ,

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