In the present paper, we built a rectangle cell with an aspect ratio of 1:1:2, where the height of the cell is two times the width of the cell. The thermal convection of liquid Galinstan with a low Prandtl number, 0.03, is systematically investigated with/without the influence of the magnetic field. During the experiments, two thermostatic water baths are adopted to keep the upper copper plate and lower copper plate with constant temperature difference where the temperature of the upper copper plate is always lower than that of the lower copper plate. Thermoprobes are inserted in copper plates to record the temperature signal in the copper and liquid metal, while the ultrasonic sensors are inserted through the sidewall of the cell to measure the velocity distribution of liquid metal. From the velocity distribution in the cell, we observe three typical flow structures. Without the influence of the magnetic field, time-dependent large-scale convection is observed in the liquid metal thermal convection, with large velocity fluctuations near the cold and hot copper plates. The characteristics of velocity are similar to the thermal turbulence obtained from the normal liquid which is called as flow structure I. Under the influence of the horizontal magnetic field, the thermal convection is strongly suppressed by the magnetic field, the stable single roll convection, namely the flow structure II, and the double oscillation roll convection, namely the flow structure III are observed. Furthermore, the characteristics of these two flow structures with the action of the horizontal magnetic field are investigated in detail through the velocity information obtained from three ultrasonic sensors.
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