The displacer immersed in the liquid is subjected to downwards gravity, upwards buoyancy and upwards spring force, when the three forces reach to a balance, the displacer will be static at a certain position. The displacer is connected with the magnetic stem, when the displacer move upwards which make the magnetic stem get into the control point's magnetic field, under magnetic induction and magnetic coupling, the magnetic-controlling switch's contact acts to close or open; when the displacer move downwards which make the magnetic stem leave away the control point's magnetic field, the magnetic-controlling switch's contact resets.