PURPOSE:To show the characteristics of the waveform of a water wave by writing an accurate trochoid wave on the blackboard and to easily write the circular motion of water particles in an advance wave or the single vibration of water particles in a standing wave on the blackboard by fixing a rotary shaft to a disc fixed with a chalk and rolling the disc along the linear rod placed on the blackboard. CONSTITUTION:As a support rod when a disc is rolled, a hollow aluminum rod having a square pillar shape is used and a rubber magnet is applied on the surface in contact with a blackboard and the support rod can be fixed to the blackboard by magnetic force. A bearing is inserted in the center of the disc, a rotary shaft is passed through the bearing and an anti-rotation handle is fixed to said shaft. A cylindrical container receiving a freely rotatable protruding by about 1/4 is supported at three fulcrums arranges closely to a circle, and an anti-slip rubber is bonded to the periphery of the disc and a double cylinder middle container receiving a spring in which chalk is inserted is provided. By using this describing device, various waves can be described according to the position of the hole in the disc in which the double cylinder container provided with chalk is inserted.
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