The transition between the bulb of the thermometer and the capillary is bent and narrowed, therefore, for the movement of mercury in the bulb of the thermometer, it is necessary to apply some force to it. When measuring the temperature, this is the pressure force of the heated mercury in the flask (this force is small, so the process of measuring the temperature takes some time (at least 3 minutes). When shaking, an inertial force appears and the more vigorously shaking, the greater this force. In 1-2 shakes, mercury descends to the desired level in the flask.
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