Physics

Bulb thermometer


The device shown on video at this link, despite the unconventional format, acts as a thermometer.

First let's interpret the object as a physical device: The lower bulb acts as a reservoir of liquid, while the connection between the two bulbs is through a spiral capillary tube.

This "thermometer" is able to measure the difference between the temperatures of each bulb and its operation is easily explained based on the isometric transformation of the air contained in each reservoir:
Considering that initially both bulbs are subjected to the same temperature, the pressure must be kept equalized and no movement of the liquid is observed.

In a second step the lower bulb is heated using the heat of the hand, as shown in the video. In this case, the temperature of the lower bulb becomes higher than that of the upper bulb, as does the pressure in each of the two.

The device tends to equalize the pressure in both bulbs by "pushing" liquid into the upper bulb. Liquid movement is still facilitated due to the capillary of the spiral tube. This has the effect shown in the video.

The bubbling of the liquid in the upper part happens when, besides the liquid, the air contained in the lower reservoir is sucked from the upper part of the device.
Watch the experiment video:
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