Galileo thermometers (or Gallimeters)
consist of a glass cylinder filled with a liquid. Within the liquid
are several hollow glass spheres called cruets, which
are also filled with liquid. The cruets have tags that indicate a
specific temperature. Depending on the exterior temperature, the
cruets either float, remain suspended, or sink; the
lowest suspended cruet indicates the current exterior temperature.
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| o| <-- Indicates temperature
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The Galileo thermometer is named after Galileo Galilei (of
course), who discovered that the density of a liquid changes
with temperature. These devices have been made since the end of the
17th century. Early manufacturing was difficult since the density of
the cruets has to be controlled very accurately for proper temperature
measurements.
The workings of the Galileo thermometer are as follows. The cylinder
is filled with a liquid with a density that changes considerably with
changing temperature (nowadays water can be used, but probably an
alcohol was used in older models). The cruets are made of glass, and
have different average densities (total mass of cruet and
content over the total volume of the cruet). The colored liquid filling
serves to set the correct weight; any type of liquid or solid can be
used for this, but this also serves a cosmetic role.
The densities of the cruets are comparable to the density of the
surrounding liquid in the temperature range that needs to be measured.
When the device is in thermodynamic equilibrium with its surroundings,
the lowest suspended cruet will indicate the temperature. At very low
temperatures, the density of the liquid increases, and all the cruets
float. At very high temperatures, the density of the liquid decreases,
and all the cruets will sink. At intermediate temperatures, some
cruets will float, and some cruets will sink.
So why don't the cruets change their density, just as the surrounding
liquid? Indeed, the temperature of the cruets follows that of the
liquid. Since the cruets are made of glass that expands or
contracts little with changing temperature, the volume of
the cruets won't change much. Therefore, the average density of the
cruets will remain relatively constant over the temperature range of the
thermometer.
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