A
GalileoGalileo Galilei was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations, and support for Copernicanism...
thermometerA thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor A thermometer (from the Greek θερμός (thermo) meaning "warm" and meter, "to measure") is a device that measures...
,
Galilean thermometer (named after Italian physicist
Galileo GalileiGalileo Galilei was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations, and support for Copernicanism...
), is a thermometer made of a sealed
glassIn general Glass refers to a solid, brittle, transparent material, commonly used for windows, bottles, or eyewear. Examples of glassy materials include, but are not limited to, soda-lime glass, borosilicate glass, acrylic glass, sugar glass, Muscovy-glass, or aluminium oxynitride. The term glass...
cylinderA cylinder is one of the most basic curvilinear geometric shapes, the surface formed by the points at a fixed distance from a given straight line, the axis of the cylinder. The solid enclosed by this surface and by two planes perpendicular to the axis is also called a cylinder...
containing a clear
liquidLiquid is one of the principal states of matter. A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material. The surface is a free surface where the liquid is not constrained by a container....
and a series of objects whose densities are designed to sink in sequence as the liquid is warmed and decreases in density and vice-versa.
Suspended in the liquid are a number of weights. Commonly those weights are themselves attached to sealed
glassIn general Glass refers to a solid, brittle, transparent material, commonly used for windows, bottles, or eyewear. Examples of glassy materials include, but are not limited to, soda-lime glass, borosilicate glass, acrylic glass, sugar glass, Muscovy-glass, or aluminium oxynitride. The term glass...
bulbs containing colored liquid for an attractive effect.
A
GalileoGalileo Galilei was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations, and support for Copernicanism...
thermometerA thermometer is a device that measures temperature or temperature gradient using a variety of different principles. A thermometer has two important elements: the temperature sensor A thermometer (from the Greek θερμός (thermo) meaning "warm" and meter, "to measure") is a device that measures...
,
Galilean thermometer (named after Italian physicist
Galileo GalileiGalileo Galilei was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations, and support for Copernicanism...
), is a thermometer made of a sealed
glassIn general Glass refers to a solid, brittle, transparent material, commonly used for windows, bottles, or eyewear. Examples of glassy materials include, but are not limited to, soda-lime glass, borosilicate glass, acrylic glass, sugar glass, Muscovy-glass, or aluminium oxynitride. The term glass...
cylinderA cylinder is one of the most basic curvilinear geometric shapes, the surface formed by the points at a fixed distance from a given straight line, the axis of the cylinder. The solid enclosed by this surface and by two planes perpendicular to the axis is also called a cylinder...
containing a clear
liquidLiquid is one of the principal states of matter. A liquid is a fluid that has the particles loose and can freely form a distinct surface at the boundaries of its bulk material. The surface is a free surface where the liquid is not constrained by a container....
and a series of objects whose densities are designed to sink in sequence as the liquid is warmed and decreases in density and vice-versa.
Typical design
Suspended in the liquid are a number of weights. Commonly those weights are themselves attached to sealed
glassIn general Glass refers to a solid, brittle, transparent material, commonly used for windows, bottles, or eyewear. Examples of glassy materials include, but are not limited to, soda-lime glass, borosilicate glass, acrylic glass, sugar glass, Muscovy-glass, or aluminium oxynitride. The term glass...
bulbs containing colored liquid for an attractive effect. As the liquid in the cylinder changes
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
its
densityThe density of a material is defined as its mass per unit volume. The symbol of density is ρ .- Formula :Mathematically:where: is the density, is the mass, is the volume....
changes, and those bulbs are free to move, rising or falling to reach a position where their density is either equal to that of the surrounding liquid or where they are brought to a halt by other bulbs. If the bulbs differ in density by a very small amount and are ordered such that the least dense is at the top and most dense at the bottom, they can form a
temperatureIn physics, temperature is a physical property of a system that underlies the common notions of hot and cold; something that feels hotter generally has the higher temperature. Temperature is one of the principal parameters of thermodynamics...
scale.
The temperature is typically read from an engraved metal disc on each bulb. Usually a gap would separate the top bulbs from the bottom bulbs and then the temperature would be between the tag readings on either side of the gap. If a bulb is free-floating in the gap, then its tag reading would be closest to the ambient temperature. To achieve this requires manufacturing the weights to a tolerance of less than 1/1000 of a
gramThe gram , ; symbol g, is a unit of mass.Originally defined as "the absolute weight of a volume of pure water equal to the cube of the hundredth part of a metre, and at the temperature of melting ice" , a gram is now defined as one one-thousandth of the SI base unit, the kilogram, or...
(1 milligram).
Theory of operation
The Galileo thermometer works due to the principle of
buoyancyIn physics, buoyancy is the upward force that keeps things afloat. The net upward buoyancy force is equal to the magnitude of the weight of fluid displaced by the body. This force enables the object to float or at least seem lighter....
. Buoyancy determines whether objects float or sink in a liquid, and is responsible for the fact that even boats made of
steelSteel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten...
can float (of course, a solid bar of steel by itself will sink).
The only factor that determines whether a large object will rise or fall in a particular liquid relates the object's density to the density of the liquid in which it is placed. If the object's mass is greater than the mass of liquid displaced, the object will sink. If the object's mass is less than the mass of liquid displaced, the object will float.
Suppose there are two objects, each a cube 10 cm by 10 cm by 10 cm (i.e., 1 litre). The mass of water displaced by an object of this size is 1 kg. The brown object on the left is floating because the mass of water it would displace if completely submerged (1 kg) is greater than the mass of the object. It floats half submerged because that is the point where the mass of the water displaced (0.5 kg) is equal to the mass of the object. The green object on the right has sunk because the mass of water it is displacing (1 kg) is less than the object's mass (2 kg).
Not all objects made of the green material above will sink. In Figure 2, the interior of the green object has been hollowed out. The total mass of the object is now 0.5 kg, yet its volume remains the same, so it floats half way out of the water like the brown object in Figure 1.
In the examples above, the liquid in which the objects have been floating is assumed to be water. Water has a density of 1 kg/L, which means that the mass of water displaced by any of the above objects when fully submerged, is 1 kg.
Galileo discovered that the density of a liquid is a function of its temperature
http://www.csgnetwork.com/h2odenscalc.html. This is the key to how the Galileo thermometer works. (As the temperature of water increases or decreases from 4
oC, its density decreases.)
http://www.lsbu.ac.uk/water/anmlies.html
Figure 3 shows a 1 kg hollow object made of the green material. In the left hand container, the density of the liquid is 1.001 kg/L. Since the object weighs less than the mass of water it displaces, it floats. In the right hand container, the density of the liquid is 0.999 kg/L. Since the object weighs more than the mass of water it displaces, it sinks. This shows that very small changes in the density of the liquid can easily cause an almost-floating object to sink.
In the Galileo thermometer, the small glass bulbs are partly filled with a different (coloured) liquid. Once the handblown bulbs have been sealed, their effective densities are adjusted by means of the metal tags hanging from beneath them. Even though these bulbs expand and contract with changing temperatures, the effect on their density is negligible. The heating and cooling of the coloured liquid and air gap, called
ullageUllage refers to the unfilled space in a container of liquid.-Etymology:The word comes ultimately from the Latin oculus, “eye”, which was used in a figurative sense by the Romans for the bung hole of a barrel. This was taken into French in the medieval period as oeil, from which a verb ouiller was...
, inside the bulbs will affect their density. However, the overall density of each bulb will not vary greatly since the volume of the glass bulb will change only very little due to temperature rise. The clear liquid in which the bulbs are submerged is not water, but some inert
hydrocarbonIn organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. With relation to chemical terminology, aromatic hydrocarbons or arenes, alkanes, alkenes and alkyne-based compounds composed entirely of carbon and hydrogen are referred to as "pure"...
(probably chosen because its density varies with temperature more than water does). This change of density of the clear liquid, with temperature change, causes the bulbs to rise or sink.
Figure 4 shows a schematic representation of a Galileo thermometer at two different temperatures (the temperature markings on this example are in
FahrenheitFahrenheit is the temperature scale proposed in 1724 by, and named after, the physicist Daniel Gabriel Fahrenheit . Today, the scale has been replaced by the Celsius scale in most countries; it is still in use for non-scientific purposes in the United States and a few other nations, such as...
).
If there are some bulbs at the top (Figure 4, left) and some at the bottom, but one floating in the gap, then the one floating in the gap (green 76
o) tells the temperature. If there is no bulb in the gap (Figure 4, right) then you take the temperature of the bulb at the bottom of the gap, add it to the temperature at the bulb at the top of the gap, and divide the result by two. This will give you an approximate measurement.
The bulbs and weights should be sized so as not to jam with each other, either by being at least half the size of the tube diameter to retain their stacking order or, as an alternative, much less than the tube diameter to freely pass each other in the tube.