Medieval method of measuring temperature

Question

What methods could a medieval society (technology level around 1475-1500 AD) use to measure the temperature of a liquid?

• The world is similar to medieval Britain/Europe, but with more trade so technologies are essentially shared across the different nation states
• Magic does not exist
• The measurement does not have to be 100% accurate by today's standards, a crude method would do as long as it was reliable
• Money is not an issue in terms of discovery - the church and a fair amount of merchants are invested in this line of research, but the final method should be fairly affordable (i.e. doesn't require expensive chemicals/materials to make)
• The device will be used for measuring the temperature of water around 10-50°C

EDIT: I'm not looking for a mercury thermometer, I've edited the bullet points to be a bit clearer :)

Answer 1

The simplest method of measuring temperature would be to use a bimetallic strip: You put together a layer of copper and a layer of steel, and you get a spring that will change form significantly with temperature. The two layers have different thermal expansion coefficients, so when you heat the strip, one layer will expand a little more than the other, leading to a very visible deformation. Attach a pointer and put it next to some scale, and you have a quite decent thermometer.

The only technological requirement for this thermometer is the ability to produce relatively thin strips of metal. This may be difficult for a medieval black-smith, but I guess it's doable with enough effort.

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To the question of precision and calibration:

• First you manufacture the strip, then you calibrate it. Some ice and boiling water should suffice to fix the 0°C and 100°C marks, then you proceed with marking a scale in between for easier reading.

• As you don't need to know where the resulting scale will be located before you calibrate, you don't need any precision when it comes to the form of the strip.

• As you don't need to know how wide the resulting scale will be before you calibrate, you don't need any precision when it comes to the thickness of the two layers. Thicker layers just mean less deformation, and thus less precision of reading, so you want to produce thin layers. But you don't need to produce a specific thickness.

Bottom line: You just do your best manufacturing the strip, then you see how well it works. Strips that turn out not to move much will be sold off as cheap, imprecise thermometers, strips that move more will demand a higher price...

Answer 2

Boiling water, ice cold water and a binary search

You did say crude would do...

Theory

Humans can't do absolute temperature measurement, but they can do relative - I can't tell you how hot my mug of tea is, but I know it's hotter than my hand and cooler than the kettle. This means that if we have a range of objects whose temperatures we know, we can tell if the thing we want to measure is hotter or cooler than them, and so get a range of possible temperatures - with some reservations.

Method

This is where the boiling and ice cold water come in. Let's assume the ice cold water is about zero degrees on an arbitrary scale of temperature. (we can't use solid ice because of latent heat, as AlexP pointed out), but we can use water that's just above freezing. Let's then assume the boiling water is 100 degrees on the same arbitrary scale. If we mix equal quantities of boiling water and ice cold water, we should get a liquid that is 50 degrees on our arbitrary scale.

Now check the temperature of your water against this liquid. Is it warmer or cooler? If it's cooler then your water is somewhere between 0 and 50 degrees.

Repeat this process again with a 3:1 mixture of ice cold:boiling water, and keep adjusting the fractions until you've narrowed it down enough.

Issues

This only works between 0° and 100°C at best. In fact, because humans burn easily, it only works well below about 45°C. Helpfully this is about the range specified.

Another issue is thermal conductivity - if two objects have different conductivities they will feel different even if they're the same temperature. Fortunately once again, we're comparing water to water so their thermal conductivities are the same.

It also suffers from the fact that the ice cold water will be slightly warmer than 0°C, that the mixture will cool fairly rapidly from its nominal value, so your measurements will be less than exact, and that ice isn't always easy to come by.

Other than that, this method should be fairly accessible and uses practically bronze age technology.

Answer 3

You just need to anticipate by 1-2 centuries the studies that would lead to the appearance of the first thermometer

EDIT: THIS IS AN EXTRACT FROM THE WIKILINK REGARDING EARLY THERMOMETERS:

Various authors have credited the invention of the thermometer to Hero of Alexandria. The thermometer was not a single invention, however, but a development. Hero of Alexandria (10–70 AD) knew of the principle that certain substances, notably air, expand and contract and described a demonstration in which a closed tube partially filled with air had its end in a container of water.[5] The expansion and contraction of the air caused the position of the water/air interface to move along the tube.

Such a mechanism was later used to show the hotness and coldness of the air with a tube in which the water level is controlled by the expansion and contraction of the gas. These devices were developed by several European scientists in the 16th and 17th centuries, notably Galileo Galilei.[6] As a result, devices were shown to produce this effect reliably, and the term thermoscope was adopted because it reflected the changes in sensible heat (the concept of temperature was yet to arise).[6] The difference between a thermoscope and a thermometer is that the latter has a scale.[7] Though Galileo is often said to be the inventor of the thermometer, what he produced were thermoscopes.

The first clear diagram of a thermoscope was published in 1617 by Giuseppe Biancani (1566 – 1624): the first showing a scale and thus constituting a thermometer was by Robert Fludd in 1638. This was a vertical tube, closed by a bulb of air at the top, with the lower end opening into a vessel of water. The water level in the tube is controlled by the expansion and contraction of the air, so it is what we would now call an air thermometer.[8]

The first person to put a scale on a thermoscope is variously said[by whom?] to be Francesco Sagredo (1571–1620) or Santorio Santorio in about 1611 to 1613.

The word thermometer (in its French form) first appeared in 1624 in La Récréation Mathématique by J. Leurechon, who describes one with a scale of 8 degrees.[9] The word comes from the Greek words θερμός, thermos, meaning "hot" and μέτρον, metron, meaning "measure".

The above instruments suffered from the disadvantage that they were also barometers, i.e. sensitive to air pressure. In 1629, Joseph Solomon Delmedigo, a student of Galileo, published what is apparently the first description and illustration of a sealed liquid-in-glass thermometer. It is described as having a bulb at the bottom of a sealed tube partially filled with brandy. The tube has a numbered scale. Delmedigo does not claim to have invented this instrument, nor does he name anyone else as its inventor.[10] In about 1654 Ferdinando II de' Medici, Grand Duke of Tuscany (1610–1670), actually produced such an instrument, the first modern-style thermometer, dependent on the expansion of a liquid, and independent of air pressure.[9] Many other scientists experimented with various liquids and designs of thermometer.

However, each inventor and each thermometer was unique—there was no standard scale. In 1665 Christiaan Huygens (1629–1695) suggested using the melting and boiling points of water as standards, and in 1694 Carlo Renaldini (1615–1698) proposed using them as fixed points on a universal scale. In 1701, Isaac Newton (1642–1726/27) proposed a scale of 12 degrees between the melting point of ice and body temperature.

Answer 4

Maybe they could build a primitive alcohol thermometer, with ethanol, a metallic little bottle and a reed to work as "straw". By scrapping some reeds or cleaning them with a caustic substance you can make them quite translucid.

I have no idea how you could callibrate it, though. And it doesn't work for boiling water.

Answer 5

If we're only trying to measure the temperature of water then we can make an inverse Galileo thermometer. A Galileo thermometer works because the density of water changes based on the temperature of the water. Instead of putting spheres with precisely calibrated density inside of a tube of water we just need to put a precisely measured and standardized amount of water on one side of a scale. Then we balance the scale to get the mass of the water. Since we have the mass and the volume density is easy to calculate. The density measure can be used directly like we did for mmHg, or you can translate that to some arbitrary scale.

Answer 6

I suggest an inflatable bladder full of air.

As air increases or decreases in temperature it expands and contracts, meaning you can measure the size of the balloon to get a sense for the current temperature. Make marks on a stick to find relative temperatures of say.. a hot sunny day or a chilly winter.

The bigger your balloon the more accurate this will be.

The tricky parts are

• Making a consistent balloon
• Maintaining the mass of air inside it against any losses to osmosis.

Inflated pig bladders were used historically as footballs amongst other similar roles, however if you can manufacture reasonably consistent rubber then manufacturing a balloon wouldn't be awfully difficult.

The important factor is that all the balloons must be as similar in size and thickness as possible when deflated, which may make them impractical for medieval engineering.

Maintaining the consistent air mass inside it is somewhat trickier, due to leakage (inevitable) it'll slowly deflate with time, not as much as if it were full of helium, but still enough to affect it over time. The solution is to make a lot of them and when inflating them make sure to match the size of the other balloons. They should then remain a reasonably consistent measuring tool. If a balloon is noticeably the wrong size compared to the others, then re-inflating it to match is easy enough.

It also is unable to account properly for changes in air-pressure and changes to the barometer will affect the apparent temperature.

Answer 7

Imagine you take a proper boiling bowl of water and place it outside in a freezing environment. You carefully check how long it takes before it freezes.

During that process, insert a few different materials and see where on the time scale they solidified (because our starting point is boiling). It would require some research on which material melts when on the scale, but you can fine tune this method as long as it freezes outside.
The time might differ (e.g. 1 ór 2 minutes), but the percentage wont change. If material Y freezes at the 30% passed time, it's 70°C.

Your thermomether would consist of a few tubes, with the know materials. If material A melts, its 25°C. If material B melts it's 35°C and material Z could be 90°C. This method would become more accurate as more tests are performed, with different amounts of water etc.

Answer 8

Everyone are so hung up on fancy technological inventions.

Since the range is 10-50 °C - just stick a finger into the liquid. If you've tested a few different temperatures you will know if it's 10, 20, 30, 40 or 50 degrees.

• No magic necessary.
• Crude but reliable. Will not break. Body is always very close to 37 °C.

Answer 9

You could do something with the varying density of water at different temperatures. A set of wood and/or metal spheres say with densities that span the range of densities of water at the target temperatures. To measure, you would see which one just floated, or just sank, or just stayed at the same location. The density of water changes by about 4% between 0 and 100C, I think.

Answer 10

I'm not sure if this would meet your needs or not. Here's my method for what it's worth. It assumes that you can obtain ice at a reasonable cost. This ought to be feasible if you are nearish to a glacier or if you pack chunks of ice in sawdust and bury them in winter.

You have a material that is used as a thermal standard. Say an ingot of copper of a standard size. You place it in the water whose temperature you want to measure until thermal equilibrium is achieved. Take the ingot out and place it on a cube of ice of a standard size. Collect the melt water in a graduated cup over a standard length of time and see how much water runs off. This should give you a rough measure of the temperature of the water. You can measure time with a water clock.

This assumes you can do your measurement in a "room temperature" environment. Also, it assumes you aren't worried about how the water temperature is affected by sticking a copper ingot into it. This would work for some applications but not for others.

If ice is not available, you could measure the time it takes for the ingot to return to room temperature. You can feel the difference between someone with a normal temperature and someone with a fever which is probably more accurate than you need. Just have someone compare the "water temperature" piece of copper (or some other known metal) with a room temperature "control" piece.