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 :)

  • Comments are not for extended discussion; this conversation has been moved to chat. – Tim B Jun 14 at 14:19

10 Answers 10

up vote 39 down vote accepted

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.

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...

Boiling water, ice cold water and a binary search

You did say crude would do...


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.


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.


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.

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    Two words: latent heat. Phase transitions consume or liberate huge amount of heat, especially for water. What's more, the latent heat of solidification and the latent heat of vaporisation are not equal. – AlexP Jun 13 at 9:49
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    @WGroleau you decant it from a barrel of icewater. Ice was available by transporting it from glaciers and storing it in icehouses. &walrus This has a problem with things of different thermal conductivity. If they submerge the object in water first to compare water to water you need a theory of specific heat capacity to determine what the temperature was before you added it to the water. – user25818 Jun 13 at 15:57
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    @notstoreboughtdirt yeah, that would be an issue. Fortunately the OP only wants to measure the temperature of some water, so comparing it with some other water of known temperature should work. – walrus Jun 13 at 16:10
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    This will not work. Exhibit A: Find two objects that are at room temperature (maybe 20 C) - one that is an insulator like a block of wood, or a book, and one that is a conductor like a metal water bottle, or even just water. They are the same temperature, but the conductor feels cooler than the insulator if you touch them. The opposite is true if the two objects are warmer than your body temperature. – BlackThorn Jun 13 at 19:22
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    @BlackThorn @not store bought dirt already pointed this out. It's true that this won't work for generic objects; fortunately the question only asks for finding the temperature of some water, so in this instance it isn't a problem. I've now edited the question to address this directly. – walrus Jun 13 at 19:34

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


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.

  • 3
    Could you describe the principles involved in your answer and why this would work with the described technological level? Links can get outdated in theory which would leave your answer basically meaningless with just another term to google, but people searching for answers should find their answers here and not be required to go somewhere else. Also, one-liners are rarely well received on the site so you might want to edit your answer to expand it a bit and explain your idea in greater detail. – Secespitus Jun 13 at 8:29
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    In a strict sense this is excluded by the constraint medieval technology of course. However I agree that usually in fiction this is how they would do it basically – Raditz_35 Jun 13 at 8:32
  • I'm sorry for the short answer, but the thermometer is just that: a simple measuring instrument that came to be thanks to a series of development in natural studies and production of materials. There is nothing to be updated in that wikilink, unless they discover a functioning thermometer dateable before Christ, so it remains the best reference you can find for general information – Valerio Pastore Jun 13 at 8:42
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    @Korthalion: You are looking for a thermometer by definition. A thermometer is a device for measuring temperature. A mercury thermometer is simply a closed glass tube with some mercury inside; there is nothing fancy about it, and it could have been made in the 15th century, supposing that 15th century physicists had a notion of quantitative measurement of temperature. For example, less then half a century after your target period, Torricelli invented the barometer, which is basically the same thing except the tube is open to outside air at one end. – AlexP Jun 13 at 8:54
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    Whether or not that's all you really need, you shouldn't give the bare minimum. I suggest that you include a bit more details in the body of the answer. Rephrase content from the link if you must. Besides, there are probably numerous considerations and side-notes you could give to the OP that would be helpful. Even if you disagree with my advice, I'm afraid this answer will most likely be deleted without improvement. – SealBoi Jun 13 at 9:54

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.

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.

enter image description here

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    Given alembics and drawn glass beads existed by the 1500s, e.g. 1001inventions.com/glass , there's no reason it couldn't be made out of glass. – Pete Kirkham Jun 13 at 14:39
  • @PeteKirkham True, I obsessed with the "simple technology" part. – Alberto Yagos Jun 13 at 15:48

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.

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    This isn't measuring temperature, it's measuring air pressure... and it depends really heavily on having the ability to reliably consistently produce balloons of identical thickness, thin enough to respond to very minor changes in air pressure. I doubt medieval technology is up to this. – Benubird Jun 13 at 16:15

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.

  • I like this idea, but it relies on the freezing environment being constant. I suppose you could determine when it is freezing outside/ in the room when a control bowl of water begins to freeze? – Korthalion Jun 13 at 15:36
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    While modern science knows a number of "Material A, B and Z" which have a sharp melting point in the interval between 0 and 100C, they were not known (or not readily available) in medieval times. – Alexander Jun 13 at 16:47
  • @Korthalion: I've added it to the 2nd paragraph. Think in percentages :) – Martijn Jun 13 at 17:05
  • @Alexander You are correct, but the materials do exists, they just have to find them by trial and error :) – Martijn Jun 13 at 17:06
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    The problem with this is that cooling rates are highly non-linear, and depend strongly on the exact temperature of the "freezing environment". For example, at an outside temperature of -10C, the temperatures of 75C, 50C, 25C, and 10C correspond to temperatures of 10%, 25%, 47%, and 71%, while at an outside temperature of -2C, they correspond to temperatures of 7%, 17%, 34%, and 54%, and at -1C, they correspond to 6%, 15%, 29%, and 48%. Not too usable when you can't tell 10C from 25C. – Mark Jun 13 at 19:58

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.

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    Good start, but water does not expand linearly, and designing spheres of variable density within one-hundredth of 4% in a medieval society is going to be fairly difficult. Could you build on your answer to include these issues? – nzaman Jun 13 at 13:11
  • Well the linearity of the expansion coefficient doesn't really matter (calibration would take of that) and we're not aiming at 100% accuracy. So about 10 different devices giving different temperatures over the 10-50C range requested might well be achievable. Failing that, coconut oil. 10 phials containing more or less pure oil (different levels of hydrogenation) could indicate the temperature by whether or not they melted. link – ParallelPanda Jun 13 at 15:14
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    I beleive you are trying to describe a Galileo thermometer – James Jenkins Jun 13 at 17:14
  • @nzaman, you don't need linearity, you just need each density to correspond to exactly one temperature. For water, that's true above 8C or so. – Mark Jun 13 at 20:03

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.

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.
  • 1
    Bold. A bit too basic for my purposes but I've +1 because "Will not break" made me laugh. – Korthalion Jun 15 at 8:46

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