Erasmus Darwin

Air raising

Small waves seen from Lindisfarne

How do clouds form? How does temperature vary with altitude, and what does coffee have to do with any of it?

You put a drop of alcohol on your hand and feel your hand get cooler as the alcohol evaporates, but what has this to do with coffee, climate and physics?

Erasmus Darwin (1731-1802) was the grandfather of Charles of “Origin of the Species” fame. As a member of the Lunar Society (so-called because the members used to meet on evenings on which there was a full moon so that they could continue their discussions into the night and still see their way home) he would conduct all sorts of scientific experiments and propose various imaginative inventions. Other members of the Lunar Society included Matthew Boulton, Josiah Wedgwood and Joseph Priestley. The society was a great example of what can happen when a group of people who are interested in how things work get together and investigate things, partly just for the sake of it.

One of the things that Darwin had noticed was that when ether* evaporates from your hand, it cools it down, just as alcohol does. Darwin considered that in order to evaporate, the ether (or alcohol or even water) needed the heat that was provided by his hand, hence his hand started to feel cooler. But then he considered the corollary, if water (ether or alcohol) were to condense, would it not give off heat? He started to form an explanation of how clouds form: As moist air rises, it cools and expands until the moisture in the air starts to condense into droplets, clouds.

hole in water alcohol

There are several cool things you can notice with evaporating alcohol. Here a hole has been created in a thin layer of coffee by evaporating some gin. You can see the video of the effect here.

As with many such ideas, we can do a ‘back of the envelope’ calculation to see if Darwin could be correct, which is where we could also bring in coffee. The arabica growing regions are in the “bean belt” between 25 °N and 30 °S. In the sub-tropical region of that belt, between about 16-24°, the arabica is best grown at an altitude between 550-1100 m (1800-3600 ft). In the more equatorial regions (< 10º), the arabica is grown between 1100-1920m (3600-6300 ft). It makes sense that in the hotter, equatorial regions, the arabica needs to be grown at higher altitude so that the air is cooler, but can we calculate how much cooler it should be and then compare to how much cooler it is?

We do this by assuming that we can define a parcel of air that we will allow to rise (in our rough calculation of what is going on)¹. We assume that the parcel stays intact as it rises but that its temperature and pressure can vary as they would for an ideal gas. Assuming that the air parcel does not encounter friction as it rises (so we have a reversible process), what we are left with is that the rate of change of temperature with height (dT/dz) is given by the ratio of the gravitational acceleration (g) to the specific heat of the air at constant pressure (Cp) or, to express it mathematically:

dT/dz = -g/Cp = Γa

Γa is known as the adiabatic lapse rate and because it only depends on the gravitational acceleration and the specific heat of the gas at constant pressure (which we know/can measure), we can calculate it exactly. For dry air, the rate of change of temperature with height for an air parcel is -9.8 Kelvin/Km.

contrail, sunset

Contrails are caused by condensing water droplets behind aeroplanes.

So, a difference in mountain height of 1000 m would lead to a temperature drop of 9.8 ºC. Does this explain why coffee grows in the hills of Mexico at around 1000 m but the mountains of Columbia at around 1900 m? Not really. If you take the mountains of Columbia as an example, the average temperature at 1000 m is about 24ºC all year, but climb to 2000 m and the temperature only drops to 17-22ºC. How can we reconcile this with our calculation?

Firstly of course we have not considered microclimate and the heating effects of the sides or plateaus of the mountains together with the local weather patterns that will form in different regions of the world. But we have also missed something slightly more fundamental in our calculation, and something that will take us back to Erasmus Darwin: the air is not dry.

Specific heat is the amount of energy that is required to increase the temperature of a substance by one degree. Dry air has a different specific heat to that of air containing water vapour and so the adiabatic lapse rate (g/Cp) will be different. Additionally however we have Erasmus Darwin’s deduction from his ether: water vapour that condenses into water droplets will release heat. Condensing water vapour out of moist air will therefore affect the adiabatic lapse rate and, because there are now droplets of water in our air parcel, there will be clouds. When we calculate the temperature variation with height for water-saturated air, it is as low as 0.5 ºC/100 m (or 5 K/Km), more in keeping with the variations that we observe in the coffee growing regions†.

We have gone from having our head in the clouds and arrived back at our observations of evaporating liquids. It is fascinating what Erasmus Darwin was able to deduce about the way the world worked from what he noticed in his every-day life. Ideas that he could then either calculate, or experiment with to test. We have very varied lives and very varied approaches to coffee brewing. What will you notice? What will you deduce? How can you test it?

 

*ether could refer to a number of chemicals but given that Erasmus Darwin was a medical doctor, is it possible that the ether he refers to was the ether that is used as an anaesthetic?

†Though actually we still haven’t accounted for microclimate/weather patterns and so it is still very much a ‘rough’ calculation. The calculation would be far better tested by using weather balloons etc. as indeed it has been.

¹The calculation can be found in “Introduction to Atmospheric Physics”, David Andrews, Cambridge University Press

 

 

Ripples in the Knowledge Quarter at Pattern, Kings Cross

Pattern, coffee, Kings Cross, Kings X

Pattern Coffee, Kings Cross

In 2018, the Institute of Physics will move to Kings Cross and into what is being called the “Knowledge Quarter”, an area incorporating the British Library, the newly opened Francis Crick Institute and the University of the Arts, among others. Coffee houses have, in the past, been integral to the development of knowledge, places where scientists, artists and the generally interested would meet to discuss new ideas or groundbreaking results. So what about the cafés in Kings Cross? Where will tomorrow’s scientists, artists and the generally interested meet?

Knowing that I would be in the Kings Cross area a couple of weeks ago, I looked up the Kings Cross coffee guide by doubleskinnymacchiato and decided, for not-quite-random reasons, to try Pattern on this occasion. I had been forewarned that the first thing that I would notice would be the colourful patterns on the wall. A good call, that was indeed one of the first things you notice as you walk in. Secondly though were the hat-lampshades on the bulbs over the table at the window (visible in the photo on doubleskinnymacchiato’s review). As anyone who has met me in autumn/winter may appreciate, the lampshades immediately made me feel right at home. It was fairly crowded when I arrived in the late-morning and so I shared the bench in the window with a couple of people who seemed to be discussing history/philosophy and how to write properly referenced argumentative essays. The Americano I had ordered was brought over and, slightly self-conscious to photograph it while sharing the table, I just had to enjoy and savour the well made coffee. There is, perhaps, almost too much to notice at Pattern. But something behind me caught my eye, something that connects coffee, patterns and this café: An old style dial telephone, fixed to the counter.

telephone, dial, coffee Kings X

Patterns in the cord, patterns in the telephone. An unusual feature at Pattern, Kings Cross.

Although the history of the invention of the telephone is quite controversial, the bit that reminds me of coffee is not so contentious, it is to do with how the telephone works. Let me explain.

In the gallery the “Information Age” at the Science Museum in London, it is argued that the commercial success of the telephone was driven by the invention of the carbon microphone, simultaneously invented by David Hughes (1831-1900) and Thomas Edison (1847-1931). It is the Edison version that prompts me to think of espresso. Edison’s microphone worked by packing a cylinder of carbon granules between two metal plates. In my mind I think of Edison’s carbon microphone as similar to a perfectly tamped coffee block in a filter basket. In the microphone, one plate was fixed, the other was flexible and acted as a diaphragm. When somebody spoke into the microphone, the diaphragm would vibrate causing the carbon granules to move alternatively closer together and further apart. Carbon conducts electricity and so the resistance of the microphone changed if the carbon granules were closer together or further apart. The sound waves impacting on the diaphragm were being perfectly translated to electric current patterns that could be transmitted through the telephone lines. The packing of the carbon granules would need to be optimum to transmit the sound, just as the pressure used to press the espresso tablet needs to be just right, enough contact between coffee grains to prevent the water flowing straight through without producing a good coffee, but not so much that the water cannot percolate through the coffee tablet and what should be a lovely espresso becomes over extracted. The ground coffee pressed into the filter basket at Pattern must have fitted this optimum density very well. A well poured espresso revealing that they had achieved that optimum balance between compression and space in the espresso tablet. Good coffee, interesting physics, I’m sure the Institute of Physics will be pleased when it eventually moves to its new home with such great coffee neighbours.

IoP poster in Kings Cross

Physics is everywhere! (But coming to Kings Cross)

Although slightly off topic, a cafe-review considering telephones would not be complete without including the story about Erasmus Darwin, the Devil and his “speaking machine”. Erasmus Darwin (1731-1802) was a fairly portly man who worked hard. So it was inconvenient for him to have to go from his study to the kitchen when he wanted something to eat. Being a bit of an inventor, he installed a speaking tube in his home that connected his study to his kitchen. Desmond King-Hele in “Erasmus Darwin, A life of unequalled achievement”* described what happened next:

One day a local yokel who had arrived with a message for Darwin, was left alone in the kitchen. He was terrified when a sepulchral and authoritative voice from nowhere demanded ‘I want some coals’. Such a request could only come from the Devil, he thought, wishing to stoke up hell’s fires. The man fled and would not come near the house again.

The poor local may have been bewildered by the number of telephones and ‘voices from nowhere’ that surround us now. If you’re reading this in a café, why not look around you, notice some strange connection (the very lateral ones can be particularly fun to ponder), and then let me know what you have seen. It’s always interesting to hear the science, history and connections that people notice as they sit in cafés around.

Pattern Coffee is at 82 Caledonian Road, N1 9DN

*Desmond King-Hele, “Erasmus Darwin, A life of unequalled achievement” was published by Giles de la Mare Publishers, 1999.

Echoes of Bach at Amoret, Hammersmith

Amoret coffee Hammersmith

Amoret, so new it still didn’t have its name on the outside.

Amoret is a new addition to the coffee scene over in Hammersmith. Just up the road from the Hammersmith & City line entrance of Hammersmith tube station, I nearly missed this cute cafe when I walked past as it had no name on its frontage, nor did it have the chalk board that is characteristic of many cafes. Fortunately however, I had the address and so double backed to find a great little cafe. It appears that that majority of Amoret’s business comes from take-away orders although there is a small seating area at the back (it is small, when we visited in February, there were two chairs and a couple of tables/stools).  If you are fortunate enough though to be able to take a seat at the back of the cafe, I would thoroughly recommend doing so. Not only can you enjoy good coffee in a nice environment, the friendly people behind the bar were very happy to chat about their coffee and cafe. Moreover, there is plenty to notice from this observation post at the back of the cafe.

When we visited, the espresso based coffee was by Campbell and Syme, with V60s that featured different guest roasters (though it seems that other roasters also regularly feature for the espressos). I had a coffee from Panama, roasted by Union, which featured the word “caramel” in its tasting notes. I have simple tastes (‘caramel’ or ‘chocolate’ descriptions always go down well) but it was a great coffee. Complementary water was available at the counter with take-away cups (and water ‘glasses’ ) that were compostable and biodegradable*. As the very friendly staff brought my coffee to the table, I noticed that the ‘table’ that I had put my water on was in fact a metal drum that sounded ‘clang’ as the cup was put down. The sound of the drum immediately suggested that the drum was hollow. We all recognise the sound of a hollow drum, it is partly about the pitch of the sound, but partly about the echoes that we hear as the sound reverberates inside the metal.

Kettle drum at Amoret

After I had enjoyed my filter! The table-drum at Amoret. Does the drum sound the same in summer?

Although it appears simple, the sound made by the drum is influenced by many aspects of the drum’s construction and surroundings. The stiffness of the metal and the atmospheric pressure affect the way that the drum’s surface vibrates, while the size of the drum and the speed of sound in air also affect the note, or pitch, that we hear. How is the sound of the drum affected by a change in its surroundings? For example, if the atmosphere in Amoret got much warmer, the speed of sound would increase, how would that affect the sound of the table-drum?

A few years ago, Professor Timothy Leighton was wondering how the properties of the atmosphere affected the sounds of musical instruments. Specifically, he wondered what instruments would sound like on other planets. Take Venus. Venus is a planet with a very dense, very hot atmosphere. The surface temperature on Venus is 457C (Earth’s average is approx 14C) while the atmospheric pressure is 90 Bar (Earth’s average: 1 Bar). As it gets hotter, the speed of sound increases and so, to a first approximation, the note made by the drum-table at Amoret will sound higher as the air gets warmer. However, the metal of the drum is also hotter on Venus (so less stiff) and the density and pressure of Venus’ atmosphere will act to further complicate things. So to start thinking about how things sound on Venus, we would be more sensible to think about a simpler instrument, such as an organ, which is only affected by the change of the speed of sound†. Take the famous case of Bach’s Toccata and Fugue in D minor. Played on Venus, the researchers found that, rather than be in D minor (293.66 Hz), it would have the pitch of F minor (at 349.23 Hz). You can hear Bach’s Toccata on Venus (Mars and Titan) here.

Venus

The clouds of Venus photographed by Hubble. Image credit © NASA/JPL

What about a human voice, how would a person sound on Venus (were they able to survive)? In humans, the pitch of the voice is determined by the rate of vibration of the vocal cords. So it is possible to construct a speech synthesiser to imitate human speech by modelling such a voice ‘box’. Erasmus Darwin, (grandfather to Charles) made such a device in around 1770 with wood, leather and silk‡. Darwin’s voice synthesiser could pronouce the sounds ‘p’, ‘m’, ‘b’ and ‘a’ and so ‘mama’, ‘papa’, ‘map’ and ‘pam’, which by some accounts was convincing enough to fool people into thinking there was a small child in the room. Why did people think that Erasmus’ ‘child’ was small? It turns out that just as with the drum, when we listen to people speak, we do not just register their pitch but also the echoes on their voice. Each time we make a sound, the sound travels from the vocal cords down to the lungs (where it gets reflected upwards) and up to the mouth (where it gets reflected downwards). We subconsciously listen for these echoes and, if they take a long time to appear, we deduce that the person is large (there is a greater distance between their voice box and their lungs). If the echo comes back quickly, clearly the distance between the voice box and the lungs is smaller and hence the person is smaller. Just like the drum at Amoret, the human voice is a bit more tricky to model on Venus than Erasmus Darwin’s device allowed for.

Leighton and co-author Andi Petulescu considered the question of the sound of the human voice on Venus in their 2009 paper. Firstly they said, the density of Venus’ atmosphere would make the vocal cords vibrate more slowly, so the person speaking would sound as if they had a deeper voice. But secondly, the high speed of sound on Venus would mean that those echoes that we listen for would come back very quickly, so we would perceive the speaker as being small. What does this sound like? A few years ago, a Dutch TV show set this very topic as a question for their annual quiz and answered it by one of the co-hosts singing Banarama’s “Venus” with, and without, the Venus voice changing software of Leighton. If you understand Dutch, the full clip is below. If you don’t understand Dutch but would just like to find out how you would sound on Venus while singing Banarama, forward to 7 minutes in for the version on Earth and 7m46 in for the Venus version.

It is not easy for us to travel to Venus to investigate whether Prof. Leighton was correct. It is possible for us to repeatedly visit Amoret to investigate how the coffee cups sound as they are put on the drum as the temperature changes around us. This seems a fantastic excuse to revisit to me.

 

Amoret is at 11 Beadon Road, W6 0EA

‡”Erasmus Darwin – A life of unequalled achievement” by Desmon King-Hele was published by Giles de la Mare Publishers (1999)

* It should be noted that ‘compostable’ plastic has a very specific definition that does not mean that it can necessarily be composted in the way that you or I would understand the term, as I described in more detail here. Nonetheless, it is definitely a significant improvement from conventional plastic and I would love to see more cafes follow suit with environmentally sound packaging.

† Of course this comes with a fair few caveats, not least the fact that the organ has to have flue pipes only. I would thoroughly recommend browsing Professor Timothy Leighton’s excellent webpage on this and other aspects of acoustics which you can find here.