Jun 262021
 

Today is the birthday (1824) of William Thomson, 1st Baron Kelvin OM, GCVO, PC, PRS, FRSE, mathematician, mathematical physicist and engineer born in Belfast, and professor of Natural Philosophy at the University of Glasgow for 53 years, he did important work in the mathematical analysis of electricity and formulation of the first and second laws of thermodynamics, and did much to unify the emerging discipline of physics in its modern form. He received the Royal Society’s Copley Medal in 1883, was its President 1890–1895, and in 1892 was the first British scientist to be elevated to the House of Lords.

Absolute temperatures are stated in units of kelvin in his honor. While the existence of a lower limit to temperature (absolute zero) was known prior to his work, Kelvin is known for determining its correct value as approximately −273.15 degrees Celsius or −459.67 degrees Fahrenheit. The Joule–Thomson effect is also named in his honor. He also had a career as an electric telegraph engineer and inventor, which propelled him into the public eye and ensured his wealth, fame and honor. For his work on the transatlantic telegraph project he was knighted in 1866 by Queen Victoria. Today is also World Refrigeration Day as a salute to Kelvin’s researches into cold temperatures.

I am not going to spill a whole lot of ink on Thomson accomplishments – actually I am not going to use any real ink at all – because I want to devote most of this post to ice cream making, which was once a passion of mine, courtesy of my late wife.  We had four ice cream makers – one, an old-fashioned, hand cranked bucket machine (that was great), and three that were electrically operated (not nearly as satisfactory).  All are long gone.

By 1847, Thomson had already gained a reputation as a precocious and maverick scientist when he attended the British Association for the Advancement of Science annual meeting in Oxford where he heard James Prescott Joule making yet another of his, so far, ineffective attempts to discredit the caloric theory of heat (the theory that heat was a fluid that passed from hotter to colder bodies). Joule argued for the mutual convertibility of heat and mechanical work and for their mechanical equivalence. In 1848, Thomson proposed an absolute temperature scale (now called the Kelvin scale) in which a unit of heat descending from a body A at the temperature T° of this scale, to a body B at the temperature (T−1)°, would give out the same mechanical effect [work], whatever the number T. Such a scale would be quite independent of the physical properties of any specific substance. Thomson and Joule  began a fruitful collaboration, mostly via letters, from 1852 to 1856, making a number of discoveries including the Joule–Thomson effect, sometimes called the Kelvin–Joule effect, and the published results did much to bring about general acceptance of Joule’s work and the kinetic theory of heat.

Thomson next, in 1854, became involved in the development of cables laid under the ocean to carry telegraphic signals, and with some experiments that Michael Faraday (needs his own blog post) had conducted on a proposed transatlantic telegraph cable. Faraday had demonstrated how the construction of a cable would limit the rate at which messages could be sent – in modern terms, the bandwidth. Thomson jumped at the problem and published his response that month. He expressed his results in terms of the data rate that could be achieved. Thomson contended that the signaling speed through a given cable was inversely proportional to the square of the length of the cable. Thomson’s results were disputed at a meeting of the British Association in 1856 by Wildman Whitehouse, the electrician of the Atlantic Telegraph Company. Whitehouse had possibly misinterpreted the results of his own experiments but was doubtless feeling financial pressure as plans for the cable were already well under way. He believed that Thomson’s calculations implied that the cable must be “abandoned as being practically and commercially impossible.”  Thomson defended his own calculations and ended up spending many years sailing on cable-laying vessels and advising companies worldwide.

In December 1856, he was elected to the board of directors of the Atlantic Telegraph Company. Thomson sailed on board the cable-laying ship HMS Agamemnon in August 1857, with Whitehouse confined to land owing to illness, but the voyage ended after 380 miles (610 km) when the cable parted. Thomson contributed to the effort by publishing in the Engineer the whole theory of the stresses involved in the laying of a submarine cable, and showed that when the line is running out of the ship, at a constant speed, in a uniform depth of water, it sinks in a slant or straight incline from the point where it enters the water to that where it touches the bottom. Thomson developed a complete system for operating a submarine telegraph that was capable of sending a character every 3.5 seconds. He patented the key elements of his system, the mirror galvanometer and the siphon recorder,in 1858. Cable laying was completed 5th August 1858 despite numerous mishaps – including storms and cable breaks – and in the end the cable failed when Whitehouse sent 2000 V through it.

In July 1865, a new cable was authorized and Thomson sailed on the cable-laying expedition of the SS Great Eastern. The voyage was dogged by technical problems, and the cable was lost after 1,200 miles (1,900 km) had been laid, and so the project was abandoned. A further attempt in 1866 laid a new cable in two weeks, and then recovered and completed the 1865 cable. The enterprise was now feted as a triumph by the public and Thomson enjoyed a large share of the adulation.

Thomson took part in the laying of the French Atlantic submarine communications cable of 1869, and was engineer of the Western and Brazilian and Platino-Brazilian cables, assisted by James Alfred Ewing. He was present at the laying of the Pará to Pernambuco section of the Brazilian coast cables in 1873.

Thomson’s wife died on 17th June 1870, and he resolved to make changes in his life. Already addicted to seafaring, in September he purchased a 126-ton schooner, the Lalla Rookh, and used it as a base for entertaining friends and scientific colleagues. His maritime interests continued in 1871 when he was appointed to the board of enquiry into the sinking of HMS Captain. His interest was roused by the fact that new metal-hulled ships experienced compass errors that wooden-hulled ships were not prone to, and so he developed an improved compass that corrected for the errors. He also introduced a method of deep-sea depth sounding, in which a steel piano wire replaces the ordinary hand line. The wire glides so easily to the bottom that “flying soundings” can be taken while the ship is at full speed. In addition he added a pressure gauge to the sinker to register its depth.

For brevity I will pass over Thomson’s contributions to atomic theory, geology, and atmospheric electricity, and turn my attention to ice cream. Ice creams and sorbets were being produced by Persians and Mongols at least 2000 years ago.  The theory of making ice cream is not complicated.  Place the materials you want frozen into a metal container, submerse it in a mix of ice, salt, and water, and stir the mixture (in some fashion) until it freezes. The salt and water reduce the temperature of the ice to below that of the freezing temperature of the ice cream mix, and the constant stirring breaks up ice crystals as they form, so that the resultant product is smooth.  Easy-peasy. The issue is getting the ice in summer.

If you are a rich Persian prince you can pay workers to journey to the mountains, cut huge blocks of ice, wrap them in massive insulating layers of hay or straw, carry them back to your palace and use them for ice cream making, or bury them deep underground for later use.  This was the method used worldwide until the invention of the electric freezer at the beginning of the 20th century.  Once ice was more readily available, ice cream making became increasingly popular.

I will give you a recipe for mango ice cream, but with severe caveats. A lot hinges on ambient temperature and humidity when making the ice cream, and you need to experiment again and again with the proportions of ingredients to suit your tastes.  One of the tricks my wife discovered – based on recipes by Gaston Lenôtre – is that the butterfat content of the ice cream is most easily increased by adding a stick of butter to the custard before freezing it.  Using nothing but heavy cream simply makes the product gummy.  There is also the question of overrun – the amount of air trapped into the ice cream when churning it.  Some is necessary for a smooth texture, but too much makes the ice cream lose flavor and richness.  You have to make batch after batch after batch.  No worries – it’s all good.  Unless you stop me I will go on and on . . . and on about the difference between ice cream and Italian gelato, the wonders of Indian kulfi, the joys of sorbets, and so on. Here is a simple example of mango ice cream that most people with a freezer can enjoy. Replace the mangoes with peaches if you like.

Mango Ice Cream

Ingredients

2 large well-ripened mangoes, peeled and cut in small dice.
1 can (14 fl. oz) sweetened condensed milk
2 cups heavy cream

Instructions

Place the mango chunks in a food processor or blender, and process until they are the consistency of apple sauce (about 1 cup).

Place the mango pulp, condensed milk, and cream in a bowl and whisk on low speed until the mixture starts to thicken.  Then increase the speed to medium and whisk until stiff peaks form.

Transfer the mixture to a loaf pan, cover with plastic wrap, pressing the wrap down on to the surface of the mixture, and freeze for a minimum of 6 hours, or (preferably) overnight.

If you prefer, you can eliminate the second whisking and, instead, place the mixture in the freezer container of an ice-cream churn and process.  The resultant product tends to be rather soft, and does best if placed in a freezer for a few hours to set up fully.

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