T - Factary

Temperature

Although these days it’s a relatively simple thing to measure the temperature of anything, it is quite difficult to define exactly what temperature itself is.

We know from experience that hot things (from the oven say) will eventually cool down and that cold things (from the freezer) will warm up. What is happening is that in both cases the object is reaching equality (technically it is called thermal equilibrium) with its surroundings (the air around it). We can therefore define temperature as that quantity which is the same for both things (the initially hot/cold object and the air) when they have settled to thermal equilibrium.

There have been many different scales and devices created for measuring temperature. The three most commonly used today are the Fahrenheit, Celsius (or Centigrade) and Kelvin scales.

One of the first attempts to make a standard temperature scale happened about AD 170. Galen, in his medical writings, proposed a standard "neutral" temperature made up of equal quantities of boiling water and ice. On either side of this temperature were four degrees of heat and four degrees of cold. Crude maybe, but at least it was an attempt to make heat measurable.

The earliest devices used to measure temperature were called thermoscopes. They consisted of a glass bulb having a long tube extending downward into a container of coloured water.

So where did all this Celsius, Centigrade, Fahrenheit and Kelvin stuff come from?

It's a tale that starts with three scientists; a Dane, a Dutchman and a Swede at around the beginning of the 18th century and ends with a Belfast-born 'Scot' in the middle of the 19th century. Very cosmopolitan. Their names were Roemer, Fahrenheit, Celsius and Thomson. It begins to sound familiar. So were Roemer and Thomson the only ones to miss out in historical terms because I've never heard of Roemer or Thomson degrees? Not really. Roemer had other strings to his bow, as they say, and Thomson changed his name.

Roemer was an astronomer and was the first person to measure the speed of light by observing the satellites of Jupiter - not an insignificant contribution to science.

However he was also one of the first to successfully produce a "temperature measuring thingy" (a thermometer to you and me). He created a glass tube with a glass bulb at the end - pretty much as we have today. He filled it with strong wine, coloured with saffron to make it easier to see or less likely to be drunk? He knew that temperature could be measured by recording the volume of the liquid by seeing how far up the thin tube the liquid rose when heated to different temperatures (liquids expand when they are heated). In practice he took a measurement of freezing water, another of boiling water and then divided the scale into seven parts. He added an eighth part, as big as the others, below the freezing point and called it zero degrees. His boiling point was at 60 degrees.

Why would he choose 60 degrees as the top end? Could it have been anything to do with there being 60 minutes in an hour?

Sixty is actually a pretty convenient number for doing simple arithmetic. It is easy to calculate fractions of it because it has 10 factors (2,3,4,5,6,10,12,15,20,30) - numbers which can divide into it exactly. However, 50 only has 4 factors and 100 only 7. That probably helps explain why there are 60 minutes in an hour and so Roemer was just following common sense tradition.

So now he had a scale divided into 8 parts going from 0 to 60 - so each part was 7.5 degrees. The freezing point of water was at 7.5 degrees. On this scale his experiments showed that human body temperature was pretty constant and equal to about 22.5 degrees.

Enter Mr Fahrenheit. Roemer showed him one of his new thermometers which used his temperature scale but only measured up to about the temperature of the human body, which on his scale remember was 22.5 degrees. Building such a device makes sense - after all how often do you need to measure temperatures higher than that? Fahrenheit realised the potential of Roemer's device but didn't like fractions - who does? So he set about refining it. First of all he divided Roemer's scale more finely (to get rid of the fractions) so that one degree in Roemer's scale now equaled 4 new Fahrenheit degrees. He reset the zero degrees to be exactly where a mixture of ice, water and salt freezes. I suppose that was about the coldest thing he could easily produce and so now he had a scale from 0 (ice, water, salt) to 90 (human body, = 22.5 x 4) degrees, with pure water freezing at 30 (= 4 x 7.5) degrees.

He was happy with that for a while but eventually figured that mercury was a better liquid to deal with in these devices rather than Roemer's alcohol, which was of variable quality from thermometer to thermometer, and also he didn't like dividing by 3 rather than 2 so he reset the scale again so that water froze at 32 degrees and the human body temperature was 96 degrees.

After Fahrenheit's death it was realised that human body temperature can vary and is therefore not a good scale point so once again the boiling point of water was used and set to 212 degrees (close to Fahrenheit's original value of 205 degrees) for convenience of dividing the scale.

Meanwhile, a fresh start had been made by the Swedish scientist Celsius - he had no hang-ups about the number 60 and built a device where there were 100 degrees between the freezing and boiling points of water. Sounds an obvious thing to do, right? All the best ideas sound obvious afterwards.

One final twist in the tail - Celsius actually set freezing point at 100 degrees and boiling at 0 degrees. All scientists can be mad once in their lives. That was reversed after his death to be the more natural way around we know today and in 1948 the temperature scale was officially named the Celsius scale (the term centigrade had come into common use) at the Ninth General Conference of Weights and Measures.

So where does Thomson come into all of this? William Thomson was born in Belfast in 1824, but his family moved to Glasgow when he was six. He went to Glasgow university at the ripe old age of 10, still a record. After doing some fundamental work on heat and temperature he devised a temperature scale similar to that of Celsius. However, his scale started at -273.16 °C. There was a very good reason for that. He had worked out that that was the temperature at which matter (molecules and atoms) would have no kinetic energy and therefore no movement. For this reason this temperature is referred to as 'absolute zero' and is the coldest anything in the universe can get.

Notice that the unit of temperature on Kelvin’s scale is one kelvin NOT one degree, so it is different in that respect from Celsius and Fahrenheit. The correct expression is therefore, ‘the temperature of the Sun’s surface is about 6000 kelvin’ not ‘the temperature of the Sun’s surface is about 6000 degrees kelvin’.

For his work in physics, Thomson was knighted and eventually created Baron Kelvin of Largs, which is where the unit of temperature gets its name.

So there you are, you can take your choice and say:

'Phew it's hot today, it's 89 degrees Fahrenheit'

'Phew it's hot today, it's 32 degrees Celsius'

'Phew it's hot today, it's 305 kelvin'

Tera (t)

A prefix indicating one million million, or one thousand billion, of something. In numbers that is 1,000,000,000,000 or 10¹²

When used before a unit abbreviation, it is abbreviated to ‘t’. For example tW indicates a terawatt - that's a very big light bulb.

One common use of the prefix tera today is in Information Technology where very large computer memory or disk space is discussed e.g. a terabyte of memory. But (as ever) we have to be a little careful here. Kilo, mega, giga and tera are all prefixes that normally refer to powers of 10; a kilometre is 1000 or 10³ metres. However computers and computer people work in powers of 2. So in computer-speak a kilobyte is 2¹⁰ (=1024) bytes rather than 10³ (=1000). The bigger the numbers, the bigger the difference. By the time we reach 'terabyte' we might think we are talking about:

10¹² = 1,000,000,000,000 bytes

but actually the number of bytes will be

2⁴⁰ = 1,099,511,627,776 bytes (nearly 10% more)

Can you work out the difference between the expected and actual size of a petabyte?

Thermonuclear Fusion

The process by which the nuclei of atoms can join together so that the mass of the joined-up nucleus is slightly less than the mass of the parts that went into it. The energy originally locked up in the mass that is ‘lost’ is transferred into radiation and particle energy by the famous E=mc² equation.

In the Sun, atomic nuclei (nucleuses) of hydrogen combine to form helium. The same process happens in a hydrogen bomb. Many laboratories are also trying to make fusion work in a controlled way (bombs are generally considered to be uncontrolled). If this succeeds it will provide a very convenient and pollution-free source of energy.

The word fusion is used in biology as well. It is used to describe the event when two cells combine (or stick) together.

When things 'split apart' the word 'fission' is used. When atomic nuclei split (the famous 'splitting the atom') energy is also given out in vast amounts. This is called 'Thermonuclear Fission'. Uncontrolled, it is the way in which an atomic bomb works. An atom bomb is used to set off a hydrogen bomb (which uses fusion) because you need very high temperatures to get the nuclei fusing together.

More peacefully, controlled nuclear fission is the way in which our present atomic power stations work. The energy given off is used to boil water into steam, which in turn drives the electrical generators.

Thomson, Joseph John (1856-1940)

English mathematician and physicist who graduated from Cambridge before working at the Cavendish laboratories. Most of his work was in the field of electricity, but he is most famous for discovering the electron. He also deduced a surprisingly accurate estimate of its mass (1/2000th the mass of a proton - now known to be 1/1836th). In 1906, he received the Nobel prize for Physics for this discovery.

Thomson, William (1824-1907)

William Thomson (aka Lord Kelvin)

Famous physicist and mathematician born in Belfast who was brought up as a strict Presbyterian. He went to Glasgow university from the age of 10 and studied astronomy, chemistry and natural philosophy (physics). William entered Cambridge university at the age of 17. He graduated from Cambridge with the second highest honours in his year: quite an achievement. Eventually he became professor of natural philosophy at Glasgow university where he had a notorious and marathon letter-writing exchange with Stokes, another famous scientist of the time, over the similarities between heat and fluids. This lead William to come up with the scale of temperature for which he is most famous, the Kelvin scale. It was called the Kelvin scale because William was later appointed Lord Kelvin by Queen Victoria.

TRACE

This is the name of a satellite launched into Earth orbit in April 1998. It studies the Sun's outer atmosphere by taking very detailed images at several wavelengths. The acronym 'TRACE' stands for Transition Region And Coronal Explorer.

Transition Region

A region in the Sun's atmosphere between the chromosphere and the corona. Since the Sun's temperature rises from 10 000 ^oC to over 1 000 000 ^oC through this region, it's called the Transition Region because it's where the temperature is in transition from fairly cool to very hot.

Transverse Wave

In a transverse wave the particle displacement is perpendicular to the direction of wave propagation.

In this demonstration (by Dr. Dan Russell Kettering University Applied Physics) you can see the wave travelling through the medium, but if you concentrate on an individual dot in the medium, you’ll see that it doesn’t go anywhere.

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