Yearly Archives: 2012

The Calutron Girls

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One of the most difficult aspects of the Manhattan Project that built the first nuclear bombs was obtaining enough enriched uranium to make the bomb work. The enrichment of uranium took place at a site near the Oak Ridge National Laboratory and used three different methods: electromagnetic separation, gaseous diffusion and thermal diffusion. The gas centrifuge method of separation that is the modern standard could not be made to work at the time.

The final stage of enrichment was the electromagnetic separation stage that took place in a building known as Y-12; the output from the S-50 thermal diffusion plant and the K-25 gaseous diffusion plant (which at the time was housed in the world’s largest building by floor space) was used as input for Y-12.

Electromagnetic seperation was carried out on calutrons, which used giant electromagnets made of silver* to deflect the paths of ionised uranium-235 by a little more than ionised uranium-238. Initially these calutrons were operated by scientists from the University of California, Berkeley where the calutron was invented by Ernest Lawrence, but when a reasonable rate of return was achieved the operation of the calutrons was turned over to operators from the Tennessee Eastman Company.

These Tennessee Eastman operators were mostly women and all of them were only educated to high school level.

“The Calutron Girls”

Major General Kenneth Nichols, the man in charge of ore procurement and feed materials, pointed out to Ernest Lawrence that Eastman’s “hillbilly girl” operators were achieving better rates of production that his scientists and engineers had and a competition took place, with Eastman’s operators beating out Lawrence’s scientists. Nichols put this down to the fact that the girls were “trained like soldiers not to reason why … [whilst] the scientists could not refrain from time-consuming investigation of the cause of even minor fluctuations of the dials”.

During the operation of the calutrons the staff from Tennessee Eastman had no idea what they were doing: they operated switches and dials and monitored meters, but had no idea what those switches and dials did or were related to.

Gladys Owens, seated in the foreground of the photograph above, only discovered what her job was when taking a public tour of the K-12 facility fifty years later. Owens stated that she was told by a manager during a training session “We can train you how to do what is needed, but cannot tell you what you are doing. I can only tell you that if our enemies beat us to it, God have mercy on us!” and said that “Everywhere you looked it told you to keep your mouth shut!”.

Gladys Owens returns to K-12, fifty-nine years later.

* Normally copper would be used to construct electromagnets but this was in short supply due to the war. Kenneth Nichols met with the Under Secretary of the Treasury and arranged to borrow 13 300 tonnes of silver (worth about £170 million at today’s prices) from the US’s West Point Bullion Depository. After the war ended the silver was melted down and returned, with only a tiny fraction being lost in the process.

 

Baobab

Baobab is the common name of the eight species of tree of the genus Adansonia. Baobabs are well-known for their distinctive broad and swollen trunks.

Two photographs of Grandidier’s Baobab, Adansonia grandidieri.

All Baobab’s occur in arid regions, and survive by storing water (sometimes as much as 120000 litres) inside their trunks, making them a useful source of water for human populations. Their bark is used for cloth and rope, their leaves for medicines and condiments, their fruit (“monkey bread”) as food and their seeds to replace coffee. It is not for nothing that the Baobab is sometimes referred to as the Tree of Life.

The unusual trunk structure makes it very difficult to date Baobab trees, as they do not produce annual growth rings, but radiocarbon dating has shown some to be more than 2000 years old.

What is a ‘Retina’ display?

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Apple describes some of its products as featuring a “Retina” display. But what does that actually mean?

The individual pixels (each one made up of three red, green and blue subpixels) that make up my laptop’s display, viewed through a magnifier.

The main claim that Apple makes of its Retina display is that the pixels used are so small that they are too small to be seen individually by the human eye. In physics terms, this means that these pixels are below the resolving power of the human eye.

The resolving power of the human eye is about 60 arcseconds, or 0.0167 degrees. This means that any two objects separated by an angle smaller than this will appear as one object to the eye. The minimum vertical or horizontal spacing between two items which are visible as separate items is therefore given by dtan(θ) where d is the distance to the items and θ is the resolving power of the eye.

Assuming that the display in question is held or viewed at a distance of 30 cm from the eye, this distance is found to be 0.0873 millimetres. This means that a person with normal vision will be able to discern individual pixels on any display with fewer than 11.5 pixels per millimetre.

As can be seen from the graph above, the screen of the iPhone 4 does possess a greater density of pixels than the human eye can perceive; but the iPad 3 and the just-released 2012 MacBook Pro do not. (None of this matters of course, because “Retina” is just a trademark that Apple uses as a marketing term.)

An argument could be made, in the case of the MacBook Pro, that the distance between the screen and the eye would usually be larger than 30 cm. If the distance was 50 cm that would make the resolution of the eye 6.88 pixels per millimetre and therefore give the 2012 MacBook Pro a “true” retina display.

An astonishing photograph

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At the recent Institute of Physics Teachers’ Group AGM, Anu Ohja from the National Space Centre in Leicester drew our attention to this incredible photograph of the Apollo Lunar Module returning to dock with the Command/Service Module.

The amazing thing about this photograph is that it contains all of humanity with the exception of one person.

Everybody alive, except for Michael Collins, who took the photograph, appears in this photograph: the entire population of Earth, plus Neil Armstrong and Buzz Aldrin aboard the Apollo LM.