Barometer of Bohr

See also: Mef=page of homonymy: [[Barometer (homonymy)]]

The history of the calculation of the Height of a Immeuble using a Baromètre by Niels Bohr is a urban Légende. It describes the inventiveness of the Niels young person, to answer a statement of physics subjected by one of its professors, of the technically right solutions, but intentionally except subject.

This history in fact would have been written in the Reader' S Digest in 1958, and the history would have been transformed with the wire of time into an anecdote presumedly real and allotted to Niels Bohr. One can wonder whether the recourse to this famous person is not a manner of transforming an anecdote amusing into a lampoon against the “rigidity of secondary education” opposed to the “creativity”.

History

I received a telephone call of a colleague in connection with a student. He estimated that he was to give him one zero to a question of physics, whereas the student claimed one 20. The professor and the student are reflected agreement to choose an impartial referee and I was selected. I read the question of the examination:

" Show how it is possible to determine the height of a building using a baromètre"

The student had answered: " One takes the barometer in top of the building, one attaches a cord to him, one makes it slip to the ground, then it is gone up and the length of the cord is calculated. The length of the cord gives the height of the immeuble".

The student was right considering which he had answered just and completely the question. On another side, I could not put his points to him: in this case, it would have received its rank of physics whereas it had not shown me knowledge in physics. I proposed to give another chance to the student by giving him six minutes to answer the question with the warning that for the answer it was to use his knowledge in physics. After five minutes, he had not still written anything. I asked to him whether he wanted to give up but he answered that he had many answers for this problem and that he sought the best of them. I excused myself to have stopped it and asked him to continue. In the minute which followed, he hastened to answer me:

- One places the barometer at the level of the roof. One drops it while measuring his drop time with a Chronomètre. Then by using the formula: $X\; =\; \backslash \; frac\; \{G\; t^\; \{2\}\}\; \{2\}$, one finds the height of the building.

At this time, I asked my colleague if he wanted to give up. He answered me by the affirmative and gave almost 20 to the student. By leaving his office, I recalled the student because he had said that he had several solutions with this problem.

- He well, says it, there is several way of calculating the height of a building with a barometer. For example, it outside is placed when there is sun. One calculates the height of the barometer, the length of his shade and the length of the shade of the building. Then, with a simple calculation of proportion, one finds the height of the building.

- Well, I answered him, and the others.

- There is a rather basic method which you will appreciate. One assembles the stages with a barometer and at the same time one marks the length of the barometer on the wall. By counting the number of features, one at the level of the building in length of barometer. It is a very direct method. Of course, if you want a method more sophisticated, you can hang the barometer with a cord, to make it balance like a pendulum and to determine the value of G on the level of the street and the level of the roof. From the difference in G the height of the building can be calculated. In the same way, one attaches it to a large cord and while being on the roof, one lets it go down until little close the level from the street. One makes it balance like a pendulum and one calculates the height of the building as from the period of précession." Finally, he concludes:

- There are still other ways of solving this problem. Probably the best is of going to the basement, knocking on the door of the caretaker and to say to him: " I have for you a superb barometer if you me known as which is the height of the immeuble".

I then asked the student if he knew the answer until I waited. He admitted that yes but that he had some enough of the university and the professors who tried to teach him how he was to think.

For the anecdote, the student was Niels Bohr (Nobel Prize Physical in 1922) and the referee Ernest Rutherford (Nobel Prize Chemistry in 1908).