# GUSTAV KIRCHHOFF (1824- 87)

1845 – Germany

‘First law (Junction law): At any junction point in an electrical circuit, the sum of all currents entering the junction must equal the sum of all currents leaving the junction’

‘Second law (Loop law): For any closed loop in an electrical circuit, the sum of the voltages must add up to zero’

In equation form the first law is I = I1 + I2 + I3 + I4 +…. where I is the total current and I1, I2, I3 etc. are the separate currents.

Second law is V = V1 + V2 + V3 + … where V is the total voltage and V1, V2, V3 etc. are the separate voltages.

GUSTAV KIRCHHOFF

These laws are an extension of OHM‘s law and are used for calculating current and voltage in a network of circuits. Kirchhoff formulated these laws when he was a student at the University of Konisburg.

Kirchhoff also showed that objects that are good emitters of heat are also good absorbers. This is Kirchhoff’s law of radiation.

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ELECTRICITY

# ROBERT BUNSEN (1811- 99) GUSTAV KIRCHHOFF (1824- 87)

1860 – Germany

‘Each chemical element, when heated to incandescence, produces its own characteristic lines in the spectrum of light’

For example, sodium produces two bright yellow lines.
Bunsen developed the Bunsen burner in 1855.
In the flame test the Bunsen burner’s non-luminous flame does not interfere with the coloured flame given off by the sample.

Kirchhoff was a professor of physics at Heidelberg. Bunsen and Kirchhoff together developed the first spectroscope, a device used to produce and observe a spectrum. They used their spectroscope to discover two new elements, caesium (1860) and rubidium (1861).

In 1860 Kirchhoff made the discovery that when heated to incandescence, each element produces its own characteristic lines in the spectrum.

This means that each element emits light of a certain wavelength – sodium’s spectrum has two yellow lines (wavelengths about 588 and 589 nanometres). The Sun’s spectrum contains a number of dark lines, some of which correspond to these wavelengths.

The Swedish scientist ANDERS ANGSTROM had, four years earlier, found that a gas always absorbs light at the same wavelength that it emits light. If the gas is hotter than the light source, then more light is emitted by the gas than absorbed, creating a bright line in the spectrum of the light source. If the gas is cooler than the light source the opposite happens; more light is absorbed by the gas than is emitted, creating a dark line.
The dark solar D lines told Kirchhoff that sodium is present in the relatively cool outer atmosphere of the Sun. This could be tested in the laboratory by burning a piece of chalk in a hot oxygen-hydrogen torch. The intensely bright limelight that is produced may be passed through a cooler sodium flame and the light emerging examined through a spectroscope. Crossing the spectrum of the artificial light occur black lines at the same wavelength that a sodium flame emits light. This solved the mystery of the FRAUNHOFER LINES.

Scientists now had a means to determine the presence of elements in stars. By comparing the dark lines in the spectra of light from the stars with the bright lines produced by substances in the laboratory, Kirchhoff had been able to identify the elements that made up a celestial body millions of miles away in space.

In England the astronomer William Huggins recorded the spectra of hundreds of stars and showed the unmistakable fingerprints of familiar elements that are found on the Earth’s surface. The stars are made of exactly the same kind of atoms as the Earth.

In 1868 Norman Lockyer described a spectral line in the yellow region very close to the wavelength of the two ‘D’ spectral lines of sodium. After repeated attempts to discover a substance that produced the same line on Earth, it appeared that the line did not correspond to any hitherto known element. Lockyer gave the element the name ‘helium’, the gas later to be found associated with radioactive decay in ores containing uranium.

Helium had not previously been found on Earth because it is both inert and lighter than air, ironic because after hydrogen, helium is the second most common element in the universe.

In 1904 RUTHERFORD would declare that the presence of helium in the Sun was evidence that sunlight was a product of radioactive processes. The absence of any FRAUNHOFER lines in sunlight that corresponded to radium dealt a blow to this hypothesis. Was there another way of releasing atomic energy than radioactivity?

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THE STARS

# CARL GAUSS (1777-1855)

1832 – Germany

GAUSS

An electric field may be pictured by drawing lines of force. The field is stronger where these lines crowd together, weaker where they are far apart. Electrical flux is a measure of the number of electric field lines passing through an area.

‘The electrical flux through a closed surface is proportional to the sum of the electric charges within the surface’

Gauss’ law describes the relationship between electric charge and electric field. It is an elegant restatement of COULOMB‘s law.

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MATHEMATICS

# GOTTFRIED LEIBNIZ (1646-1716)

1684 – Germany

‘A new method for maxima and minima, as well as tangents … and a curious type of calculation’

Newton invented calculus (fluxions) as early as 1665, but did not publish his major work until 1687. The controversy continued for years, but it is now thought that each developed calculus independently.
Terminology and notation of calculus as we know it today is due to Leibniz. He also introduced many other mathematical symbols: the decimal point, the equals sign, the colon (:) for division and ratio, and the dot for multiplication.

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MECHANICS

# FRIEDRICH OSTWOLD (1853-1932)

1894 – Germany

‘A catalyst can change the rate of a chemical reaction, but is not itself used up in the reaction’

WILHELM OSTWALD

The effect of a catalyst is known as catalysis.
The action of a catalyst is specific (a particular catalyst is necessary to catalyse a given reaction) and it can increase or decrease the rate of the reaction.

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CHEMISTRY

# HEINRICH SCHWABE (1789-1875)

1843 – Germany

‘The number of visible sunspots varies in a regular cycle that averages about 11 years’

GALILEO was the first to study sunspots. Schwabe made careful records of sunspots almost daily for 17 years before announcing his theory. He continued his observations for another 25 years.

Wherever magnetic fields emerge from the sun, they suppress the flow of surrounding hot gases, creating relatively cool regions that appear as dark patches in the sun’s shallow outer layer, the photosphere.

Sunspots vary in size from 1000 to 40,000 kilometres across and may last from a few days to many months.

Near a solar minimum there are only a few sunspots. During a solar maximum, solar flares can produce dramatic changes in the emission of ultraviolet rays and X-rays from the sun.

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THE STARS

# OTTO HAHN (Germany 1879-1968) LEIS MEITNER (Austria 1878-1968) FRITZ STRASSMANN (Germany 1902- 80)

1938 – Germany

‘Nuclear Fission. The breaking up of the nucleus of a heavy atom into two or more lighter atoms. Energy is released during the process’

A reinterpretation of the results of the mid 1930s neutron-bombarding experiments of ENRICO FERMI with uranium offered an alternative explanation to Fermi’s own idea that the uranium had transmuted into new heavier elements. The three German scientists offered the explanation that the uranium nucleus had in fact been broken down into a number of smaller nuclei

with the release of potentially huge amounts of energy under the rules of Einstein’s formula E = mc2.

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LISE MEITNER