Gustav Kirchhoff

Who Read The Sun's Chemistry · Physicist · 1824–1887

Who published the two rules that solve any electrical circuit while still a student, and then read the chemical composition of the Sun out of the dark lines in its spectrum.

Who was Kirchhoff?

Gustav Robert Kirchhoff was born at Königsberg in 1824 and published the work he is best known to schoolchildren for while he was still a student there: two rules for electrical networks, set out in 1845. The first says that the currents arriving at any junction equal the currents leaving it; the second that the voltages around any closed loop sum to zero. They follow from conservation — of charge in the first case and of energy in the second — and between them they allow a circuit of any complexity to be solved, which a simple proportionality cannot do. Kirchhoff himself regarded this as minor. The work he cared about came at Heidelberg with the chemist Robert Bunsen, where the two of them established spectrum analysis: every element, heated until it glows, emits a characteristic set of spectral lines, so a substance can be identified by its light alone. Kirchhoff then made the inference that mattered. The dark lines that Fraunhofer had mapped in the Sun's spectrum decades earlier lay at exactly the wavelengths where known elements emit — they were those elements absorbing. The chemical composition of a body ninety-three million miles away had been determined without anybody going near it. He also stated the law relating thermal emission and absorption, and coined the term black-body radiation, leaving the problem that Planck would take up thirteen years after Kirchhoff's death.

Major achievements

• Published the junction rule and the loop rule for electrical networks in 1845, as a student
• Derived both from conservation of charge and conservation of energy
• Founded spectrum analysis with Robert Bunsen, identifying elements by their spectral lines
• Determined the chemical composition of the Sun from its absorption spectrum
• Stated Kirchhoff's law of thermal radiation and coined the term black-body radiation

Life in brief

• 1824 — Born at Königsberg: Son of a lawyer; taught by Franz Neumann.
• 1845 — The two circuit rules: Published while still a student, aged twenty-one.
• 1854 — Heidelberg: The beginning of the collaboration with Bunsen.
• 1859 — Kirchhoff's law of radiation: Emission and absorption related; the black-body idea named.
• 1860 — Spectrum analysis: Every element identified by its own lines; caesium and rubidium found.
• 1861 — The Sun's composition: The dark lines read as known elements absorbing.
• 1875 — Berlin: A chair in mathematical physics, his health having failed.
• 1887 — Dies at Berlin: Planck answers his black-body question thirteen years later.

Explore the life of Kirchhoff — five chapters

1. Twenty-One — Two rules, published as a student
2. Heidelberg, With Bunsen — Why coloured glass fails and a prism does not
3. The Dark Lines in the Sun — Fraunhofer's puzzle, answered
4. The Black Body — A problem posed and left for somebody else
5. On Crutches — A body that failed, and a mind that did not

Read the five-chapter life of Gustav Kirchhoff →

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