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Across 1. A physicist who in 1821 discovered the thermoelectric effect. 6. He discovered a magnetic decennial period (ten-year cycle) and the electric current in the Earth closing the electric "circuit" creating the magnetic field in 1850. 8. His major contribution was an empirical formula for the visible spectral lines of the hydrogen atom. 11. The Hagenbach-Bischoff quota (a voting system) is named after him. 13. He developed the "Lummer-Brodhun-Würfel" (Lummer-Brodhun spectro-photometer) with Otto Lummer in 1889. 15. He played a direct role in the genesis of the quantum theory, providing the experimental results that pushed Max Planck to provide the first quantum hypothesis. 18. Recognized for his invention of the daguerreotype process of photography. 20. He is particularly well known for his work on rare earth elements.

Down 2. He was a pioneer of vacuum tubes, inventing the Crookes tube. 3. Proved that mercury vapor is a monatomic gas. 4. Who was the first to discover that a radioactive substance, later named radon, is emitted from radium. 5. Who invented the first dry galvanic cell. 7. He was elected president in 1892 of the Institute of Electrical Engineers. 9. The Bunsen–Kirchhoff Award for spectroscopy is named after him and his colleague, Robert Bunsen. 10. He edited the literary and scientific parts of the " Bibliotheque Universelle. 12. Who has been championed as an early developer of the use of X-rays for medical imaging. His contributions were largely neglected until the end of the 20th century. 14. From his published works, the most important ones were Theory of waves (1804) and Handbuch der Mechanik (1831; Handbook of mechanics). 16. Invented the Nicol prism. 17. His most prominent achievement was formulating classical electromagnetic theory. 19. Who discovered the element thallium independently from William Crookes in 1862.