EMILE THELLIER (1904-1987) a pioneer in studies of the “fossil” Earth’s magnetic field.

1. EMILE THELLIER(1904-1987)a pioneer in studiesof the “fossil” Earth’s magnetic field. Maxime LeGoff, Lucien Daly, I.P.G.P. St-Maur, France David Dunlop, University of Toronto, Canada Constantin Papusoi , University Alexandru Ioan Cuza, Iasi, Romania.

2. Throughout his long scientific career, Thellier devoted his life to the study of rock magnetism and its applications in geophysics, geology and archaeology. He developed very sensitive and accurate instruments that allowed measurements to be made undisturbed by the presence and the variations of the earth’s magnetic field.

3. Professor Thellier did not begin his studies in the University, but until 1926, at the École Normale Supérieure de Saint-Cloud. After his military service (1926-27), he taught at the École Primaire Supérieure de Bourges,in the center of France.His love for sciences led him to begin university studies in 1929, as a non-registered student. He brilliantly obtained “agrégation des Sciences Physiques” in 1932, while he was assistant at the “Faculté des Sciences de Paris” since 1931. He said then : “I became a geophysicist”

4. Some years before, some researchers have shown the interest of remanent magnetizations of volcanic rocks and baked clays, as Folgerhaiter from 1899 Brunhes in 1905, and others… His master, Charles Maurain, proposed to Thellier the idea of using the thermoremanent magnetization of ancient pottery to deduce not only thedirection but also the strength of the Earth’s field at the time of firing.

5. Working with clays from the famed Sèvres pottery factories, and later with volcanic rocks, Thellier minutely and exhaustively determined how thermo-remanent magnetization forms, producing a remarkabledocteur ès-Sciences thesis in 1938

6. During this period and until the 50’s, Emile Thellier was not only interested by rock-magnetism, but also by some other areas on geophysics : climatology, solar-terrestrial relationships, magnetic explorations methods, atmospheric electricity, ionospheric investigations, oceanography (he was on board the “Pourquoi-Pas ?”, with the Commander Charcot during the 1935 voyage in Arctic Ocean). A lot of studies was done in collaboration with his wife Odette Thellier, who was also a professor at the Faculté des Sciences de Paris

7. The rock-magnetism was then new, and only some laboratories was developing such studies over the world, in the US, Japan, England, Russia, France… All the apparatus needed have had to be designed and constructed by researchers and Emile Thellier was one of the most fertile designers. He builds very sensitive magnetometers and inductometers, always keeping in mind the protection against the laboratory field variations and the possibility for measuring big samples without cutting (specially for precious pieces).

8. 50cm Astatic symmetric Magnetometer with coils, for measurement of remanent and induced magnetization on cylindrical samples. A furnace allows heating with or without low field central magnet(+M) external (-M/2) cylindrical sample

9. 0.7 meter Astatic reversal Inductometer, for measurement of remanent magnetization on very big samples (more than 25 cm without cutting)

10. And one of the most famous, the big sample spinner inductometer, for measurement of remanent magnetization on decametric sized sample. 40cm

11. In addition with large furnaces in controlled-field, and alternating high-field demagnetizer, Emile Thellier discovered the laws of “Magnetic Memory”, which were later confirmed theoretically by Louis Néel and are known as the Thellier-Néel laws : The baked clay retains a memory of the temperature, and of the direction and intensity of the field that was responsible for its thermoremanent magnetization (TRM).Let’s precise that the exact validity is limited to dispersed fine grains in the single-domain size, and for low fields.

12. 1)TRM is proportional to the field for weak fields. 2) Partial TRMs are parallel to the field H in which they were acquired. 3)A partial TRM acquired by cooling in H through the temperature interval (T2, T1<T2) and in zero field elsewhere is unaffected by reheating in zero field to a temperature ≤ T1 and completely disappears after reheating to T2. 4)A partial TRM (T2, T1, H) is independent of other partial TRMs acquired in temperature intervals outside (T2, T1), which may be due to fields differing fromH. 5)All partial TRMs add geometrically but each of them retains true autonomy and an exact memory of the temperatures and field in which they were acquired. The first two properties were known or assumed before Thellier’s time. The latter three are entirely novel.

13. These important results, which have lost none of its pertinence today, launched his academic career at the Institut de Physique du Globe de Paris, where he became successively Physicien-adjoint (1943), Maître de Conférences (1945), Professor (1948)and Director (1954-66). Later, from 1967, he moved his research to the Observatoire du Parc Saint-Maur, in the suburbs of Paris, where he established a CNRS Laboratoire de Géomagnétisme which he moulded into one of the world’s leading rock magnetic and paleomagnetic research centres.

14. His work led to the field of archeomagnetism : in close collaboration with his wife he developed the first method to study the intensity of the earth's magnetic field in the recent past and used it on a large number of archeological sites. Their method known as Thellier, Thellier, 1959is still used today worldwide.

15. His last article, in PEPI-1980, shows the results on Earth magnetic field direction, in France, between 0 and 1800 AD, obtained with more than 200 ovens and 50 sets of bricks .

16. Emile Thellier's work revealed the non-cyclic secular variation of the earth’s magnetic field and the changes in its strength over the last 25 centuries. The influence of professor Thellier expand not only on archeomagnetism, but also on rock magnetism, physical investigations and paleomagnetism studies, and their applications in geology and geophysics.

17. His students, each one concerned by a particular problem on rock-magnetism :anisotropy, piezomagnetism, field reversals, magnetic viscosity, thermal, crystallization or detrital magnetization…, formed the “Thellier school”, and with many foreign visitors, established fruitful links to the broader international paleomagnetic community

18. Emile Thellier receives many honours in his lifetime. His first love, to the end of his life, remained his research and the laboratory he had created. His heritage continues undiminished

19. Recent archeomagnetic results obtained in France (directional master curve of 200 results) and in Italy (70 individual results).