EMILE THELLIER (1904-1987), a pioneer in studies of the "fossil" Earth's magnetic field.

Professor Emile Thellier was born in northern France in 1904 and passed away on May 11, 1987. Following studies at the Ecole Normale Superieure de St. Cloud he received his Doctorate from the University of Paris in 1938 for his work on the thermoremanent magnetization of baked clays and its application in geophysics. Ten years later he was named Professor at the Faculty of Sciences of the same university. From 1956 to 1966, succeeding Charles Maurain and Jean Coulomb, he held the position of Director of the Institut de Physique du Globe de Paris. Thellier received many honours in his lifetime. Throughout his long scientific career, Thellier devoted his life to the study of rock magnetism and its applications in geophysics, geology and archaeology. In the early phase of his career he developed very sensitive and accurate instruments that allowed measurements to be made undisturbed by the presence of the earth’s magnetic field. Using magnetometers of his invention in a systematic study of the magnetisation of rocks, he discovered the laws of "magnetic memory", which were later confirmed theoretically by Louis Neel and are now known as the Thellier-Neel laws: they state that 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. His work led to the field of archeomagnetism : in close collaboration with his wife he developed the first method to study the earth's magnetic field in the recent past and used it on a large number of archeological sites in Europe and North Africa. Their method is still used today worldwide. Improvements and refinements of the Thelliers’ method continue to be made in the laboratory he created at the Observatoire du Parc Saint-Maur, now reunited with the Institut de Physique du Globe de Paris. 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 and demonstrated the wealth of information contained in rocks that can be used for archeological dating and understanding geomagnetism.