EMILE THELLIER

(1904-1987)a pioneer in studiesof the fossil Earths 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.

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 earths magnetic field.

Professor Thellier did not begin his studies in the University, but until 1926, at the cole Normale Suprieure de Saint-Cloud. After his military service (1926-27), he taught at the cole Primaire Suprieure 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 agrgation des Sciences Physiques in 1932, while he was assistant at the Facult des Sciences de Paris since 1931. He said then : I became a geophysicist

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 the direction but also the strength of the Earths field at the time of firing.

Working with clays from the famed Svres pottery factories, and later with volcanic rocks, Thellier minutely and exhaustively determined how thermo-remanent magnetization forms, producing a remarkable docteur s-Sciences thesis in 1938

During this period and until the 50s, 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 Facult des Sciences de Paris

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).

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

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

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

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 Nel and are known as the Thellier-Nel 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). Lets precise that the exact validity is limited to dispersed fine grains in the single-domain size, and for low fields.

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

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), Matre de Confrences (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 Gomagntisme which he moulded into one of the worlds leading rock magnetic and paleomagnetic research centres.

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, 1959 is still used today worldwide.

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 .

Emile Thellier's work revealed the non-cyclic secular variation of the earths 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.

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

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

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

I present the authors : Lucien Daly was the successor of Professor Thellier, after his retirement in 1974. He his now also retired. David Dunlop, who is a Canadian professor and very famous in the field of rock-magnetism, was also a great friend of the Thelliers laboratory, where he was visitor two times for a year and some shorter visits. Constantin Papusoi was an ancient foreing visitor from Romania, and is unfortunately died at the beginning of Jully. I had contacts with him until june. And myself, in Saint-Maur since 38 years, continuing the instrumental development on rock magnetism. We will see later some examples of his apparatus.Professor Thellier began his studies at the Ecole Normale Suprieure de Saint-Cloud (near Paris), and since 1928 he taught at the cole Primaire Suprieure de Bourges until 1930. He was always attracted by sciences, and while he continued his teaching, he began university studies in 1929, and in 1931, he obtained a position at the Facult des Sciences de Paris.CLICK..He said : I became a geophysicist He published his first results on the magnetism of clays in 1932The interest for magnetizations of rocks and baked clays was known some years before by works of Folgerhaiter in germany or Bruhnes in France, then others in Europa or Japan. The study of magnetizations of archaelogical baked clays is the topic that Professor Charles Maurain, Director of the Institut de Physique du Globe de Paris, proposed to Thellier for his thesis.CLICK...Lets recall there elements of the Earth magnetic field : Declination D, Inclination I, Intensity F.

During his thesis and until the 50s, Thellier was interested by a lot of other research fields in geophysics. He published, alone or with some collegues, articles on climatology, solar-terrestrial relationships, magnetic explorations methods, atmospheric electricity, ionospheric investigations, oceanography In example, Commander Charcot, during the 1935 Arctic voyage of the "Pourquoi-Pas?", entrusted to him the task of carrying out regular monitoring of short-wave radio emissions. Unfortunately, The ship Pourquoi Pas ? sank the following year, in 1936, during a storm. Only one sailor survived.CLICKA lot of studies was done in collaboration with his wife Odette Thellier

Here is the magnetometer he constructed during his thesis.The principle is that of a compass, which deviate when a magnetic piece is put nearby. The calculation of the magnetization requires a geometric shape of the sample. Astatism is obtained by gluing three magnets, one central large and two others, with half magnetic moment in opposite sense and equally distant from the central, resulting in a null total moment. The sample is translated near the central magnet where the sensitivity is maximum.

Here is the reversal inductometer he constructed during his thesis.The principle is that of a fluxmeter where the magnetic object is moved inside a set of coils, producing a magnetic flux variation, then an induced current which is measured with a ballistic galvanometer. If the homogeneity volume is large enough, the sample can take any form.CLICKIn this instrument, the sample, tight in a grip, is quickly fully reversed by hand. The Use of four equal-surface coils, the central two in a sense, the two others in opposition, results in a null total magnetic surface, which is the condition for obtain insensitivity to the laboratory field variations.

Here is the model he designs later, in the 50s.In this instrument, the sample is continuously rotating, producing an alternating voltage. In order to increase the accuracy, this inductometer was used as a zero detector. Under the sample, two perpendicular small coils, very precisely constructed, supplied by currents through a rotating switch, produce two perpendicular magnetic moments, exactly the opposite of those on the rotating plane of the sample. Axis permutations of the sample bring us the three components.A new model of this instrument was built later, in 1975, and is always running now. With big samples, precisely oriented when taken on the field, the accuracy of directional measurements on archaeological artifacts or volcanic products, is one of best in the world.These laws can be expressed by the five following properties.

MagnetiteSuperpara < 0.03Single domain > 0.03, 0.2, 110 A TRM (called total TRM) is obtained by cooling until ambient from a high temperature more than or equal to the Curie temperature of the mineral concerned (580 for Magnetite, 670 for Hematite)A partial TRM is created when the field is applied only inside a limited temperature interval, less than the Curie temperature of course. The first property is the proportionality to the field; the second concerns the directional fidelity; and the latter three shows the total independence and autonomy of each partial temperature interval in which any (but weak) magnetic field can be applied or not, in any directions.This is really the result of an exceptionally precise work, obtained essentially thanks to the special instrumentation that Thellier has designed.This method compares the natural remanent magnetization (NRM) to a thermo-remanent magnetization acquired in laboratory with a known field. The ratio between both magnetizations allows the knowledge of the ancient field. Thellier established a step by step procedure with intermediate monitoring of partial TRM, assuming the check on possible mineralogical modifications during heatings, or multi-domain behaviour, which is not suitable for such intensity determination.The figure shows a comparison (without geographical reduction) of the results obtained by three laboratories in the beginning of the sixties. (attention lchelle des champs, 0.4 0.8 Gauss)Thellier waited, year after year, before publish these results He was always hoping that archaeologists would be able to precise their datations, whereas, on the contrary, some of them was more interested by the reverse question : Can you date my artifacts ?This figure represents both declination and inclination on spherical equal area projection.Determinations of direction and intensity of the Secular variation of the Earth Magnetic Field are always pursued in a lot of laboratories over the world. This knowledge during a multi-millennium period is of first importance in order to constrain the theoretical parameters of the terrestrial dynamo.The use in dating archeological artifacts or volcanic products is now possible, in some countries, where a precise master curve has been established.Thellier has also discovered the phenomenon of magnetic viscosity, which is a spontaneous magnetization acquired during extended influence of a magnetic field, even as weak the magnetic terrestrial field. This viscous remanent magnetization, for geological time, can have a high value, more than some detrital ones.Thellier have published a lot of articles and some big ones, but only very little in English.Here in April 1968, with Dr Nagata, at Newcastle University.