Absolute Gravimetry at BIPM, Sèvres (France), at the Time of Dr. Akihiko Sakuma

Abstract

The Bureau International des Poids et Mesures (BIPM) was created and established in 1875 at Sèvres, (France). The history of Absolute Gravity (AG) measurements at BIPM started soon after and can be divided into three periods. The 1st period went from 1886 to 1960, starting with the decision of CIPM to measure the gravity acceleration at BIPM. First AG measurements were made with a time device: i.e., a pendulum. But as soon as the end of World War I, Amédée Guillet considered monitoring the free fall of an object; a concept also adopted by Charles Volet and Åke Thulin who elaborated an in situ instrument. The second period started in February 1960, when Akihiko Sakuma arrived at BIPM. A. Sakuma was at that time a 29 year old scientist, coming from the National Laboratory of Metrology in Japan. He had already constructed an absolute gravimeter, observing the free fall of a divided line scale with a chronometer of his own invention. His first observation of the "g" value was obtained in 1959, at Tokyo. At BIPM Akihiko Sakuma started to develop a new gravimeter, based on the symmetrical free fall of a reflecting body. For that purpose, he designed new original parts: catapult, seismometer,... and obtained his first result in October 1966, increasing by two orders of magnitude the precision of former measurements. Thanks to this precision, his observations led to the first observation of the solid earth tides with an absolute instrument. An improved copy of this instrument was installed at the International Latitude Observatory of Mizusawa (Japan), in order to establish new international reference absolute gravity acceleration stations, following the recommendations of the IAG. The BIPM gravimeter was a stationary one, and A. Sakuma worked continuously on the development of a transportable gravimeter. Between 1968 and 1975 he collaborated with the National Institute of Metrology in Turin (Italy), to finalize a transportable absolute gravimeter aiming to calibrate the European Gravity Network. Then between 1978 and 1983, A. Sakuma collaborated with the French Jaeger Co., to produce a transportable absolute gravimeter (GA-60), according to BIPM ideas. The BIPM copy of this instrument measured "g" to 10-9, for the first time. It was also used to establish the 0th order network of the French Gravity Network. We enter then in the world of geophysics and geodesy to which A. Sakuma gave precise foundations. He retired from BIPM in 1996, and died in 2004, at the age 73. By that time, BIPM was in its third period of AG measurements, renouncing its efforts to develop AG instruments and acquiring in 1993 the commercial FG5#108. From then the instrument plays the role of a stationary instrument, monitoring the time variation of gravity at BIPM, and acting as a reference during the International Comparisons, every 4 years. © Springer-Verlag Berlin Heidelberg 2010.