The Sixth International Comparison of Absolute Gravimeters was held from 5 June to 28 August 2001 at the Bureau International des Poids et Mesures (BIPM), Sèvres. Seventeen absolute gravimeters were used to make measurements at five sites of the BIPM gravity network. The vertical gravity gradients at the sites and the ties between them were also measured using seventeen relative gravimeters. For the first time the ties were also measured using absolute gravimeters. Various methods of processing the absolute and relative data were tested to calculate the results. The final results of ICAG-2001 are presented. The acceleration due to gravity at a height of 0.90 m is given as (980 925 701.2 ± 5.5) µGal* and (980 928 018.8 ± 5.5) µGal for sites A and B, respectively,

Results of an international comparison of iodine absorption cells organized by the BIPM are reported. Ten of the thirteen member laboratories of the Comité Consultatif pour la Définition du Mètre (CCDM) participated. Two experimental methods were used to estimate the purity of the iodine in the cells: first, the cells were incorporated in BIPM He-Ne lasers and the laser frequencies thus obtained at λ = 633 nm were compared by the beat frequency method with those of the BIPM reference lasers and, second, direct measurements of iodine purity by a laser-induced fluorescence method were made at λ = 502 nm. Of the thirty-nine cells investigated, more than half are pure enough to realize a frequency reproducibility to within 10 kHz (2 parts in 1011 in relative terms). A contamination of 129I2 has been observed in certain cells containing 127I2. The comparison confirms that observed frequency shifts of the hyperfine components in iodine are strongly linked to impurities in the iodine cells.

An international comparison of 12712 He-Ne stabilized lasers, used as length standards, has been made. Five laboratories from Sweden, Denmark (two laboratories), Finland and Germany were represented. In addition, two lasers from the Bureau International des Poids et Mesures took part in order to relate the results to earlier comparisons. The reproducibility of the frequency mean of the four components d, e, f and g in the transition R(127)ll-5 in iodine 127 for the different lasers was found to be 15 kHz, taken as the standard deviation of the distribution of frequency among the group of ten lasers. Using laser BIPM4 as reference the frequency average of the group of lasers was +3,6 kHz, making this a good reference laser for the current ensemble. Measurements of frequency dependence on modulation amplitude, iodine pressure and, for some lasers, intracavity power are reported. The relative frequency partition of the four components d, e, f and g of each laser was also determined and shown to exhibit characteristics which are new for this type of stabilized laser.

The fifth in the series of International Comparisons of Absolute Gravimeters (ICAG) was held at the Bureau International des Poids et Measures (BIPM) in November 1997. Fifteen absolute gravimeters participated in the comparison. The mean gravity value obtained at station A (0.9 m) at the BIPM was found to be 980 925 707.8 µGal with a standard uncertainty of 2.8 µGal. This is consistent with the results obtained in previous comparisons at this site. Conclusions based on the analysis of the present results and proposals for future activities are presented.

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