  In 1998, two centuries after Cavendish, a conference on theory and experiment
of the G measurement pointed out the progress made in various experimental
methods and discussed the effects on the accuracy of G. In spite of several
measurements with torsion balance in vacuum to the aim of reducing some
disturbances, no mention was made about a possible pressure effect in calm air.
In 2000 J.Luo and Z.K.Hu firstly denounced the presence of some unknown
systematic problem on G measurement. In the present work a new systematic error
is analysed which arises from the non-zero balance of the overall momentum
discharged by the air molecules upon the test mass within the vacuum chamber.
This effect is normally negligible, but when the pressure is so low that the
molecule mean free path is comparable to the thickness of the air meatus
surrounding the test mass, the drawing force may become greater than the
gravitational force. Considering the usual size of the meatus, the molecular
effect becomes maximum when the pressure drops to about 0.01milli bar. Before
Heyl measurement at 1 millibar (1927), the experiments were made at higher
pressures. Conversely those made with recent vacuum techniques show pressures
down to 0.1 nanobar (Gundlach and Merkowitz, 2000) and 0.01 nanobar (Gershteyn,
2002). In these experiments the effect of the vacuum pressure appears very
little. As a matter of fact, we were not able to find in the literature some
measurements made at vacuum pressures between the millibar and the nanobar. Why
? This lack appears embarrassing in absence of an adequate physical
explanation.
