A new determination of the numerical value of the gravitational constant G was carried out by means of an apparatus consisting of a fibreless torsion balance which was carried by the buoyancy of a floater in a liquid; the angular deflection of the balance was compensated with the aid of a control loop and an electrostatic torque transmitter; the reacting bodies were cylinders. The result obtained for G differs from the CODATA value by about +6 × 10-3 in relative value. No dependence on the materials or on the distance between the cylinders was found. The apparatus is described, the methods of evaluation and the results are reported, the possible sources of errors are discussed.

A new optical diffractometer has been developed and set up at the Physikalisch-Technische Bundesanstalt (PTB). It offers the possibility of high-accuracy calibrations of the lateral period of gratings (pitch) in the micro- and nanometre scale. The measurement principle is based on a modified Littrow configuration, where the incident and the diffracted laser beams are almost collinear. The grating is mounted on a rotary table, and a high-precision rotary encoder is used to measure its angular positions. The profiles of the diffracted laser beams are recorded by means of a line array image detector. To determine the centre positions of the imaged laser beam profiles, different analysis methods can be applied, among others a new correlation method. A variety of laser wavelengths, ranging from 266 nm to 633 nm, can be used. Due to the optional UV wavelength, the smallest measurable pitch is about 150 nm. Depending on the quality of the sample, the measurement uncertainty can be smaller than 10 pm. For two-dimensional gratings the pitch of the two main and the diagonal directions can be measured and thus, also the angle between the two main grating orientations can be determined.

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Measurements of the Newtonian gravitational constant yielded the value G = 6.667 x lo-" N m 2 kgP2 with a 1 0 uncertainty of f 7 xThe report describes the set-up and function of the measuring equipment. The basic principle underlying the experiment is the classical principle of the torsion balance. New components are the mercury bearing of the balance beam and the control system with a quadrant electrometer according to Maxwell, which compensates the gravitational forces. The uncertainty of the measurement results and the intended improvements are discussed.

In addition to earlier experiments on the determination of G that were carried out in the PTB, supplementary investigations of the electrostatic torque transmitter unit have been performed. We have identified a previously overlooked effect that is capable of reducing the deviation from other determinations by an order of magnitude. Additionally, the uncertainty associated with the former value has to be enlarged.