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.

We describe the results of a comparison of optical-power meters undertaken by the National Institute of Standards and Technology (NIST, USA) and the Physikalisch-Technische Bundesanstalt (PTB, Germany) at nominal wavelengths of 1300 nm and 1550 nm. Both laboratories used thermal detectors as reference standards, which were compared using a germanium trap detector as a transfer standard. Measurement results showed differences of less than 1 part in 10 3 , well within the combined uncertainty for both laboratories.

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.

We describe the results of a comparison of reference standards between the National Institute of Standards and Technology (NIST-USA) and Physikalisch-Technische Bundesanstalt (PTB-Germany) at nominal wavelengths of 1300 nm and 1550 nm using an optical-fiber cable. Both laboratories used thermal detectors as reference standards. A novel temperature-controlled, optical-trap detector was used as a transfer standard to compare two reference standards. Measurement results showed differences of less than 1.5 × 10−3, which is within the combined uncertainty for both laboratories.