A cryogenic torsion pendulum: progress report

A cryogenic torsion pendulum for gravitational experiments is being refined at its remote operation site near Richland, Washington. Features of the ap paratus include: four stages of temperature control, an angular readout of the pendulum about three orthogonal axes and a remote operation capability. Currently, we are testing the apparatus with a nominally symmetric pendulu m suspended from a 25 mu m diameter Al5056 fibre. Since installing the appa ratus in June 1999, we have been tearing and fine tuning the instruments an d software. Most recently (December 1999) magnetic shielding was added, red ucing the eddy current damping of the pendulum by similar to 10(3). Magneti c contamination of the rest pendulum, with a net magnetic dipole moment of similar to 6 x 10(-9) N m T-1, served in measuring the effectiveness of the shielding.