Torsion balance experiments for the measurement of weak forces in nature

An ongoing TIFR programme of torsion balance experiments for the measurement of weak forces is described. The extreme weakness of the forces searched for necessitates careful shielding of the torsion balance from sources of noise. The passive thermal and magnetic shields that were developed. The concepts underlying them are set forth. To minimize gas damping and radiometric forces, the torsion balance is suspended in an ultra high vacuum chamber. The design, construction and performances of this large uhv system are detailed, including the development of a bakeable uhv seal for a quartz optical window. An autocollimating optical lever with an operating resolution of ∼3x10-9 rad is also described. Some results from the recent searches for a 'fifth force' are mentioned. (author). 19 refs., 19 figs