A relatively robust torsion balance which will withstand baking in vacuo at 450°C, has a period of 6 seconds and can detect a force of about 2 × 10^-8 dyn is described. It consists of a light metal vane mounted on a taut tungsten wire 10 μm in diameter. The rotation of the vane is measured by using a mirror and a light-beam amplifier of the type developed by Jones and Richards. The smallest torque detectable produces a rotation about equal to the r.m.s. Brownian deflection.