An apparatus for measuring the damping to torsional oscillations of a glass fibre is described which permits automatic recording of the temperature of the fibre and of the damping curve. With this apparatus it is possible to measure very high values of damping. Measurements have been extended into the transformation range where the damping is shown to arise from the same mechanism as the background, or network, contribution to the damping, i.e. that part of the damping which remains after the packs caused by the stress induced diffusion of particular ions have been subtracted. Attention is drawn to the great sensitivity of this background damping to thermal history. From an approximate expression given by Zener, which relates the logarithmic decrement to the tangent of the loss angle, it is shown that the frequency dependence of the back ground damping at constant temperature can be accounted for in terms of a stress-relaxation function s/so=exp-(t/m)b, where the value of b changes slowly with temperature from about 1/2 at lower temperatures to 1 at the upper end of the transformation range.
