![]() ![]() This filling material was chosen for its high density and in order to avoid electrical charging of the material that might disrupt homogeneous flow. The upper container consists of a cylinder and a cone characterized by R 1 = 2.75 cm, R 2 = 1.0 cm, and z 2 = 3.5 cm, initially filled up to z 1 = 8.2 cm with M = 800 g bronze powder (our “sand”) of density ρ = 5, 230 kg / m 3, a grain size of approximately 150 μ m, and angle of repose α = 28.3 °. But even for the simplified experiment 5 the full time dependence of the change of weight could not be measured due to perturbations of the opening mechanism. ![]() Whether this result, shown for a single orifice using different approaches, 15–17 holds true also for flow through a sieve remains unclear. Indeed, unlike fluids granular matter shows flows at a rate independent of the filling height. For the given geometry, however, a constant increase of the weight is expected from theory whereas the measurement shows a clear trend that may be understood either from the large uncertainty (error bars) of the measurement or from the non-stationary flow rate. Using a multiple-orifice setup 5 where the upper cylindrical container was separated from the lower cylindrical container by a sieve, a quantitative measurement was presented. Even in an experiment with a very large device of height 120 cm, emptying in only 10 s, a maximal change in weight of only 49 mg was measured, 2 and the time dependence of this effect could not be determined. In the literature, the effect was described as tiny, perhaps smaller than the change of gravity when turning the hourglass over and thereby changing the distance of the center of mass of the sand from the center of Earth, 3 an effect that is on the order of some μg and cannot be measured with usual physics lab equipment. From the analysis of the motion of the center of mass, 2–5 it follows that the weight of the hourglass is a function of time that depends on the specific details including the geometric shape of the hourglass, the outflow rate, and the angle of repose of the sand.
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