A Very Long Period Pendulum (Abstract)

Volume 24 number 1 (1996)

Casper H. Hossfield

Abstract

(Abstract only) A very low frequency gravity wave antenna requires two swinging pendula with a frequency (1/period) lower than the antenna frequency. Since no suitable pendulum for the antenna existed, I invented one by laying a simple pendulum on its side and suspending it on three wires so it swings in an exactly horizontal plane and its period is infinite. Simple pendulum supported horizontally by three wires so its gravitational period is infinite. The central vertical wire, which carries most of the weight, becomes a torsion rod that provides the restoring force that gravity provides in the simple pendulum. The torsion rod has one very important advantage over gravity, however: it can be wound up like a spring to produce enough tension without changing its period. The period depends only on the rodís spring constant and the moment of inertia. In a practical device, the pivot points for the two support wires and the torsion rod's suspension must all lie on the same exactly vertical axis. Furthermore, the rod's suspension will usually need to be above the upper support wire's anchor. A bypass in the torsion rod allows all three pivot points to lie on the same vertical axis. The torsion rod is wound up equally on both ends so the bypass does not turn and hit the upper support anchor. The tension provided by the wound up torsion rod can be calculated. The torsion rod also suspends the pendulum and the stresses add up. The torsion rod should also be made as long as possible so its diameter can be increased to make it stronger for a given period. The support wires need to bend at a predictable point that can be placed on the vertical axis. This can be done with a hinge made from a thin flat spring. A spring should be used as a hinge rather than a bearing which would absorb energy. The spring stores the energy and gives it back as it swings and there is no stick-slip friction for small movement. All components of an outdoor device must be protected from the wind, because even the lightest draft will set up vibrations in the antenna.