to that of the line, and greatly attenuate oscillation of the main cable. Since Stockbridge dampers were economical, effective and easy to install, they became used routinely on overhead lines. Live-line working using hot stick tools meant it was possible to retrofit dampers to lines while energised. Modern designs Modern designs use metal bell-shaped weights rather than Stockbridge's concrete blocks. The bell is hollow and the damper cable is fixed internally to the distal end, which permits relative motion between the cable and damping weights. To provide for greater freedom of motion, the weights may be partially slotted in the vertical plane, allowing the cable to travel outside the confines of the bell. Some more complex designs use weights with asymmetric mass distribution, which enables the damper to oscillate in several different frequency modes and ranges. Another modern design is the Dogbone invented by Philip Dulhunty in 1976 is so called due to its configuration, a larger metal sphere attached to the end of the damper, with a smaller sphere projecting sideways from it, resembling a dog's bone. The damper offsets the weights sideways in order to introduce a third degree of freedom, twisting the damper cable in addition to bending it up and down. Additional intra-strand friction was created in the damper cable, dissipating significantly more energy. The most vulnerable section of the cable is where it is clamped to the end of an insulator string, so dampers are typically installed at the nearest anti-nodes (points of maximum displacement) either side of the clamp. There are thus normally two dampers per span, though more can be installed if necessary on longer spans. Overhead transmission lines form a catenary for which vibration is predominately in the vertical plane. When more than one plane of vibration is anticipated, Stockbridge dampers may be mounted at right angles to each other. This is common when the cable runs in a vertical or off-horizontal plane, for example in cable-stayed bridges or radio mast guy-wires. See also Conductor gallop Tacoma Narrows Bridge - destruction once thought to be a case of resonance References External links Vibrationdata article "Cable clingers" New Scientist Q&A Aerodynamics Electric power systems components