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Dancing Wires

Suspended telephone and power cables and other objects sometimes are seen to vibrate without apparent cause, as several readers including Ginger Gauss of Fairbanks have noted. Especially on windless winter days it seems that one most often notices dancing wires; but perhaps any motion on such days is unusual and therefore easily recognized.

Every solid object and every stretched wire has one or more resonant frequencies at which it will vibrate easily. A person can make the tip of a rather sizable tree oscillate by repeated pushing on the tree trunk at just the right rate.

The telephone wire or power line is the same. It has a resonant fundamental frequency that depends upon the length and weight of the wire and upon how taut it is. If the wire can be repeatedly tickled ever so slightly at the proper frequency, it will build up a major oscillation. To oscillate, the wire must receive energy from one of two places: either from the ends of the wire or from the strumming of the wire itself by air movement or a falling tree branch.

Dancing wires probably get their energy most often from vibrations of the poles or trees to which they are attached. When two or more vibrating objects are coupled, they can transfer the energy of vibration from one to another. One part of such a coupled system may vibrate for a time and then cease vibrating as another part of the system takes up the vibration.

The ultimate energy source for the dancing wires is air movement, occasional plucking of the wire somewhere along its length or by motion of the earth transmitted to the wire by the poles to which it is attached. The fact that the wire may be carrying electricity has nothing to do with its vibration.