They’re called Stockbridge dampers.
I agree, vortex-shedding. It does not take a high wind, just the right combination of exciting force and resonant frequency. (Surprisingly no one has mentioned the Tacoma Narrows bridge, which developed a side-to-side sine wave and then broke apart. The widely accepted reason is vortex-shedding combined with a narrow cross-section.)
Today I saw a wire fence bouncing. It had about 3" of snow hanging from it, and this was enough to catch the wind and get it moving.
New power lines were recently installed near my home. These are on big towers (not as big as “high tension” line towers) with maybe six lines per tower. The upper most line has these things that look like coiled springs placed every 20-25’ along the line. It hard to tell but I’d guess they are about 2’ long. When viewed from the side, they look something like this —I–I--I–I--. I’m guessing that they are some sort of dampers but why only on the one line? Aren’t the other lines subject to the same oscillations?
I’ve been told that “coiled springs” look like “snakes” to birds, and deter perching. I don’t claim any personal knowledge.
Also, I don’t think that “vortex shedding” off of a wire will cause that long-wavelength bouncing. I think that long-wavelength bouncing caused by vortex shedding is cause by vortex shedding off of nearby mountains and structures.
Based on your “anti-bird” idea, I was able to find that these things are call flight diverters and are intended to make the power lines more visible to birds. I live in coastal New Jersey and all sorts of birds migrate through here.
I found this answer on an electrical engineering site:
Originally the spiral shaped devices were fitted to the earth wires on top of pylons as vibration dampers (the very top single wire on pylons is the earth wire). It was however noted that they also had the effect of discouraging birds from landing on the wires. Therefore the spiral bird diverters can be used as dual purpose devices. Keeping birds away and effectively managing vibration issues on lines in particularly windy areas. Although these days newer weighted vibration dampers are used on most problem lines.
They use spirals so that during periods of vibration the spiral will bounce around and hit the wire and disrupt any vibration patterns (specifically they were designed to prevent a vibration type called aeolian vibration) in the line. The weighted dampers can’t do that. Generally they use the weighted dampers on thicker cables and spirals on thinner ones.
That only sort of makes sense. I think that if the problem with lighter wire is so bad that it is causing clashing with the vibration dampers, the problem is galloping, not aeolian vibration. (I suspect that the author doesn’t understand coupled tuned systems). But a lighter wire will require a vibration damper that is tuned differently than the weighted (stockbridge) dampers, so no reason to expect it to be the same size and shape.
A company that make these things doesn’t mention vibration damping on its site. Also, nothing about discouraging birds from perching. Its all about minimising birds hitting the wire in flight and being injured/killed.
I know an other kind of spiral is also still used – I’ve got them at HV power poles here. I don’t know what their function is, but since they are only at the attachment to the poles, I think that they are vibration or galloping or anti-perching or small-animal devices: since these lines don’t have anything on the span, I don’t think I’ve got bird visibility markers (or airplane visibility markers, which are more common around here)
I witnessed the same bouncing line phenomenon this week. And can add one more oddity. Each Wednesday I meet an old classmate for a dog walk in the bush. We parked our cars in a pull-off by a rarely used road. I pointed out that the telephone line was oscillating up and down with an amplitude of 20 - 25 cm (8-10 inches), but only the lower thick telephone cables (the power lines remained still). There was absolutely no wind, but it was very cold -17 C (1 F), with very light snow, falling straight down. We walked the dogs as long as we could take the cold - maybe a half hour. When we returned to the cars the line was still bouncing in exactly the same manner. We watched from the cars, and it simply kept bouncing. Some of the other entries refer to snow or cold temps. Could this have something to do with it?
I’ve never heard it called that.