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Can anyone explain why a vertical steel tube, standing with one end in contact with the ground under a $230 \space kV$ electric transmission tower, would physically vibrate at high frequency? I'm not well-grounded in electricity laws, but I suspect it may have to do with eddy currents.

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The high-voltage line is inducing eddy currents in the steel tube, and these are reacting against the cyclic magnetic field that surrounds the wire- and the steel tube becomes "microphonic". If the excitation frequency is close to one of the tube's fundamental (mechanical) harmonics, then the tube will "sing". This occurs in power transformers that have poorly-clamped steel laminations; They hum with varying degrees of loudness when power is applied to them.

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