The piezoelectric effect works both ways; that is, if you apply an electric current to a piezoelectric substance, you can make it vibrate; but also if something makes a piezoelectric substance vibrate, it will generate current. The former is also frequently used, for example, in piezoelectric speakers, the tiny flat speakers that you find in musical greeting cards, mobile phones and even recently musical condoms. It was used in both capacities on mercury delay lines.

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