Object, Instrument, Technology

Vibration microscope

Authors
Hermann von, Helmholtz, Hermann von Helmholtz
Jules Antoine, Lissajous, Jules Antoine Lissajous
Date(s)
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The vibration microscope is an electromagnetically-driven adaptation by Hermann von Helmholtz (1821-1894) of the earlier optical comparator by Jules Antoine Lissajous (1822-1880). The device allows one to determine the frequency of a tuning fork or other vibrating object with respect to a fork of known frequency, by way of Lissajous figure analysis.

Vibration microscope by Rudolph Koenig
Fig. 1: Vibration microscope by Rudolph Koenig
Vibration microscope from Koenig’s Acoustic Catalogue, 1865. Cost: 120 francs
Fig. 2: Vibration microscope from Koenig’s Acoustic Catalogue, 1865. Cost: 120 francs

The standard fork has a small glass lens attached to one of its prongs. If the standard fork is sounded, objects visible through the lens will appear to vibrate. A second tuning fork can then be positioned behind the hole, so that its plane of vibration is at right angles with that of the standard. When both forks are driven, a point on the second fork’s prong, viewed through the lens, will appear to trace out a curve. If the tuning fork frequencies are a fraction multiple of one another, simple Lissajous figures will appear (see left diagram below). However, if the frequencies are slightly mismatched, the curve will not remain constant, but change in a repeating way. Even if the frequencies are not identical, the difference between them can be deduced from the rate at which the pattern changes. 

Simple Lissajous figures for various frequency ratios, in different stages of their cycles. From Koenig’s Acoustic Catalogue, 1865
Fig. 3: Simple Lissajous figures for various frequency ratios, in different stages of their cycles. From Koenig’s Acoustic Catalogue, 1865

Text & figures by Brian Tinker, republished with the kind permission of Case Western Reserve University and William Fickinger, Prof Emeritus of Physics.

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Tuning forks
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