Sonometer

A SONOMETER is the standard laboratory apparatus for studying and verifying the three laws of a vibrating string. It consists of a hollow rectangular wooden SOUND BOX, whose purpose is to set a much larger mass of surrounding air vibrating in sympathy with the (much lighter) vibrating wire, thereby amplifying the sound -- exactly the same principle used in the resonating bodies of stringed instruments such as the violin, guitar, or tanpura. Along the top of the box runs a single metal wire of uniform cross-section, passing over two BRIDGES P and Q that can be slid along the box's length; the wire is fixed at one end, while its other end passes over a small pulley and hangs down carrying a weight hanger, so that the TENSION in the wire can be varied simply by adding or removing slotted weights, and the VIBRATING LENGTH AB (the portion of wire between bridges P and Q) can be changed simply by sliding the bridges.

Plucking the wire midway between P and Q sets up transverse stationary waves in the length AB, by exactly the reflection-and-superposition mechanism of section 6.7.5. The wire can also be excited instead by holding a vibrating tuning fork's stem in contact with the sound box: if the fork's frequency happens to match one of the wire's own natural frequencies, the wire responds with a large-amplitude stationary wave (resonance). To DETECT this resonance precisely, a small, light paper RIDER is balanced on the wire at the midpoint of AB; when the length AB is adjusted (by moving the bridges) until the wire's own fundamental frequency exactly equals the tuning fork's frequency, the midpoint sits at an ANTINODE of large amplitude and the paper rider is thrown clear off the wire -- a clean, visible signal that resonance has been achieved. …