Tonometer

From Ccpi

A tonometer consists of a series of steel cylinders that resonate at specific frequencies upon being struck. They are frequency standards, used in the same manner as tuning forks. The vibrational modes (i.e. fundamental and higher harmonics) of a given cylinder are proportional to its thickness, and to the square root of its elastic property (Young's modulus); and inversely proportional to its length squared, and to the square root of its density. In the tonometer, only the length varies among cylinders; the shorter the cylinder, the higher the sounded frequency.