Tonometer

Tonometers are frequency standards, and usually occur in sets. Rudolph Koenig made two types: resonant bars and tuning forks.

The Tonometer below consists of a series of steel bars, suspended from strings at a distance of 22.4% of the overall length from the ends. Under these circumstances, the rods vibrate transversely in their fundamental modes when struck with a small hammer. For a set of round rods of constant diameter, the frequency is proportional to the inverse square of the length. The 1889 Koenig catalogue lists the apparatus at 80 francs. The longest bars ought to be audible to everyone, but the shortest has a frequency of 32,768 Hz, inaudible to everyone. The hammer was extra.