This rugged hand-held precision instrument is unlike any tachometer I’ve ever seen. It’s more musical than mechanical, and it needs no power source other than the piece of machinery whose speed the user wants to check.
Musical? Perhaps that’s drawing a long bow, but the tachometer does embody the principle of resonant frequency. Hold it to a machine, observe which reed inside it vibrates most strongly, and note its frequency from the scale. Think soldiers marching over a bridge out of step so they don’t set it vibrating at a frequency that could destroy it. Watch amazing film footage of the Tacoma Narrows Bridge swinging itself to destruction, taking with it an abandoned car and a terrified cocker spaniel.
Imagine the quartz crystal in your watch keeping time by vibrating at a precise frequency like a tuning fork. Listen as musicians set reeds, strings, membranes or columns of air vibrating, making your eardrums and the tiny hair-like sensors in your inner ear vibrate in resonance.
The logo on the tachometer is a visual analogue of the instrument’s action. The five squares represent the visible ends of adjacent vertical reeds, which are bent so they sit horizontally. And the short lines beside the squares represent their vibration, which is strongest for the central reed, the one whose resonant frequency is closest to that of the machine being tested.
In the process of acquiring this delightful object, I discovered not just how it works but also the names of its inventor, designer and manufacturer. The maker’s name, James G Biddle, appears on the face of the tachometer, along with a patent number. That 1960 patent, issued to Frederick Lombard of Pennsylvania and assigned to Biddle, covered the ‘ornamental design’ of the device. This type of patent is akin to a registered design in Australia; it protects an object’s form, rather than its function, from competition for a set number of years.
An online patent search revealed that the inventor was Hermann Frahm of Hamburg in Germany. Other patents awarded to Frahm in the early twentieth century show that he was very interested in minimising the harmful effects of vibration. His most important inventions aimed to stabilise ships in rough seas and damp the vibrations of machinery and buildings.
And a search of our own collection records revealed this Frahm’s tachoscope, also called a resonance gyroscope, an educational device that demonstrates the principle behind the tachometer. When the slightly unbalanced wheel is set spinning, it wobbles and causes the steel reeds to vibrate in turn, as the wheel’s declining rotational frequency matches each reed’s resonant frequency. While it doesn’t have the vibe of a funky six-string guitar, the two do share some basic physics.
Written by Debbie Rudder, curator