Back in 1894, Olaus Henrici invented a machine called the Harmonic Analyser. Way ahead of its time, it could pick out all the individual frequencies that make up complex sound waves—a technique we now rely on for everything from compressed audio to digital images.
In this video, Engineering Guy explains how the harmonic analyser was invented and used, so we'll leave the explanation of its mechanics—including multiple pulleys and glass spheres called rolling-sphere integrators—to him. But let's reflect on its lasting legacy for a moment.
This device turned Joseph Fourier's (much earlier) work from theory to practice. While Fourier's math explained how we could deconstruct complex signals and identify their component frequencies, Henrici's machine actually did it.
So what? Well, that MP3 you're listening to? It was compressed thanks to that very technique. Algorithms used to compress audio, you see, work out which frequencies you don't need to hear, identify them in a file using frequency analysis, and then get rid of them. Bingo, reduced file size. The same trick is used spatially to remove the fine-grained detail in photographs that you can barely see anyway—and the result is the JPEG.
All of which is quite impressive, for a machine invented in 1894. [Engineering Guy]