I did the annual maintenance on my Hammond Organ over Thanksgiving, reminding me of what an incredibly cool instrument it is. I think it represents the highest evolution of one part of the tree of musical instruments.
For one thing, it was not ever considered to be an electronic instrument by its manufacturer. They always referred to it as an "electric organ." Why? Because the sounds, although amplified and processed by various analog circuits, was actually produced mechanically. Laurens Hammond was a successful inventor and tinkerer. He had developed the first synchronus electric clock in the 1920s, which used the steady frequency of the electric power system to ensure high accuracy. When the Depression hit, the demand for clocks dropped, so he was trying to think of other ways to use his motors. He remembered that a lot of churches couldn't afford an organ, and the crude electric and electronic models of the time had lots of problems, the most pressing one was how they couldn't stay in tune. He developed a system of rotating tonewheels, little wheels that passed through a magnetic field to create sine waves. There are 96 of these, with their rotation controlled by a series of gears, and running off one of his synchronous motors. Nothing to ever go out of tune!
The next trick was a way to combine these to create pleasing tones. By mixing the various harmonics of a tone through a series of drawbars (linear volume controls), very rich sounds can be created. This was the first successful additive synthesizer, way back in 1934!
A criticism of many electronic instruments is that their tone is too regular, making it dull. Since this is a mechanical system, very slight and subtle variations of the tones are created as the wheels go in and out of phase. Many early instruments used a master high octave, and dividers to create the lower octaves. When mixed together, they were always perfectly in phase, giving a lifeless sound. Not the Hammond Organ; the pricipal tones and harmonics all come from separate tonewheels. Read more »