The Hammond organ was invented by Laurens Hammond in 1935 as a lower-cost alternative to wind-driven pipe organs for churches. Instead of pipes, it used 'tonewheels'—rotating metal discs that created electromagnetic signals captured by pickups. While intended for liturgical use, it became a powerhouse in jazz, blues, and rock music. A critical component of the 'Hammond sound' is the Leslie speaker, invented by Donald Leslie. The Leslie speaker contains a rotating horn for high frequencies and a rotating drum for the woofer. As these components spin, they create a Doppler effect, causing the pitch and volume to fluctuate slightly. This 'rotary' effect adds a shimmering, three-dimensional quality to the music. Musicians can control the speed of the rotation (slow 'chorale' or fast 'tremolo') to build tension and emotion, a sound that defined the music of artists like Jimmy Smith and bands like Deep Purple.