text: From the start, tonewheel organs operated on a radically different principle from all previous organs. In place of reeds and pipes, Robb and Hammond introduced a set of rapidly spinning magnetic wheels, called tonewheels, which excite transducers that generate electrical signals of various frequencies that are mixed and fed through an amplifier to a loudspeaker. The organ is electrically powered, replacing the reed organ's twin bellows pedals with a single swell (or "expression") pedal more like that of a pipe organ. Instead of having to pump at a constant rate, as had been the case with the reed organ, the organist simply varies the position of this pedal to change the volume as desired. Unlike reed organs, this gives great control over the music's dynamic range, while at the same time freeing one or both of the player's feet to play on a pedalboard, which, unlike most reed organs, electronic organs incorporate. From the beginning, the electronic organ has had a second manual, also rare among reed organs. While these features mean that the electric organ requires greater musical skills of the organist than the reed organ has, the second manual and the pedalboard along with the expression pedal greatly enhanced playing, far-surpassing the capabilities of the typical reed organ.