View source for Tonewheel

{{Short description|Electromechanical apparatus for generating electric musical notes}}
{{Refimprove|date=August 2011}}
[[Image:Tonewheel-p.svg|thumb|Simplified diagram of how a tonewheel works]]
[[Image:Goldschmidt tone wheel.jpg|thumb|Goldschmidt tone wheel (1910), used as an early [[beat frequency oscillator]]]]

A '''tonewheel''' or '''tone wheel''' is a simple electromechanical apparatus used for generating electric [[musical note]]s in electromechanical [[electronic organ|organ]] instruments such as the [[Hammond Organ|Hammond organ]] and in telephony to generate audible signals such as [[ringing tone]]. It was developed by [[Thaddeus Cahill]] for the [[telharmonium]] c. 1896 and patented in 1897.<ref>{{US Patent|580035A}}</ref> It was reinvented around 1910 by [[Rudolph Goldschmidt]] for use in pre–[[vacuum tube|vacuum-tube]] [[radio receiver]]s as a [[beat frequency oscillator]] (BFO) to make [[continuous wave]] [[radiotelegraphy]] ([[Morse code]]) signals audible.

==Description==
The tonewheel assembly consists of a synchronous [[AC motor]] and an associated [[Transmission (mechanics)|gearbox]] that drives a series of rotating disks. Each disk has a given number of smooth bumps at the rim; these generate a specific frequency as the disk rotates close to a [[Pick up (music technology)|pickup]] assembly that consists of a [[magnet]] and [[electromagnetic coil]].{{efn|This is electrically and magnetically similar to a [[guitar pickup]], in that a permanent magnet is placed within the coil and the moving element is unmagnetized. Unlike most generators or dynamos, there is no external field applied through the moving part.}}

As each bump in the wheel approaches the pickup, it temporarily concentrates the magnetic field near it, and thus strengthens the magnetic field that passes through the coil, inducing a current in the coil by the process of [[electromagnetic induction]]. As the bump moves past, this concentrating effect is reduced again, the magnetic field weakens slightly, and an opposite current is induced in the coil. Thus, the frequency of the current in the coil depends on the speed of rotation of the disk and the number of bumps.

[[Image:US580035A Thaddeus Cahill, Telharmonium patent p.04.jpg|thumb|Rheotome-cylinders and electric-brushes used on Telharmonium (1896)]]
Typically, the coil is connected to an amplifier through a network of switches, contacts, resistor banks, and transformers which can be used to mix the fluctuating current representing the note from one coil with similar currents from other coils representing other notes. A single [[fundamental frequency]] can thus be combined with one or more [[harmonics]] to produce complex sounds. Tonewheels were first developed for and used in the impractical [[Telharmonium]] circa 1896<ref name=uspat580035>{{US patent reference |number=580035 |y=1897 |m=04 |d=06<!-- issued; filed in 1896-02-04; former patent was filed in 1895-08-10 as Serial No.558,939 --> |inventor=Thaddeus Cahill |title=[https://patents.google.com/patent/US580035A Art of and apparatus for generating and distributing music electrically]}}, filed 1896-02-04.</ref> and later in the original [[Hammond organ]]s.

''Tonewheel leakage'' occurs in the Hammond organ and in similar situations, where the large number of tonewheels causes pickups to overhear tonewheels other than their own. This causes the organ to add [[chromatic scale|chromatics]] to played notes. In some kinds of music this is undesirable, but in others it has become an important part of the Hammond sound. On some digital simulations of [[Hammond organ]]s tonewheel leakage is a user-set parameter.

== Early uses ==
The tonewheel was independently invented in 1910 by [[Rudolph Goldschmidt]] as a beat frequency oscillator in early [[radio receiver]]s to make [[continuous wave]] [[radiotelegraphy]] ([[Morse code]]) signals audible, before the existence of the [[vacuum tube]].

== See also ==
* [[Alexanderson alternator]]
* [[Savart wheel]]

==Notes==
{{notelist}}

==References==
{{reflist}}

{{Electronic organs}}

[[Category:Electronic organs]]
[[Category:Electric and electronic keyboard instruments]]