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The Compton Organ Company - Electrostatic Generators - Part Two of Three

This week’s entry is the second of three posts discussing the instruments produced by the Compton Organ Company Ltd. This post will look at the underlying principles behind the Electrostatic tone generation system invented by Compton engineer Leslie Bourne in the early 1930s.


During the 1920s Leslie Bourne had worked on developing an electromagnetic tone generation pickup system that was similar in principle to the Tonewheel design, most famously used in the Hammond organ from 1935 to 1975.


The first Tonewheel tone generator was invented by Thaddeus Cahill in 1896 and used in his early electrical organ the Telharmonium. The concept proved popular but the instrument itself was incredibly large and therefore was never viable as a commercial instrument. It wasn’t until the use of Tonewheels in the Hammond organ in 1935 that the technology would enjoy widespread success.

Thaddeus Cahill's Telharmonium - inverse.com (2015)

Tonewheel generators comprise of a rotating disc with smooth bumps on the outer edge and a pickup consisting of a magnet and electromagnetic coil. As the disc rotates the smooth bumps passing by the pickup create a current in the coil. The coil is connected to an amplifier where the signal is mixed with additional signals from Tonewheels representing the fundamental frequencies and harmonics from other notes in the instrument.


Bourne ceased work on his Tonewheel inspired work and instead focused his efforts on designing what would come to be known as the Electrostatic tone generation system. Bourne filed US patent US1996669A for his invention in 1932, it was listed simply as an ‘Electrical Musical Instrument’ and was granted in 1935. The original patent is available to view in the sources section below.


Diagram of stator engraving taken from patent US1996669A - Bourne (1935)

Electrostatic generators comprise of a circular insulating plate know as a stator, on which many sine waveforms are engraved concentrically into a conductive metal coating. A single generator plate will typically consist of at least eight partials, specifically the fundamental frequency in addition to a further seven harmonics (integer multiples of the fundamental). It will also include multiple iterations of these partials at additional octaves. For example, a six-octave organ with eight partials would have 48 different waveforms engraved on each generator disc. There were twelve generators in a single organ, each representing a separate note of the chromatic scale.


Positioned less than one thousand of an inch from the face of the metal plate there are a series of electrodes on a rotor which can be energised in correspondence to one of the engraved waveforms. As the electrode is energised and the stator plate rotates, an electrical capacitance is produced between the two surfaces due to the close proximity and an AC tone is generated. The AC tones frequency generated is dependent on the cycles per revolution of the engraved waveform in addition to the speed that the stator is rotating.

Stator and Rotor from Electrostatic tone generator - Electrokinetic.org (2012)

If a note is held on the organ as it is turned on its pitch will quickly rise in frequency until it reaches standard tuning. This is a contextual illustration of the electrostatic generator’s tuning being directly related to the speed of the generator pulley motor.


In the same way that different combinations of pipes are selected in a pipe organ to alter the timbre of the note produced, different combinations of sine wave partials can be used in an Electrostatic organ by controlling which electrodes are energised. This is controlled by the organist using a series of stops in the same way one would with a traditional pipe organ.


The AC signal is taken from the rotor by capacitive coupling using a separate pickup electrode on the opposite side of the disc. This signal is then sent to an amplifier system before being reproduced over loudspeaker. This is the basic underlying principle of operation underpinning Electrostatic tone generation.

The main benefit of Electrostatic tone generation, besides facilitating lower costs and more compact instruments, is the perfect tuning stability. As all the waveforms are engraved into metal plates and the generators are run by a single motor via a pulley it is impossible for any tuning issues to arise. It could also be argued that Bourne’s design of a system built around engraved waveforms was essentially the first example of sampling seen in a musical instrument, years ahead of its time.


Sources:


Banton, H. (2015). MAKIN ORGANS HISTORY 1972 – 1992, Retrieved from

https://www.organworkshop.co.uk/images/files/Makin_history_1972-1992.pdf


Bourne, L. (1935). Electrical musical instrument, US1996669A, viewed 01 December 2018, Retrieved from https://patents.google.com/patent/US1996669A/en?inventor=Bourn+Leslie+Edwin+Alexander&sort=old


Bourne, L. (1936). Electrical music instrument, US2032044A, viewed 01 December 2018, Retrieved from https://patents.google.com/patent/US2032044A/en?inventor=Bourn+Leslie+Edwin+Alexander&sort=old


Curtis, S. (2010). Compton Electric Organs, Retrieved from http://www.stancurtis.com/compton.htm


Electrokinetica. (2018). Introducing the Compton Electrone - the definitive British electric organ, Retrieved from http://www.electrokinetica.org/d8/1/index.php


Electrokinetica. (2018). How it works: The electrostatic generator. Retrieved from http://www.electrokinetica.org/d8/2/index.php


Electrokinetica. (2018). Introduction to the 1950s Electrones, Retrieved from

http://www.electrokinetica.org/d8/4/index.php


Electrokinetica. (2018). Model 357, Retrieved from http://www.electrokinetica.org/d8/4/2.php

Lawton, C. (2013). A BRIEF HISTORY OF THE JOHN COMPTON ORGAN COMPANY LTD, Retrieved from

https://comptonorgans.yolasite.com/the-john-compton-organ-company-ltd---brief-history.php

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