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Replicating Bell's 19th Century Photophone

Introduction

In 1880, Alexander Graham Bell and Charles Sumner Tainter put forward the patents to their most recent series of inventions which encompassed the photophone. Directly following his invention of the telephone, Bell realized the heavy reliance on direct electrical transmission that the telephone implied. Fearing a future in which telephone wires engulfed cities, Bell aimed to create a cleaner method of transmitting sound -- one which would rely on light, rather than electricity. Bell consequently described the photophone as his most important invention of all, although his genius was left relatively unappreciated for nearly another century. 


Overview of Bell's transmitter and receiver apparatus.

Background of the Photophone

Bell’s creation of the photophone operated on the concept of modulation of sound waves, light waves, and electronic current; this eventually served as the predecessor to modern fiber optics (Bellis). US patents 235497 and 235590 (Bell) put forth by Alexander Graham Bell and Charles Sumner Tainter describe in limited detail the design specifications of the selenium cell receiver employed in the photophone’s design. Although Bell’s notebooks dated from 1875 to 1879 provide insight on the photophone's operation (Bell), there have been lacking consolidations of the mechanics and processes involved in creating the photophone, particularly with regards to its use of selenium.

Accounts of the photophone give a basic layout of its design. The transmitter operated upon solar power; a series of lenses manipulated incoming rays to reflect off a flexible mirror. The reflected rays served as the “transmission,” as messages were spoken into the flexible mirror which would vibrate in accordance to the incident sound waves (Bellis). These vibrations were then propagated through the transmitted light waves until they reached the receiver apparatus.

The receiver of the photophone employed a selenium cell which encompasses the focus of this experiment. A concave mirror focused the incoming transmission onto the selenium cell, which itself was a resistor in a simple circuit consisting of the cell, a battery, and an amplifier or speaker (Bellis). Selenium was used due to its photoconductive properties; selenium’s electrical conductivity varies in relation to light exposure (Bell). As modulated transmissions impacted the selenium cell, its contribution of resistance to the circuit modulated accordingly. The amplifier would finally convert the modulations back into sound waves.

Bell's use of selenium was a particularly innovative facet of the photophone that took advantage of the element’s high resistance and effective photoconductivity. Below is a 1914 study regarding selenium's behavior in relation to light exposure and annealing processes.

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Additional sketch of the photophone transmitter.

Additional sketch of the photophone receiver.
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Visit to the Smithsonian

Over the course of this research project we were fortunate enough to be shown some of the original iterations of Bell's photophone. The transmitter and receiver apparatuses shown below were held in the reserves of the Smithsonian in Washington DC.

The two most salient pieces of information gathered from this visit was a second form of transmitter as well as additional insight into the creation of Bell's selenium cell. Physical interaction with the experiments as well as a collection of Bell's handwritten notes confirmed the following observations:

  • The original selenium cell employed a double-comb form of parallel disks, each with alternating voltage (recreation in Fusion 360 displayed below).
  • Bell employed a double grating pattern (occasionally referred to as a Moire pattern) attached to a vibrating diaphragm in order to convert sound waves into modulated sunlight. The gratings were created via evaporating silver onto a glass plate and then scraping away lines of silver.

Several pages of Bell's notes are also included below in pdf format.

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A Fusion 360 recreation of a simplified and blown-up selenium cell. Positive leads are red; negative leads are blue. The gray ring represents the thin layer of selenium that bridges the opposing leads, completing the circuit. The white disk is an insulating spacer. This design is iterative and compacted such that there is no space between each disk.

Bell's grating-based transmitter.

Close-up of vibrating grating.

Parabolic receiver apparatus.

Selenium cell at the parabolic focal point.

Handling of disks (~0.5mm thick).

Singular conducting disk.
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Recreating the Photophone

In order to more easily recreate Bell's photophone, I have opted to employ several technological advances that Bell did not have access to in the 19th century. Below is a quick summary of these design choices:

  • A helium neon gas laser was used instead of sunlight as the mode of transmission.
  • A signal amplifier was used to more easily detect transmissions.
  • Instead of a selenium based photocell, proof of concept was demonstrated using a smaller cadmium sulfide cell with analogous properties of photoconductivity.
  • To recover AC signal from the DC photocell circuit, a 1 micro-farad capacitor was used in series with the amplifier. 

Below is a circuit diagram for the recreated receiver along with several images of the recreation.


Receiver circuit diagram.

Light source and modulator.

Receiver assembly with ambient light shield.

Receiver wiring and amplifier.

Photocell circuit and condensing lens.
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Operation

The assembly shown above was easily manipulated using a simple set of computer speakers such that test audio can be chosen at ease. Bell used two basic variations of audio input for his own tests: the most famous was a tube and diaphragm that was spoken into, while the other was simply a speaker attached to the modulator. Using the speaker variant was more convenient for testing purposes.

To operate the photophone, the following steps were taken:

  1. Ensure that all wiring is complete (note: photocell circuit, amplifier, and speaker).
  2. Turn on oscilloscope, amplifier, laser, and modulator speakers.
  3. Adjust the laser, modulator, and convex lens such that the modulated beam falls upon the photocell.
  4. Cover the photocell circuit using the ambient light shield assembly.
  5. Play test audio through the modulator speakers and adjust amplifier settings accordingly.

Special attention was paid to the amplifier settings to avoid over-driving the low-impedance speaker in the receiver circuit. Likewise, the modulator speaker settings were adjusted to increase treble and lower bass balancing (due to the receiver circuit picking up low frequencies more easily).

Taking these steps, audio was more or less easily heard through the receiver assembly. The following audio sources were used to test fidelity:

  • Audiobook recording of The Art of War by Sun Tzu.
  • Various music recordings (e.g. Claire de Lune).
  • Percussion instrument recordsings (e.g. Carolina Crown drum cadence).

Conclusion

This project is of importance because it covers a gap in scientific literature regarding the origin of fiber optics technology, as well as a study of selenium’s photoconductive properties. The recreation of the photophone gives an opportunity to re-evaluate the impact the historic 1880 patent. Similarly, it simplifies the concepts of fiber optics to an easily optimizable level of complication. 

Bibliography

Bell, Alexander G., and Charles S. Tainter. Selenium Cells. A.G. Bell & C.S. Tainter, assignee. Patent 235497. 14 Dec. 1880. Web.

Bell, Alexander G., and Charles S. Tainter. Selenium-Cell. A.G. Bell & C.S. Tainter, assignee. Patent 235590. 14 Dec. 1880. Web.

Bell, Alexander G., Gardiner Greene Hubbard, Lewis Monroe, and Thomas A. Watson. "Notebook by Alexander Graham Bell, from 1875 to 1876." The Library of Congress. United States Legislation, n.d. Web. 10 July 2017.

Bellis, Mary. "Alexander Graham Bell's Photophone: An Invention Ahead of Its Time." ThoughtCo. ThoughtCo, 24 Apr. 2017. Web. 10 July 2017.

Greenslade, Thomas B. "Selenium Cell." Selenium Cell. Kenyon College, n.d. Web. 10 July 2017.