On June 30, 1875, Bell wrote a triumphant letter to Hubbard: "I shall have ready tomorrow afternoon an instrument modeled after the human ear--by means of which I hope...to transmit a vocal sound...I am like a man in a fog who is sure of his latitude and longitude. I know I am close to the land for which I am bound and when the fog lifts I shall see it right before me." The instrument was a second version of the Gallows telephone, constructed by Watson; it worked little better than the first, but Bell wrote his patent anyway. His reference to 'an instrument modeled after the human ear' can be understood only by looking closely at another line of research in Bell's network of enterprises.
Bell's interest in teaching the deaf kindled his interest in devices used to visualize sound. At the Institute of Technology, he had experimented with the manometric flame capsule, a device that had a speaking tube and membrane on the other side of which was a chamber through which gas was fed to a small flame. As one spoke, the gas was alternately compressed and decompressed by the vibration of the membrane, resulting in higher and lower flames, respectively. Four mirrors were typically rotated as a unit to show the wave shapes: "when we speak to the apparatus, an undulatory band of light makes its appearance in the mirror. The upper edge of the luminous band appears to be carved into beautiful waves of various shapes and sizes, and when we sing different vowel sounds into the mouth-piece of the instrument, retaining the voice on a uniform level, the form or shape of the undulations visible in the mirror changes with every vowel. I thought that if I could discover the shape or form of vibration that was characteristic of the elements of English speech, I could depict these upon paper by photographic means for the information of my deaf pupils." (Bell, 1908, pp. 24-5)
Since Bell could not physically record the manometric flame patterns using photography and since the patterns were difficult to discern, he concentrated on another device, the phonautograph, which he also saw at the Institute of Technology. It consisted of a cone and membrane with a lever attached to the membrane; when one spoke into the cone, the lever vibrated. At the end of the lever was a bristle brush which traced the shape of the sound wave on a piece of glass covered with lampblack; the glass was moved horizontally in a direction perpendicular to the motion of the lever. "I proposed to use these glass plates as negatives, and by photographic means, print off copies of the tracings for the use of my pupils." (Bell, 1908, p. 26)
However, a comparison of phonautograph tracings and manometric flame shapes suggested to Bell that the phonautograph device needed extensive modification so that the tracings would match the flame shapes of the manometric capsule. Considering the phonautograph's geometry--with its thin, light membrane and the relatively heavy wooden lever and style--Bell was struck by the resemblance between the device and the structure of the human ear. The ear analogy suggested the sorts of modifications he might undertake to successfully replicate the flame shapes in the tracings of this device. The modifications aimed to make the analogy between technology and nature more literal. Bell sought to duplicate "the shape of the membrane of the human ear, the shapes of the bones attached to it, the mode of connection between the two, etc." (Bell, 1908, p. 29)
Following a suggestion from Dr. Clarence Blake, a distinguished aurist, Bell built an ear phonautograph in the summer of 1874, roughly the same time as he was conceiving his harp apparatus (see Figure 14). When one spoke into the cone, the eardrum and stapes, malleus and incus (all taken from a preserved human ear) were set into vibration; these vibrations were traced on smoked glass by a bristle brush attached to the end of the incus.
Figure 14: Bell's ear phonautograph. One speaks into A at top right, which causes the bones of the middle ear (B) to vibrate, tracing the pattern of the wave on the smoked glass (C).
From the phonautograph, Bell gained a tactile, 'hands-on' understanding of how speech was translated into an undulating wave by the vibrations of the bones of the ear. From his multiple telegraph experiments, Bell gained a similar understanding of how the vibrations of a reed or a combination of reeds could be translated into what he called an undulating electric current that would reproduce the sinusoidal pattern of the sounds.
At the urging of Gardiner Hubbard, Bell had begun keeping a notebook at around the time he applied for his patent. Notebooks are one of the ways in which inventors establish priority for their ideas. On February 18, 1876, Bell drew an ear with two different mechanical representations next to the bones (see Figure 15). On the left was an electromagnet, suggesting that the bones would serve a function similar to the steel reed he had so often placed over an electromagnet to transmit and receive complex tones. On the right was an iron cylinder attached to the bones and this vibrated in the center of a magnetized helix with an iron core. Bell had conducted experiments with such an arrangement, verifying that it could produce an undulatory current; he would later develop this mechanical representation into a telephone receiver (Bell, 1908). Beside the sketch, Bell wrote, "Make transmitting instrument after the model of the human ear. Make armature after the shape of the ossacles. Follow out the analogy of nature" *(Bell, 1876b, p. 13).
Figure 15: Bell's ear mental model. 'a' denotes the bones of the middle ear. The text under 'Fig 5' reads '(Helix & core, iron cylinder vibrated in helix)' and at the bottom right Mabel Bell notes that she copied the figure on February 21st.
The ear analogy provided Bell with a mental model that suggested possibilities and problem areas. Consider Bell's ear diagram (see Figure 15, again). It shows two possible arrangements of electromagnets that could be used to translate the vibrations of the ossicles into an electric current, one of which he had already used in building his Gallows telephone (the one on the left in the figure). Bell knew he could not include the ossicles in an actual speaking telegraph. Therefore, this sketch served as a reminder of the mental model he had been working with at least since June 30th, 1875, and possibly before.
Why did Bell need to state his mental model a year after he had designed a device which apparently embodied it? He could have been using his notebook to remind himself of his 'latitude and longitude' at this point. As noted above, this kind of reflection on one's representation and strategies frequently occurs when an expert is stuck or moving into a new domain. One could argue that Bell was simply making a statement for use in possible patent disputes, but the entry is not witnessed, nor did Bell ever use it in court. For Bell, his notebook was not just a record designed to establish his priority--it was a thinking tool.
As of February, 1876, Bell had a patent, but not a device which actually transmitted speech. Therefore, for Bell, the patent was not the end-point of a long process; it was the beginning of a new stage of research, of going from a patentable idea to a marketable product. At the beginning of this stage, he needed to reflect on his goals and remind himself of his mental model of how to achieve it.
The statement that he was 'following the analogy of nature' suggests that he was also reflecting on the heuristic he should use to accomplish this goal. Like confirmation and disconfirmation, 'follow the analogy of nature' is a higher-order heuristic used by other inventors as well as Bell. For example, the inventor of nylon, Wallace Hume Carothers, went back to nature and roughly imitated the way in which the silkworm puts together the building blocks for a protein (Friedel, 1994). A more recent example is provided by Misha Mahowald, who has made a career out of following the analogy of nature: she has built both a silicon retina and a silicon neuron, trying to imitate nature's solution as closely as possible (Mahowald & Douglas, 1991; Mahowald & Mead, 1991).
But telling a budding inventor that he or she should 'follow the analogy of nature' gives little guidance about what experiments to perform. This heuristic, like all heuristics, depends heavily on having an appropriate problem representation, in this case a mental model of nature's solution, and also some idea of how to map nature's solution onto the current problem domain. In order for Bell to copy nature, he had to have a detailed understanding of how the bones of the ear translated speech into undulating, sinusoidal waves, and how such waves could be reproduced electromechanically. Bell obtained this understanding from devices like the ear phonautograph.
The central claim of Bell's patent was that this undulating current was the best method to transmit sound, as opposed to the intermittent, or on-off, current commonly used in telegraphy. The undulatory current preserved the gradual changes in intensity produced by speech or musical tones; the intermittent current reduced these often subtle variations to an on-off code. In the first claim at the end of his patent 174,465, Bell claimed "a system of telegraphy in which the receiver is set in vibration by the employment of undulatory currents of electricity" *(Bell, 1908, pp. 459-60); his main example of how to do this was his reed mechanical representation, which could send telegraph signals, musical notes, or even speech. Bell's patent was breathtakingly broad: anyone who used the undulating current to transmit information could potentially be in conflict with Bell's claims.
This page was last edited: Wednesday, July 14, 1999