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Mike Gorman: Bell's Path the the Telephone--Home Page
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Mike Gorman: Bell's Path the the Telephone--Home Page


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Alexander Graham Bell's Path to the Telephone

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Introduction

Michael E. Gorman
Technology, Culture & Communications, SEAS
University of Virginia

To organize and depict, in abbreviated form, Alexander Graham Bell's
invention of the telephone, we [1] have created a series of flowchart "maps"
that include every sketch we have been able to locate from Bell's
experimental notebooks, patents, depositions in court and correspondence. As
the dates on the map indicate, time advances as on the maps from top to
bottom. Multiple boxes spreading from right to left at the same time
indicate that Bell was pursuing several lines of research at that point.

When we say that Bell followed a path to the telephone, it makes his
innovation process sound more linear and goal-directed than it really was,
though Bell tried very hard to be scientific in his approach [2] and
therefore was more linear than his competitors Edison and Gray. We refer to
this flowchart as a map because the term flowchart implies more logical
structure than does map, which may reflect the wanderings of an inventor.

This series of maps is arranged hierarchically. The top level depicts the
major experiments along Bell's path to a patent and to a device that
successfully transmitted speech. When you click on one of the sketches in
boxes on this top level, you will move to a lower-level map, depicting a
series of experiments that were subsumed under that higher-level box. Some
of these lower level maps will be combined with text which describes the
depicted experiments and/or sketches. These maps were originally developed
using a program called TopDown on the Macintosh; exporting them to the World
Wide Web has caused us to make format changes that we think will result in
improvements over the longer term, but in the short term, may make them more
difficult to use. We welcome feedback as you attempt to explore Bell's path.
What follows is a narrative that will help you to understand the structure
of the top-level map, and will also provide references for further study.

When Bell started the early experiments depicted in the top level of the
map, he wasn't thinking about a telephone. Indeed, as the
parallelogram-shaped box on the upper right-hand part of our map suggests,
he was thinking about the cutting-edge technology of his day: the multiple
telegraph. Throughout the maps, this shape will indicate an inference about
Bell's goals at a particular stage. When the goal is explicitly stated, we
use a box with a wedge pointing toward the goal. At the time, everyone knew
that the inventor who could create a device that would send multiple
messages over the same wire would reap fame and fortune. A duplex that could
send two messages simultaneously was available by the early 1870s, but Bell,
Edison, and Gray were all in pursuit of a device that could send four, six,
eight, or more messages.

Bell's first idea for a multiple telegraph stemmed from his observations of
Helmholtz's apparatus for producing vowel sounds electromechanically. The
oval to the right of the "Mental Model for a Multiple Telegraph" box
suggests the Helmholtz influence; a sub-map under this oval suggests how
Bell found out about Helmholtz. Bell wanted to use Helmholtz's scientific
discoveries as the basis for a working device. Here we appear to have a
clear-cut case of an inventor borrowing his ideas from others. If Bell had
simply taken Helmholtz's apparatus and tweaked it a bit to create a multiple
telegraph, there would be no need for a cognitive map--one could trace an
invention path that required virtually no mental processing. Bell, however,
did more than modify the Helmholtz apparatus, he transformed it. Indeed, he
misunderstood it in a creative way. Helmholtz's device used a series of
tuning forks and resonance chambers to simulate vowel sounds. A single
tuning fork continually interrupted the circuit, which kept all the other
forks in constant vibration. Bell could not read Helmholtz in the original
German; instead, this apparatus was described to him by Alexander Ellis, and
Bell derived much of his understanding from complex diagrams. Therefore,
according to Robert Bruce,[3] he made an important error. He assumed the
lower interrupting fork was transmitting vowel sounds which were reproduced
by the other forks.[4] Bell assumed that if vowels could be transmitted over
wires, so could other sounds, including musical tones and consonants.

Therefore, this mapping method allows us to represent how an inventor
transforms knowledge from outside sources and contacts. An inventor's or
user's mental representation of a device is not always the same as--or even
similar to--the way the designer intended it to be represented. In the case
of invention, two kinds of representation are particularly crucial.

Mental Models and Mechanical Representations

In terms of our cognitive framework,[5] the Helmholtz interrupting fork and
resonator served as mental models for Bell's harmonic multiple telegraph.
Mental models are dynamic visual representations of devices, objects, or
forces that an inventor or scientist can "run" in his or her "mind's eye."
There is a growing literature on mental models in cognitive psychology; the
term is not always used in the same way by different authors [6]. Our
meaning and use are best illustrated by examples.

Consider the interrupting fork and resonator. For Bell, these devices served
as mental pictures of how a multiple telegraph might be achieved. The box
below the goal statement on the upper right contains Bell's first attempt to
build a multiple telegraph transmitter and receiver. The transmitter closely
resembles Helmholtz's interrupting fork; it made or broke contact with a
dish of Mercury, which alternately completed and interrupted the circuit.
Similarly, the receiving end resembles Helmholtz's upper resonator; the
electromagnets attracted the tuning fork every time the circuit was
completed, causing the fork to vibrate. (see lower level maps subsumed under
the "Mental Model for a Multiple Telegraph" box).

We have put boxes around the transmitter and receiver to indicate what we
call "slots",[7] or areas an inventor can concentrate on. For example, if
one divides Bell's multiple telegraph into transmitter and receiver slots,
one can then imagine putting different devices in place of the Helmholtz
fork and resonator.

Bell intended to set up a series of tuning forks which made or broke contact
with mercury cups, and match them with tuning forks on the other end of the
circuit that would vibrate at the same frequency. Four, six, eight or more
tuning forks could send separate tones over the same wire to the same number
of matching tuning forks, each of which would respond only to the vibrations
sent by its "twin" on the transmitting end.

In his subsequent experiments, he began with devices that looked like
Helmholtz's, but gradually developed alternatives that accomplished the same
goals. These alternate devices are what we call mechanical representations;
they can be inserted into slots like different values into a variable or
function. Experiments with different mechanical representations often
suggest alterations in the mental model, as we shall see below.

Below the "Mental Model for a Multiple Telegraph" box is a conclusion,
marked by a box with a wavy line on the bottom. This indicates what Bell
learned from the line of experiments associated with the box: he now had a
clear idea how to transmit musical tones, though he had not mastered the
complexities of the circuits involved in turning this idea into a successful
multiple telegraph. Indeed, in response to his difficulties with circuits
and connections, Bell made an important decision about his cognitive style:
"It became evident to me, that with my own rude workmanship, and with the
limited time and means at my disposal, I could not hope to construct any
better models. I therefore from this time (November, 1873) devoted less time
to practical experiment than to the theoretical development of the details
of the invention."[8]

To show how Bell evolved a new mechanical representation, let us explore one
of the sub-maps associated with the box labeled "Mental Model for a Multiple
Telegraph." Figure 3* shows a lower-level, more detailed map of how Bell
developed his tuned reed relay, a mechanical representation he used
repeatedly as a kind of "transceiver" He began with two tuning fork
arrangements that were attempts to reproduce Helmholtz's apparatus and
effects as closely as possible. Then he switched to a steel plate, vibrating
over twin electromagnets, an idea he got from reading J. Baille"s The
Wonders of Electricity.[9] Finally, he left one end of the plate free to
vibrate. The result is displayed in the box labeled "Universal Transceiver."

Here we think Bell alters his mental model to accommodate his insight that
the same device can be used as both transmitter and receiver. In effect, he
merges his transmitter and receiver slots into kind of a "transceiver" slot.

The simplicity of this approach is appealing--and unique to Bell. Edison,
for example, recognized immediately that one had to perfect a distinct
transmitter. Gray also had a series of separate transmitters and receivers,
and only used a "transceiver" in one of his later attempts to get around
Bell's successful patent that included a speaking telegraph. [10] Bell's
experiments with reed relays and similar devices in complex telegraph
circuits continued throughout this period. Indeed, the line that branches
back to a circuit with two of these reed relays suggests how this line of
research played a critical role in Bell's first telephone--but we will say
more about that later (in the June 2nd Experiment section).

Inventors need not be limited to a single mental model; indeed, they can
consider several alternatives at one time. Bell was no exception. The
"Alternate Mental Model for a Multiple Telegraph" box shows that, in
addition to imagining a multiple harmonic telegraph in which the same device
served as both transmitter and receiver, Bell also thought about, and
experimented with, transmitters and receivers that could handle multiple
tones. In this particular sketch,[11] a cylinder of bar magnets is rotated
rapidly in front of a magnet and induces a current. This intermittent,
make-or-break current, is transmitted to a coil of wire in the center of
which a nail vibrates, making a crackling sound: "The sound issuing from the
helix is a kind of crackling noise, and cannot be called a musical note
although its pitch is quite apparent."[12]. The pitch of the sound coming
from the vibrating nail can be altered by rotating the cylinder more rapidly
or more slowly. Hence, multiple operators could send distinct tones over the
same wire simply by rotating their cylinders at different rates, and the
same kind of receiver could be used to translate these different
rotation-rates into distinct messages. This scheme obviated the need for
separately tuned forks on either end.[13]

Vibratory Analyzer Slot

How would a telegraph operator be able to distinguish different tones
reliably? Remember that the operator might have to distinguish eight or more
separate messages sent either to separately tuned forks or to a single
universal receiver. Both Bell and his competitor, Elisha Gray, confronted
this problem after they had established that the transmission of distinct
tones was possible. The box labeled "Vibratory Analyzer Slot" shows Bell's
solution, which he was able to patent.[14] The "P" inside a circle next to
the box indicates that the sketch is part of a successful patent and flags
patents in our mapping methodology.

The new slot is shown to the right of one of Bell's tuned reed relays. As
the steel reed vibrates, it makes contact with a lever, the other end of
which dips in and out of two cups of mercury, alternately making and
breaking a circuit. (See Figure 4*) Therefore, the harmonic vibrations of
the reed were translated into on-off impulses suitable for telegraphy. This
lever and twin cups resembled Morse's original telegraph portrule,
illustrating the way in which mechanical representations can readily be
borrowed from another source.[15]

The Morse device provided Bell with a mechanical representation that he used
in his March 6 application. But in his patent, Bell made it clear that it
was the principle he was after, not just a specific mechanical
representation: "Many forms of circuit breakers for the purpose may be
employed such as membranes &c., all that is required being that the circuit
breaker shall be capable of vibratory or oscillatory movement, and that its
normal rate of movement, when in oscillation or vibration, shall be slower
than that of the receiver by which it is actuated."[16] Indeed, Bell later
patented an improvement in this vibratory circuit-breaker.[17]

So, by the spring of 1875, Bell had a mental model for a complete system of
multiple telegraphy and had succeeded in patenting parts of it. A
significant part of this mental model was the idea that the goal in multiple
telegraphy was to transmit what Bell called an "undulatory current" as
opposed to the intermittent or make-or-break current used in single
telegraphy. To trace this important aspect of Bell's mental model, we will
have to go to the upper left-hand box of the Master Map.

The Ear Phonautograph

Bell lacked the electrical knowledge and expertise of other multiple
telegraph inventors like Edison and Gray. But he did posses a unique area of
expertise. He was a teacher of the deaf, and therefore understood the
importance of speech in communication. His father, Alexander Melville, had
invented a "Visible Speech" alphabet that would help the deaf learn to
speak. Bell was similarly interested in devices that would help the deaf
"see" speech, as is indicated by the goal box above "The Ear Phonautograph"
(see the top left-hand corner of the Master Map).

Bell's interest in teaching the deaf kindled his interest in devices used to
visualize sound; these devices are represented on sub-maps beneath the ear
phonautograph box. At MIT, he experimented with Koenig's manometric flame
and a version Scott's phonautograph that had been improved by Charles Cross'
pupil, Charles Morey. Bell planned to use the devices to give a deaf child
feedback. Bell would make templates of speech sounds and then instruct the
child to speak into one of the devices and reproduce the template pattern.
Since Bell could not physically record the manometric flame patterns using
photography and since the patterns were difficult to discern, he
concentrated on the phonautograph.

Controlled tests of the two devices revealed differences in curves produced
in response to the same sound. Bell concluded 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 moved by the membrane--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."[18]

Bell built an ear phonautograph in 1874 following a suggestion from Clarence
Blake (a more detailed picture of this device can be found by clicking on
the top level phonautograph box and going to a lower level). It consisted of
the bones of an actual human ear, mounted on a wooden frame. When one spoke
into it, the bones vibrated; a bristle brush descending from the bones
traced the shape of the sound waves on a piece of smoked glass, which could
be rolled back and forth underneath..

The box with the wavy line below the phonautograph shows the conclusion Bell
drew from this experiment--that sound could be translated into visible
waves. From this phonautograph, Bell gained a tactual, "hands-on"
understanding of how speech is translated into sinusoidal or undulating
waves. From his multiple telegraph experiments, Bell gained a similar
understanding of how the vibrations of a reed could be translated intoelect
ric current, and reproduced as sound.

The Harp Apparatus

Bell's background in telegraphy and in the visible reproduction of sound
come together in the box labeled "Harp Apparatus", which represents a new
mental model of how undulating sound waves might be translated into electric
currents of the same form. Bell sketched, but never built, this device in
the summer of 1874. To understand its function, it is better to refer to its
precursor, stored in a sub-map (found by clicking on the top level box).