The Accidental Telephone
Alexander Graham Bell's work (mid 1874) with harmonic reeds, specifically in the context of his "Harmonic Telegraph (HT)," was a crucial step in his journey towards inventing the telephone. It led him to understand various ways to transmit sounds electrically. His initial motivation was to improve the telegraph, not invent the telephone.
Bell’s goal was to send simultaneous telegraph symbols (multiple telegraph) over a single wire using tuned reeds at each end. The reeds, like musical tuning forks, were designed to vibrate at specific frequencies. So, the symbols/sec rate could be increased. Think of each reed as representing a separate symbol. Certainly, a worthy goal and one with commercial promises.
To understand the idea of the HT, let’s first look at one component of such a device. A full Harmonic Telegraph is explained later in this section. In Bell’s famous patent 174,465 (March 7, 1876), he describes a harmonic sender and receiver, Fig 1. “The armature c (on A) can be set in vibration in a variety of ways, one of which is by wind, and, in vibrating, it produces a musical note of a certain definite pitch at I.” The armature has a natural, tuned, resonant frequency.
Fig 1, From Bell patent 174,465, Fig 5
The patent continues, “The moment the armature [also called a reed] is set in vibration to produce its musical note, a powerful inductive action takes place, and electrical undulations traverse the circuit “g b e f g”. So, armatures “c and h” of the two instruments are in unison with one another.” Incidentally, this is only true if the transmitting armature is magnetized. The purpose of the battery is discussed below.
Thomas Watson, Bell’s able assistant, delivered an address at the annual meeting of the AIEE in New York on May 18, 1915 [Watson]. He discussed an experiment (based on Fig 1) in June 1875, 40 years earlier, where he and Bell made an important accidental discovery. He said:
“On the afternoon of June 2nd, 1875, Bell was [near a receiver] at that very moment I happened to snap the steel reed of an instrument. I gave it several vigorous plucks, and out Bell came in great excitement to see what I had been doing, telling me that he had heard in the receiver and the unmistakable timbre of the sound of one of the reeds.
His excitement came from his realization that he had heard the first real sound that had ever been transmitted electrically (Endnote A). It needed but a slight examination of the apparatus to reveal the fact the steel reed I had snapped, magnetized by its long use in connection with magnets, was functioning as a magneto electric generator and by its vibration had generated in its [coil] an electric current that was molded into undulations exactly analogous to the sound waves of the plucked reed.
Bell saw at once that he had been wrong in thinking that the vibration of a steel reed could not produce electric waves of any practical value and that here was the solution, not only of his harmonic telegraph but also for the speaking telephone.”
Importantly, if the metal reed had not been partially magnetized, plucking the sender reed would not have caused the receiver's reed to vibrate. So, it was by chance that the effect of sound transmission was discovered, at least by Bell and Watson. See Appendix B- Flying Blind: The Early Inventors of Electrical Devices.
Fig 2 shows replicas of the apparatuses in Fig 1, built by Bell Laboratories in 1924.
Fig 2, Replicas of a “tuned reed” Transmitter/Receiver (London Science Museum)
Importantly, the concept of transmitting speech was not discovered accidentally but the actual method to do so was by chance and not by design. Source [IEEE] states, “In the summer of 1874, while visiting his parents in Brantford, Ontario, Canada, Bell hit upon a key intellectual insight: to transmit the voice electrically, one needed what he called an “induced undulating current (an analog signal).”
The first person known to propose the idea of transmitting speech by electricity was Charles Bourseul in 1854, a French telegraph engineer [Bourseul]. This was ~20 years before Bell's first thoughts on the matter. So, the concept was germinating for almost two decades before the first sprouts of a method appeared from Bell and others.
The Harmonic Telegraph
It’s worth considering what a Harmonic Telegraph could have looked like based on Bell’s ideas. Fig 3 shows an image from [Deposition]. It was never built but was a guiding principle in his search for an improved telegraph. Each reed was tuned to a different frequency.
In a sense, it could be played like a piano for sending telegraphic alphabet symbols. If, say, reed 3 was plucked then only reed 3, in theory, on the receiver would vibrate in sympathy with the varying received current. Note the large magnet, M. This magnetized each reed and was necessary to modulate the line current (coil E) when one or more reeds were plucked.
Fig 3, Bell’s Harmonic Telegraph, from [Deposition]
Bell also imagined the device in Fig 3 functioning as a "harp telephone." Utter a sound near harp H and select reeds would vibrate accordingly. Then the corresponding reeds in H' would vibrate in sympathy but with less vigor. Bell could see that this was not practical (many hundreds of reeds needed for speech) but the concept encouraged him to continue his search for a workable telephone [Kingsbury].
Based on Watson’s speech of 1915, Bell redirected his energy to the “speaking telephone.” Bell and Watson made incremental steps in this direction and abandoned improving the telegraph.
From the same patent (174,465) Bell advanced the idea of a telephone. “One of the ways in which the armature c, Fig. 1, may be set in vibration has been stated above to be by wind. Another mode is shown in Fig. 4, whereby motion can be imparted to the armature by the human voice or by means of a musical instrument.”
Figure 4, from Bell’s patent on the speaking telephone
The Fig 4 image has become an icon of invention. Note that Bell’s initial telephone concept was only one way, from speaker on the left to receiver on the right. Although it could have worked bidirectionally. This instance was not pragmatic because the transmitter side was not sufficiently sensitive.
Fig 5, USA postage stamp issued 1976
Figure 4 is Figure 1 with cones attached to focus the speaker’s sound toward its reed and to focus the receiver’s reed vibrations toward the listener.
You may wonder what the purpose of the battery is. The answer is not obvious. See Appendix A for a complete discussion.
An improved telephone transmitter
So, what transmitter design did Bell use when he spoke his famous words, “Mr. Watson, come here, I want to see you" ? It was based on the changing resistance caused by a vibrating needle inserted into a small bath of conductive acid. See Fig 6, from Bell's notebook.
This model was also not practical for many reasons, one being the strict need to keep the needle a hair's breadth below a level surface of the liquid. Apparently, he tried many liquids including water (no sound), cod liver oil (no sound), salt water (sound). A mixture of water and sulfuric acid seemed to work best. Notice the receiver is like Fig 4 minus the cone.
Fig 6, A.G. Bell's first demonstrated telephone liquid transmitter.
The Centennial Exhibition Telephone
Bell wanted to make a sensational public telephone demonstration at the Centennial Exhibition in Philadelphia on 25 June 1876. So, he crafted a handsome telephone transmitter of yet another design. Bell knew the liquid transmitter was not practical for a live demo so he modified Fig 4 above by adding a flexible diaphragm seen in Fig 8. This model is sometimes called "Bell's first telephone" despite that the Fig 6 liquid model was used to utter Bell's famous words.
Fig 7, Bell and Watson with their liquid-based telephone (portrayal, Library of Congress)
Fig 8, A replica of Bell's 1876 Centennial telephone transmitter, made by Science Museum Workshops, South Kensington, London, England, 1959.
The black wire coil is obvious in Fig 8. Voice power moves the diaphragm with a magnet attached to generate current in the coil that is sent via a wire to the receiver. According to experts at the London Science Museum, “Although this was an improvement, it was still problematic, as the parchment was hygroscopic, meaning that just a few minutes of talking allowed the parchment to absorb enough moisture from the breath to cause it to lose its tightness, resulting in a reduced quality of sound, and eventually the iron disk becoming stuck to the electromagnet, so ending the telephone call.”
Bell and Watson tried many variations of transmitters. One with a powerful horseshoe magnet "in series" with the two pickup coil's soft iron cores. Apparently it worked quite well (Nov. 1876). See [Bell] for some images from his excellent notebooks hosted by the US Library of Congress. See Bell patent 186,787 from January 1877 for an image of this device. It was Bell's first practical transmitter that could send speech over 100 miles and was manufactured for several years before carbon-based transmitters took over.
Exponential telephone growth
Thomas Edison is given credit for inventing the carbon granule transmitter (US222,390, 1879) and a pressure-sensitive carbon button microphone (US203,016, 1878). Francis Blake is also credited with inventing the carbon button microphone with a 1879 patent in England (US250,126 patent,1881). Emile Berliner, a Bell employee, also had a hand in carbon transmitter improvement as did David Edward Hughes. These transmitter inventions were catalysts for the aggressive commercialization of telephony.
Within three months of Bell, Hubbard, Sanders and Watson founding the Bell Telephone Company in July 1877, about 1,300 devices were in operation in North America. By January of 1880, there were about 30,000. [Heritage]
Another interesting metric is the growth of "toll lines." These are inter-office lines for linking local offices. By 1883 there were 13,653 miles of toll wire and by 1884 it reached 29,359 miles . This is amazing growth considering there were no toll lines just 6 years earlier. Toll lines grew at an average compound rate of 20.4%/Yr for the next 32 years [Kingsbury] in North America.
Bottom line, Alexander G. Bell was determined to improve the telegraph. His knowledge of sound (he was a speech therapist and audiologist) gave him a head start. While tinkering with his Harmonic Telegraph apparatus, he and Watson accidentally discovered that sounds could be transmitted electrically. This famous “ah ha” moment was the start of a communication revolution.
Endnote A: History records that Bell likely was not the first to transmit sound using electricity. Johann Philipp Reis deserves recognition for his pioneering work in sound transmission as does Elisha Gray. See who invented the telephone. Regardless of paternity, Bell had momentum, good business sense, and patents to change the world.
References
Bell Notebooks, 1875-1876, https://www.loc.gov/resource/magbell.25300201/?st=gallery
Bourseul: https://en.wikipedia.org/wiki/Charles_Bourseul
Carbon: Vintage Microphones
Deposition: The Bell Telephone: The Deposition of Alexander Graham Bell, AT&T, 1908 (book). This document contains many explanations of Bell’s early experiments.
IEEE: https://insight.ieeeusa.org/articles/engineering-hall-of-fame-alexander-graham-bell/
Heritage: The story behind the worlds first telephone
Kingsbury, J.E., The Telephone and Telephone Exchanges, 1915. (page 425)
Watson: How Bell Invented the Telephone, Thomas Watson, AIEE PROCEEDINGS, volume 34, number 8, August 1915, pages 1503-13.
Acknowledgments
Many thanks to Remco Enthoven for editing suggestions and insights on Bell's early work.
Appendix A
Principles of the Harmonic Telegraph
As stated in the main article above, the invention of the telephone is directly linked to the invention of the Harmonic Telegraph. Bell, Gray and Edison were at the forefront of inventing ways to increase the number of symbols-per-second that could be telegraphed. There was a solid business model for this. Efforts to develop the harmonic/acoustic telegraphy led to the invention of the telephone.
How can a vibrating reed (audible or not) produce an electric current into a wire? This was Bell’s problem to solve. In the main article, both Fig 1 (basic) and Fig 3 are examples of his harmonic telegraph and, in theory, were solutions to his problem. Interestingly, only Figure 1 has a battery. We will see why later.
![Fig A1, Replica of Harmonic Transmitter [London Science Museum]](https://static.wixstatic.com/media/2167e7_e3550a8d4b05407faf1fdfda6f9a8c52~mv2.png/v1/fill/w_511,h_372,al_c,q_85,usm_0.66_1.00_0.01,enc_avif,quality_auto/2167e7_e3550a8d4b05407faf1fdfda6f9a8c52~mv2.png)
Fig A1, Replica of Harmonic Transmitter [London Science Museum]
A harmonic transmitter can be actuated manually by plucking its reed to set it into momentary vibration, as shown in the main article. The diminishing signal output is sinewave-like following the magnetized reed’s swinging motion.
Another method to create a signal (Fig A1) is with a generator. In this case, the reed is set in motion “like an ordinary vibrating bell” [Watson] with the coil repeatedly attracting and releasing the reed via the current interrupting terminal (top center). This means is battery powered and generates a continuous output signal [Hounshell]. The vibration frequency can be set using reed length, stiffness, and other ways. The output voltage is not a smooth signal but repeating on/off pulsing, a buzzy sound. Some authors call this a “single-tone” sound.
Fig A1 is an example of Bell’s continuous output transmitter. The middle terminal is part of the circuit for interrupting the current to the coil. A search of US patents shows that Alexander G. Bell, Thomas Edison and Elisha Gray had accumulated 9 patents related to acoustic telegraphs (1875-1880).
Fig A2, Elisha Gray’s Harmonic Telegraph Transmitter. Steel reed hangs in front of one pole of an electromagnet. It can be made to constantly vibrate at a “single resonate frequency” using a similar means as with Fig A1. [National Museum of American History]
So, the inventors of these devices used both the manual method (Figures 3 and 3A) and the continuous signal method depending on the need. With the manual method, one important design variable is the output voltage of the coil during vibration. What methods are used to maximize the output? Let’s see.
Optimizing the output voltage of the coil
Faraday’s Law (1831) states that a moving magnet induces a current into a wire (or coil of wire). Bell stated such in his patent US174,465:
“It has long been known that when a permanent magnet (the reed) is caused to approach the pole of an electro-magnet a current of electricity is induced in the coils of the latter, and that when it is made to recede a current of opposite polarity to the first appears upon the wire.”
If the flexible armature is not magnetized, then plucking it will induce zero current into the coil. During Bell’s early experiments it seems he and Watson did not clearly understand this.
Fig A3 shows nine transmitter/receiver “instruments” (from Bell patent US174,465). When armature A is plucked, devices A1 and A2 will vibrate in sympathetic resonance momentarily. The higher the output voltage from the transmitter the more vigorous the receiver’s reed will vibrate. Likewise, the more efficient a receiver, the louder its generated tone.
According to the same patent, “A reed can be set in vibration in a variety of ways, one of which is by wind, and, in vibrating, it produces a musical note of a certain definite pitch…. Another mode is when motion can be imparted to the armature by the human voice or by means of a musical instrument.”
Fig A3, Harmonic Telegraph transmitters and receivers
With a battery the transmitter coil performs two separate functions: one is to magnetize the iron core, which in turn keeps the reed/armature magnetized. This is especially needed if the reed is made of flexible soft iron. The other is to act as a pick-up coil for inductive generation. When the reed is permanently magnetized, the battery can be eliminated, and the coil acts only as a pick-up [Meyer].
Incidentally, with a soft iron core, the reed's motion causes strong flux variations, leading to a higher generated voltage. With strongly magnetized steel at the core, the flux variations will be dampened, reducing the generated voltage.
These scenarios add insight because Bell, Watson, and other inventors of the time likely did not completely understand electromagnetic theory or the design trade-offs necessary for improving device performance. There was plenty of guess work. See Appendix B- Flying Blind: The Early Inventors of Electrical Devices.
Electromagnetic Theory
The development of Maxwell’s equations [Maxwell] for electromagnetism in the 1860’s was the pivotal moment when the “density of the medium” (soon to be called permeability) was rigorously defined. It’s the measure of magnetization produced in a material in response to an applied magnetic field. Permeability was coined by William Thomson (Lord Kelvin), in 1872, just a few years before Bell’s experiments. While Bell did know a great deal about vowel sounds, he was a novice working with electricity [Franklin].
It would have helped Bell to know that a soft iron core quickly magnetizes when exposed to a magnetic field due to its high permeability, allowing magnetic flux to pass through easily. It may have helped Bell to know that hard iron is 50X more permeable than steel.
Another aspect to confuse inventors is that soft iron is highly susceptible to induced magnetism, meaning it quickly magnetizes in the presence of an external field but loses its magnetization just as fast when the field is removed. Steel, on the other hand, tends to retain polarized magnetism due to its higher coercivity, making it better suited for permanent magnets. Bell, Watson, Gray and others were likely in the dark regarding polarized verses induced magnetic fields and some of their notes indicate so [Meyer].
Through the process of trial and error, Bell, Watson and contemporaries figured all this out.
[Meyer] says. “Bell sought … to design an instrument that would work without a battery. He knew that the magnetic field could be provided by adding a permanent magnet to the design rather than using a battery in the circuit.”
The 1888 Supreme Court cases against Alexander Graham Bell, known as the Telephone Cases, were legal battles challenging Bell’s telephone patents. During one argument [Supreme, page 172] a Bell opposing attorney stated: “The efficient (part) is the magnetic force and its source is the magnetic field; and the battery current, where a battery is used, is not in any sense the cause of work, being used merely to magnetize the cores of the electro-magnets.”
Conclusion
Bell and Watson's valuable experimental work on the harmonic telegraph laid the groundwork for their subsequent breakthroughs in telephony. Chance played a big role in their many incremental advancements.
The Library of Congress notes that Bell compiled over 200 volumes of laboratory notebooks just from 1879 to 1922. One notebook selection, Fig A4, “Experiments made by A Graham Bell”, contains ~225 separate drawings related to his experiments.
When Bell famously said on March 10, 1876, ‘Mr. Watson, come here, I want to see you,” (Fig A5) Watson recalls, “I came at once”. During a speech in 1915 Watson said, “Commonplace as it was, the sentence seemed to break the spell, and the telephone progressed after that by leaps and bounds” [Watson 2].
Fig A4, Pages 1 and 2 of Bell’s 252 page notebook (Vols I, II and III) from Oct 1875 to Nov 1876 (Library of Congress)
Fig A5, Page 35 of Fig A4 notebook. Bell writes, shouting into microphone M, “Mr. Watson - come here - I want to see you”, March 10, 1876
References for Appendix A
Darrow, Floyd, The Boys’ Own book of Great Inventions, 1918.
Franklin Institute: Search “Alexander G Bell”. https://fi.edu/en
Hounshell, David, Elisha Gray and the Telephone: On the Disadvantages of Being an Expert, Technology and Culture, Vol. 16, No. 2 (April,1975)
Maxwell, James Clerk, A Treatise on Electricity and Magnetism, 1873
Meyer, Ralph, Bell, Watson, Soft Iron, and The Insight That Commercialized the Magneto Telephone, PROCEEDINGS OF THE IEEE, Vol. 108, No. 12, December 2020.
Supreme Court, Telephone Cases, 126 U.S. 1 (1888) https://supreme.justia.com/cases/federal/us/126/1/
Watson, Thomas, The Birth and Babyhood of the Telephone, 1913, Page 15
Watson 2: How Bell Invented the Telephone, Thomas Watson, AIEE PROCEEDINGS, volume 34, number 8, August 1915
Appendix B
Flying Blind: The Early Inventors of Electrical Devices
Long before the electron was discovered in 1897 by J.J. Thomson, inventors and engineers had built an electrical world guided by models of “invisible fluids and frictions.” By 1876, when Alexander Graham Bell patented his telephone, “electricity” was still widely described as a fluid substance moderated by friction, induction, or chemical action [Ref 1].
“An imponderable fluid” is a term used to describe electricity by Professor A. E. Dolbear in 1877 [2]. These fluid theories, despite being incorrect, were sufficient to spark a cascade of inventions that transformed communication, power, and lighting.
James Clerk Maxwell, famous for his Laws of Electromagnetism, never imagined electricity as particles migrating through wires. In his very first electromagnetic paper, “On Faraday’s Lines of Force” (1855), he modelled both electric and magnetic phenomena as mechanical stresses and flows in an all-pervading ether. In Maxwell’s view, an electric current was a perturbation (a displacement or flow) of this elastic fluid.
When Bell and his assistant Thomas Watson transmitted speech at the Centennial Exposition in Philadelphia in 1876, they relied on electromagnetism and conducting wires without knowing about electrons. Likewise, designers of the early magneto ringing generators were mostly blind to how lines of force operated inside of materials and coils. Builders of devices relying on magnetism based their work on experiments often without understanding the importance of permeability (theoretically grasped 1865-1872), hysteresis/retentivity (1880s), and coercivity (1890s), for example. Once these and other parameters were understood, new apparatuses were developed and device efficiency increased dramatically.
Thomas Edison’s 1879 practical incandescent lamp achieved over 1,200 hours of a glowing filament by optimizing vacuum, filament material (carbonized bamboo), and current density without any concept of electrons drifting through a lattice. He was a proud experimentalist and had little time for mathematical descriptions of concepts. He once said, “I can always hire a mathematician – they can’t hire me” [4].
It was J.J. Thomson’s careful deflection measurements of cathode rays (electrons) at Cambridge’s Cavendish Laboratory (England) that finally solved the mystery of conduction: the mobile electron carried a discrete negative charge in a vacuum, gas, metal wire and other structures [3]. Only then could the scientific community discard fluids, ether, and frictions and embrace a particle theory of electricity.
Did the new understanding of electron flow advance electrical device development? Absolutely. Consider the vacuum tube (1904, triode 1907) and the incredible transistor (1947). By 1913 vacuum tubes were just starting to gain traction as amplifiers and this enabled signal repeaters for the first transcontinental telephone call in 1915 [5].
Bell’s telephone, Edison’s light bulb, and Maxwell’s field theories greatly succeeded despite the meager understanding of electricity and magnetic properties at the time. This underscores a profound insight; engineering leaps can outpace scientific fundamentals. By endless experimentation coupled with bold guess work, the inventors of the 19th century constructed an electrical revolution with only modest knowledge of the currents they were commanding.
References for Appendix B
1 https://electricityforum.com/a-timeline-of-history-of-electricity
2 The Telephone, Prof. A.E. Dolbear, 1877 (page 40)
3 Cathode Rays, Philosophical Magazine and Journal of Science, by J.J. Thomson, Series 5, Volume 44, October 1897, pp. 293–316
4 A brief History of Light and Those that Lit the Way, Richard Weiss, 1996, page 66.
5 A History of Engineering and Science in the Bell System-The Early Years (1875-1925), M.D. Fagen (editor), Prepared by Members of the Technical Staff, Bell Telephone Laboratories. (page 260).