top of page
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 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 sending armature has some residual magnetism. The battery, g, provides a constant bias current to bend the armatures slightly. This bend allows any additional current modulations to move the reed about its resting place.
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.
Fig 2 is a replica of the apparatuses in Fig 1, built by Bell Laboratories in 1924.
Fig 2, Replica of the “tuned reed” receiver/transmitter (London Science Museum)
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) 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.
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. The receiver was similar to Fig 2. Fig 7 shows Watson and Bell (actors portrayed) with their latest invention. 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 often 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 Centennial 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.”
By 1877, Thomas Edison had improved the telephone transmitter by developing a microphone with a pressure-sensitive carbon button. This invention and that of Francis Blake’s carbon transmitter (loose carbon granules) were the catalysts for the aggressive commercialization of telephony [Carbon]. Emile Berliner, a Bell employee, also had a hand in transmitter improvement.
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 toll lines . 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
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.
Heritage: The story behind the worlds first telephone
Kingsbury, J.E., The Telephone and Telephone Exchanges, 1915. (page 425)
Watson: Who Bell Invented the Telephone, Thomas Watson, AIEE PROCEEDINGS, volume 34, number 8, August 1915, pages 1503-13.
bottom of page