A few hours later on the same day, Elisha Gray submitted his caveat for a speaking telegraph. His mental model for the transmission of speech was based on a device called 'the lover's telegraph', or what we would now call a 'string telephone'. According to Gray, this device "proved to my mind that all the conditions necessary for the transmission of an articulate word were contained in any single vibrating point... I saw that if I could reproduce electrically the same motions that were made mechanically at the center of the diaphragm... such electrical vibrations would be reproduced on a common receiver in the same manner that musical tones were" (1880, part II, 124-5).
In his caveat, Gray designed a speaking telegraph that looked like a lover's telegraph with a cylinder of water between transmitter and receiver (see Figure 13). Gray intended to use water as a medium of high resistance. Hanging from the bottom of the speaking tube and diaphragm into which one spoke was a thin wire or rod. When one spoke into the resonant cavity, the diaphragm vibrated, causing the wire hanging from it to get alternately closer to, and farther away from, a contact on the bottom of the water; this motion caused a fluctuation in the current passing to the receiver that mirrored the movement of the diaphragm. The idea of using liquid variable resistance, Gray claimed, was 'old in the art at the time' (1880).
Figure 13: Gray's caveat for a speaking telegraph. The man with the moustache speaks into A, causing the needle attached to the membrane below A to vibrate, going alternately deeper and less deeply into the water at B. At the bottom of B is one end of the circuit; the other is attached to the needle. The water serves as a resistance medium; it conducts electricity, but poorly enough so that the small motion of the needle makes a big difference in the amount of current that flows across the ends of the circuit. The electricity continues to the receiver , which consists of a resonant chamber F and an electromagnet). As the current fluctuates, the strength with which the magnet pulls the bottom of the resonance chamber also fluctuates, causing it to resonate in a manner similar to the numan voice (from Prescott, 1884, p. 455).
Gray used familiar mechanical representations in constructing his speaking telegraph. For example, his receiver consisted of a resonant cavity he had used to receive single tones and a double pole electromagnet he had used in an analyzing receiver.
Because he did not have a working device, Gray filed a caveat or preliminary disclosure instead of a full application, and he was not especially concerned if some details of the apparatus were left somewhat vague. For example, in his caveat, Gray raised the possibility of employing multiple diaphragms just as he had used multiple transmitters in his harmonic telegraphs: "I contemplate, however, the use of a series of diaphragms in a common vocalizing chamber, each diaphragm carrying an independent rod, and responding to a vibration of different rapidity and intensity, in which case contact points mounted on other diaphragms may be employed" (Gray, 1977, p. 79).
In his technical history of the telephone, J.E. Kingsbury cited Gray's preference for multiple chambers to argue that in 1876 Gray was only at the level of understanding that Bell reached with his harp apparatus in 1874, in that each of these diaphragms would function like one of Bell's reeds and it would take a large number of them to reproduce the human voice (Kingsbury, 1915). But as we have seen, Gray's mental model was the lover's telegraph, which did not require multiple diaphragms.
Furthermore, in 1875 Gray had developed a mechanical transmitter with which "we obtained a great variety of sounds on the receiver, not unlike the human voice, imitations of vowel sounds, and also imitations of a groan as if in distress...This experiment with the mechanical transmitter confirmed what my previous experiments had led me to believe: that not only could the receivers that had been named be used as receivers of articulate speech transmitted electrically, but that such speech could be transmitted through a single point. I mean by single point, without the intervention of a series of reeds or points differently tuned, and one that would be a common or universal transmitter, in the same sense that the receivers were universal or common" (1880, p. 124).
So it is not clear why Gray still thought he might have needed multiple diaphragms to transmit speech. Recall Holmes' observation that "in moving from an existing conceptual framework to a new one, scientists often cannot make a single leap from one coherent mental framework to another. They may have to endure, for extended periods of time, deep fissures within their mental worlds" (Holmes, 1989). Similarly, an inventor may simultaneously consider alternate mental models which contradict one another in important respects. Gray's caveat is largely consistent with a mental model for the transmission of speech based on the lover's telegraph, but his remark about multiple transmitters is more consistent with a mental model derived from his musical telegraph experiences, where composite tones were produced by combining single-tone transmitters. Similarly, each of the multiple diaphragms in Gray's speaking telegraph would respond "to a vibration of different rapidity and intensity".
Mental models are provocatively incomplete, often fuzzy in important details. That fuzziness is the key to their creativity--it allows them to contain contradictions that spur the inventor to resolve them.
This page was last edited: Wednesday, July 14, 1999