Bell's Liquid Variable Resistor Experiments: March 8th-10th

A New Slot Diagram

Bell's conversation with the examiner might have encouraged him to conduct more experiments with resistance in the circuit. This map shows the next line of experiments conducted by Bell. He puts the tuning fork in the circuit and dips it into a dis h of water. Bell (1908) claimed he got the idea from a spark arrester he attempted to patent in January 1876, which involved breaking a circuit and inserting the ends into water. But the idea of trying this sort of arrangement at this stage in his pro blem-solving process may have come from his conversation with the examiner about Gray's caveat.

When he introduces the dish of water, Bell changes the resistance slot: now, instead of focusing on number of coils, he now must think about properties of a liquid. So, the upper left-hand corner shows that Bell removes the electromagnet and replaces it with a dish of water, resulting in a new slot diagram, which serves to remind us that this shift to liquids represents a transformation of the problem space, opening up new potential paths to a device that will transmit speech or any other sound.

Does this shift in the problem space suggest that Bell abandoned his ear mental model? His goal is still to produce the undulating current. The tuning fork still serves a role similar to the ossicles, and could be replaced by other armatures. Even the water has an analog in the ear: Bell almost certainly knew that the vibrations of the ossicles are transmitted to the auditory nerve through a liquid medium. So one could argue that the overall ear mental model has not changed, but that Bell is expl oring a new arrangement of slots which might, in turn, lead to changes in his overall mental model.

Alternatively, this line of experiments might have been directed more at understanding the principles behind the transmission of speech than creating a working device. Bell has a very general hypothesis, namely, that any device or combination of devices which can transmit or receive an undulating current should be able to reproduce speech; therefore, the most effective strategy, according to Klayman & Ha, would be a positive test heuristic, for two reasons:

(1) It is most likely to produce disconfirmations, because a large number of possible target rules are likely to be embedded within this very general rule, e.g., only specific sorts of devices that could generate such a current would be sufficient for sp eech.

(2) It is an effective strategy in environments where the possibility of obtaining false negatives is high.

In the top righthand slot diagram, there is a new 'contacts' slots which represents the fact that Bell focuses on the relative sizes and depths of the contacts and also a 'resistance medium' slot, indicating that he experimented with acidulating the water and, later, substituting mercury . Bell began with the experiment shown in the slot diagram; one tine of the vibrating fork was placed in the water.. This produced a 'faint sound'; hence, the diagonal arrow to the next experiment below. Next, he added a bit of acid to the water. This produced a much louder sound. Increasing the distance between the tuning fork and the conducting wire had no effect, which meant that Bell could ignore distance as a variable. He then added a strip of brass to t he conducting wire; this made the sound 'much louder' and completely immersing this wire in the liquid made the sound 'very loud' (Bell, Unpublished Notebook, Vol. I, p. 37).

Note how Bell in this sequence of experiments carefully employs the holding-constant heuristic, systematically altering one variable at a time to improve transmission. It is harder in this sequence to infer that successes are equivalent to confirmations and failures to disconfirmations of his hypothesis that the undulating current can really be used to transmit speech. Bell has far less experience with contacts in liquids than with coils and tuning forks. He certainly expected that changing the resista nce would have a major effect on the undulating current, but it was not clear at the outset what the optimal level would be--hence, he tried to acidulate the water and vary the distance between contacts.

When tests failed to yield positive results, Bell did not abandon his hypothesis--he simply altered his procedures and refined his search for conditions that would produce positive results. Similarly, Faraday obtained more negative than positive results when he first attempted to induce an electric current with a permanent magnet (Tweney, 1985). In both cases, there were plenty of corollary assumptions that could account for any individual failure to obtain results consistent with the hypothesis, incl uding the possibility of a number of kinds of error (cf. Gorman, 1992). Repeated failures over a range of conditions, however, would suggest a problem with the hypothesis.

Returning to Bell's experiments, once he had established that increasing the size of the contact on the conducting wire would improve transmission, he decided to increase the size of the vibrating contact; to do this, he substituted a bell for the tuni ng fork. No sound resulted, nor was transmission improved when he substituted a steel wire for the brass one.

At this point, Bell pauses, apparently at the end of his working day, writes down a hypothesis under the heading "Thoughts', which can be re-stated as follows: the best results so far were obtained when the vibrating contact was smallest and the conta ct on the other end of the circuit largest. This is shown in the protocol by a box shaped like a trapezoid, which we use to note hypotheses or goals Bell actually wrote down. This constitutes a corollary governing conditions that effect whether this so rt of device can produce an undulating current. Bell's positive test heuristic is revealing the circumstances under which his current can be reproduced.

Right below this thought, Bell sketched a device he thought would allow him to test his corollary assumption. It includes a speaking tube and membrane, borrowed from devices Bell had constructed earlier Attached to the membrane was a needle which dippe d into water; at the bottom of the wire was a long, flat contact--Bell subsequently used a brass ribbon for this purpose. The receiver is his familiar steel reed relay. Bell here has used a needle for the vibrating contact and a long, flat ribbon for th e other, thereby minimizing the area of the former with respect to the latter.

On the next day (March 9th), Bell and Watson built a version of this apparatus, using a sounding box instead of a speaking tube in the 'ear and membrane' slot, and a cork to attach the needle to the membrane. One of his reed receivers was placed in anot her room. Bell listened to the receiver while Watson sang, and could hear the pitch of Watson's voice. When Watson spoke, Bell heard "a confused muttering sound like speech but could not make out the sense. When Mr. Watson counted--I fancied I could pe rceive the articulations 'one, two, three, four, five'--but this may have been fancy--as I knew beforehand what to expect. However that may be I am certain that the inflection of the voice was represented." (Bell, Unpublished Notebook, Vol. I, p. 39). As far as Bell was concerned, this result counted as a success--a similar result had convinced him that the Gallows telephone represented a patentable idea.

On March 10th, the two substituted a platinum pipe for the brass ribbon and a speaking tube for the sounding box. Bell spoke the famous words "Mr. Watson--Come here--I want to see you." (Bell, Unpublished Notebook, Vol. I, 40-41). After this success, the two switched places and Watson read to Bell from a book: Bell could make out only a few words, but heard Watson say "Mr. Bell, do you understand what I say?"(Bell, Unpublished Notebook, Vol. I, p. 41).