15.1 Galvanism. Electricity and chemistry are closely inter-related. A chemical reaction can produce a supply of electricity for as long as the reaction continues. This, the first source of a continuous supply of electricity, an electric current, is the principle of the electric battery. Conversely, an electric current can produce a chemical reaction, usually the decomposition of a chemical compound into its simpler elements, the process of electrolysis. Both processes involve the conversion of energy from one form to another; in the first case, chemical energy becomes electrical energy; in the other, the reverse takes place. Every living cell produces electricity. The functioning of living tissue today is studied through its electrical action. The study of electricity in living tissue, which began quite accidentally about one hundred and fifty years ago, led to the development of the electric battery, for many years thereafter the standard method of producing electricity About 1750, it was noted that pieces of lead and silver placed above and below the tongue, respectively, with their outer edges in contact, produced an unpleasant and pungent taste not encountered when the metals were placed separately upon the tongue. The phenomenon was attributed to some excitation of the nerves of the tongue. By this time, various physicians and experimenters had demonstrated that electricity could be used as a muscular stimulant in man and animals. This fact had been used to distinguish between paralyzed and atrophied muscles, an electric charge producing a contraction only in a paralyzed muscle. Before the end of the eighteenth century it was known that an electric discharge passed through the body of a freshly killed animal could cause a convulsive action in its muscles, and that the discharge of an electric eel (section 14.2) produced motion in a nearby dead fish. Identification of the origin of these effects was made by Galvani (1737-1798), a professor of anatomy at Bologna. Galvani began experimenting about 1780, using a Leyden jar [A Leyden jar was the earliest form of electric condenser, consisting of “a bottle filled with water into which was inserted a wire held in place by a cork.” p. 191] and an electrostatic machine to test the effects of the electric discharge upon the nervous system of the frog. During these experiments he made the chance observation that nearby electrical discharge caused convulsions in a freshly prepared frog’s leg in conducting contact with the earth.
Priestley then goes on to discuss the work of Michael Faraday in discovering the laws of electrolysis, which led to the development of practical cells, i.e., the batteries we now have in everyday life and which we take for granted. But what we have in this excerpt is the scientific history of the development of the modern battery – which came out of experiments which changed fundamentally how we view man, as well. The observation that we had different sensations when metals touched our tongue in different places would have gone nowhere and could have been interpreted in all kinds of ways, without the knowledge that frogs’ nerves and muscles are affected by electricity. This knowledge was the first step in our modern science of neurology, in understanding how the brain works, and in developing some of the drugs we have today (which have neurological effects because of their chemistry and electrical effects). And if not for the foundational work of Michael Faraday arising from the research of Volta and Galvani, we would not know what we do today about nutrition and the operation of the cell. What does something so everyday as Gatorade have in it? Electrolytes. Thank Michael Faraday next time you drink some. Priestley is a genius in taking us from the observation that we had certain sensations when metals touched our tongues, to the modern battery. He presents a missing side of modern scientific texts: causality. Science is about discovering cause-effect relationships. Most modern texts present physics as an exercise in mathematics – the texts could be addenda to math texts, providing word problems and applications of math. They fail miserably in presenting cause-effect relationships, and showing how scientific knowledge really develops. They fail to present the important experiments that led to modern understanding of the material world, and that make physics what it is. If there are some texts with a rational epistemology out there, please let me know!!! I’d love to have them!!
[I] had dissected and prepared a frog. [While] attending to something else, I laid it on a table on which stood an electrical machine at some distance…when one of the persons present touched accidentally and lightly the inner [thigh or leg] nerves of the frog with the point of a scalpel all the muscles of the legs seemed to contract again and again as if affected by powerful cramps. [One of my assistants] thought…the action was excited when a spark was discharged from the conductor of the machine [and] called my attention to it…I was eager to test the same and to bring to light what was concealed in it. I therefore myself touched one of the other nerves with the point of the knife and at the same time one of those present drew a spark. The phenomenon was always the same. Without fail there occurred lively contractions in every muscle of the leg at the same instant as that in which the spark jumped…
[Thinking] these motions might arise from the contact with the point of the knife rather than by the spark, I touched the same nerves again in the same way in other frogs with the point of the knife…with greater pressure [while] no one during this time drew off a spark…no motion could be detected. I [concluded] that perhaps to excite the phenomenon…needed both the contact of a body and the electric spark.
Therefore, I again pressed the blade of the knife on the nerve and kept it there at rest while the spark passed and while the machine was not in motion. The phenomenon only occurred while the sparks were passing. [In many experiments with the same knife] it was remarkable that when the spark passed the motions observed sometimes occurred and sometimes not… The scalpel had a bone handle…if this handle was led in the hand no contractions occurred when the spark passed; but they did occur if the finger rested on the metallic blade or on the iron rivet by which the blade was held in the handle…
Now to put the thing beyond all doubt we…not only touched the nerves of the leg [with a slender dry and clean glass rod] but rubbed them hard while the sparks were passing. But…the phenomenon never appeared. [It] occurred however if we even lightly touched the same nerve with an iron rod and only little sparks passed. [William F. Magie, A Source Book in Physics (New York: McGraw-Hill Book Company, Inc., 1938), p. 421.]Galvani’s “phenomenon” occurred only when the frog’s leg was in conducting communication with the earth, first by chance contact of the scalpel with the nerve, thereafter intentionally by bringing the leg into contact with a conductor grounded by contact with the human body. He continued his researches, turning to the effect of atmospheric electricity (lightning) on muscular motion. He attached frogs by the nerves to long iron wires, the feet of the frogs being grounded by similar wires. Simultaneously with a flash of lightning the muscles were markedly convulsed. In both these series of experiments the frog, place upon a body insulated from the ground, became charged by induction (section 14.11) from either the electrostatic machine or lightning. When a grounded metal object (scalpel or iron rod) touched the nerve, the sudden change of potential caused by grounding produced the observed convulsive action.
[I next laid one of the prepared frogs] on an iron plate and began to press the hook which was in the spinal cord against the plate. Behold, the same contractions, the same motions…other metals [gave] the same result, only that the contractions were different [for] different metals…more lively for some and more sluggish for the others. At last it occurred to us to use other [non-conducting] bodies…[dry] glass, rubber, resin, stone or wood. With these…no muscular contractions and motions could be seen. Naturally [this astonished us] and caused us to think that possibly the electricity was present in the animal itself…a very fine nervous fluid which during the occurrence of the phenomenon flows from the nerves to the muscle like the electric current….” [ibid., p. 424.]Galvani now recognized that here was something entirely new. “To make the thing plainer” he varied the experiment by placing the frog on a glass non- conducting plate. A curved rod connected the hook which entered the spinal cord with the muscles of the leg or feet. Convulsions occurred only when the curved rod was of conducting material and only when the hook and conducting rod were of dissimilar metals. Two possible explanations of these phenomena suggested themselves to Galvani; that there was electricity in the animal organism, or that there was involved some electrical process depending upon contact of the metals and for which the frog’s legs merely served as a sensitive detector. He leaned toward the first of these – the existence of “animal electricity,” for which the nerves had the greatest affinity and were the repository. His theory further assumed that the inner substance of the nerve served as the conductor of this electricity, while the outer layer of the nerve prevented its dispersal. The muscles were the receivers of the animal electricity, and were charged negatively on the outside and positively on the inside. The mechanism of motion was a discharge of the electric fluid from the inside to the outside of the muscle by way of the nerve (like the discharge of a Leyden jar), and this discharge provided a muscular contractional stimulus to the muscle fibers. 15.2 Volta disagrees with Galvani. Galvani’s experiments and his interpretation of the results aroused considerable interest. Among the physicists, physiologists, and medical men who obtained frogs and pieces of dissimilar metals to repeat the experiments for themselves was Volta (1745-1827), a countryman of Galvani’s and professor of physics at Paris. Volta, greatly impressed by Galvani’s work, referred to it as “one of those splendid major discoveries which…serve to usher in new epochs, not only because it is new and wonderful but also because it opens up a broad field of experiments that are especially and outstandingly capable of the application. “ [ibid., p. 443.] Volta’s original belief in the correctness of the “animal electricity” theory was weakened when he found that a muscular contraction could be produced simply by allowing a very weak electrical discharge to traverse a nerve without the discharge in anyway passing through the muscles. To produce a contraction required only stimulation of “the nerves that control the motions of the voluntary muscles concerned.” A physicist rather than a physiologist, Volta now shifted his emphasis to the function of the metallic rods used. Repeating the experiment of placing on the tongue two dissimilar metals, he “covered the point of the tongue…with a strip of tin…With the bowl of a spoon, I touched the tongue further back; then I inclined the handle of the spoon to touch the tin. I expected…a twitching of the tongue… The expected sensation, however, I did not perceive at all; but instead, a rather strong acid taste at the tip of the tongue this taste lasts as long as the tin and sliver are in contact with each other. …This shows that the flow of electricity from one place to another is continuing without interruption.” It was “not less remarkable” that reversing the experiment so that the silver touched the tip of the tongue and the tin its middle gave “a very different taste…no longer sour but more alkaline, sharp, and approaching bitter.” [ibid., p. 444] Bringing together the free ends of strips of dissimilar metal which touched, respectively, the forehead and palate produced, at the instant of contact, a bring flash clearly visible to the eye. Investigations such as these gradually convinced Volta that the metals not only served as conductors but actually generated the electricity themselves. He accordingly modified his views to the belief that the nerves were merely stimulated by a cause to be found in the metals themselves, which were “in a real sense the exciters of electricity.” By 1794 he declared his opposition to the idea of animal electricity and substituted the term “metallic electricity.” The entire effect arose from the electricity set into circulation when metals were brought into contact with any moist body. This circulation through nerves caused stimulation of associate muscles. He found that the results depended upon the nature of the substances used and drew up a series of substances (metals, graphite, an charcoal) such that the magnitude of the effect produced using any two of the substances increased with the separation of the substances in this series. Volta now dispensed entirely with the use of nerves and muscles n his investigations, and brought pairs of metals into contact with various moist substances, such as paper, cloth, etc. With a sensitive electrometer which he had previously developed, he was able to show the existence of “contact potential” – that the momentary contact of two dissimilar metals caused them to become oppositely charged, even without any moist substance present. A zinc and a copper disc after being placed in contact were both found to be charged, the zinc positively and the copper negatively. Copper also became negatively charged after contact with iron or tin, although less strongly than after contact with zinc. On the other hand, contact with gold or silver gave copper a positive charge and the gold or silver a negative charge. By numerous experiments along these lines, Volta constructed a series for the metals such that upon bringing any two of them into contact, the earlier in the list became positively charged, the later one negatively charged: zinc copper lead silver tin gold iron graphite Furthermore, the more widely separated the substances in the series, the greater was the contact charge developed between them. On the basis of his investigations, Volta originally assumed that the exciting electricity was located only at the points of contact of the metals and that the animal or other fluid served only as a conductor. But further experiments showed that an electric charge can be produced not only between metals in contact, but also between a metal and certain fluids. For instance, an insulated disc of silver or other metal brought into contact with moist wood or paper and then removed was found to be negatively charged. Experimenting further with liquids and metals, Volta found that the best results were obtained from two dissimilar metals with a moist conductor between them, a combination called a galvanic element. The effect of such a single element was multiplied by combining a large number of them to form a “pile.” In 1800, Volta described a pile which produced a constant flow of electricity. By comparison with a Leyden jar, it was “equal only to a [Leyden jar] very feebly charged; but infinitely surpasses the power of these [jars] in that it does not need, as they do, to be charged in advance by means of an outside source; and in that it can give the disturbance every time that it is properly touched no matter how often.” [ibid., p. 428.] The pile consisted of small, clean and dry discs of zinc and silver and discs of a spongy material capable of absorbing and retaining a liquid. On a table or base is placed a sliver plate, then a
plate of zinc; on this…one of the moistened discs; then another silver [plate], followed immediately by another of zinc, [then another] moistened disc…continue in the same way coupling a plate of sliver with one of zinc, always [in the same order] and inserting between these couples a moistened disc. [ibid.]Such a pile produced a slight shock when the hands were placed in contact with the top and bottom of the pile, and also the previously experienced effect upon the nerves of taste, sight, and hearing. One drawback was that the moist material between the metal discs dried out, decreasing the electric current generated. To overcome this, Volta devised his “crown of cups,” consisting of a row of beakers of non-metallic material filled with brine into which were placed alternate strips of sliver and zinc. Each silver strip in one cup was joined to the zinc strip in the next cup by a metallic jumper. “A train of 30, 40, 60 of these goblets joined up in this manner…in substance is the same as the [pile] tried before; the essential feature, of the immediate connection of the different metals which form each pair and the mediate connection of one couple with another by the intermediary of a damp conductor, appears in this apparatus as well as in the other.” [ibid., p. 431.] This crown of cups was subsequently improved by substituting copper for silver and dilute sulphuric acid for brine. Volta reported that the “tension” (potential difference) produced by the pile or cups
“is less according as they are nearer in the following series…sliver, copper, iron, tin, lead, zinc, a scale in which the first [is positive with respect] to the second, the second to the third, etc.”The importance of Volta’s discovery of a means of producing a continuous supply of electricity cannot be overemphasized. Sarton, the distinguished historian of science, compares it with the development of the telescope and microscope, with the fundamental difference that the telescope and microscope “were only means of magnifying our vision. They enabled us to see things which we could not see before, but which existed nevertheless… On the contrary, the electric cell was really a creative instrument; it opened to man a new and incomparable source of energy.” [Bern Dibner, Galvani-Volta (Norwalk: Burndy Library, Inc., 1952), p. 40.] 15.3 The simple voltaic cell. Volta’s identification of the true origin of “animal electricity” led to the familiar batteries now used in radios, automobiles, etc. In every case, production of electricity results from the conversion of chemical into electrical energy. To understand the mechanism involved, consider the simple or voltaic cell, consisting of two dissimilar metals immersed in a liquid, and in essence an element of Volta’s pile.