| Themes > Science > Physics > Electromagnetism > Electrostatics > Electric field > A "new electricity": Galvani and Volta |
About 1791, Luigi Galvani (1737-98), an Italian anatomist at Bologna, reported a series of experiments he had been conducting since 1780 when an assistant had accidentally observed that a frog's legs violently contracted if a metal scalpel were touched to a certain leg nerve during dissection. In subsequent experiments, Galvani showed that the contractions occurred when the operator made contact with a nerve by means of an electrical conductor and if the frog's legs were connected by means of an electrical conductor to ground. In other experiments he showed that contractions were produced if the frogs were placed on an iron plate and if a brass hook, making contact with a nerve, were simultaneously pressed against the iron. In general, the effects being most pronounced if two dissimilar metals were used. With nonconductors, the effects did not occur. In view of the shocklike aspect of the muscular contractions and the necessity of using electrical conductors, Galvani associated the phenomenon with electricity. [This is not so remote an inference as might at first appear. At that time, the sensation of shock and muscular contraction were, aside from electroscope deflection and passing of sparks, one of the principal ways of identifying the presence of electric charge. Cavendish made surprisingly accurate measurements of the relative conducting power of various materials by comparing the physiological shock sensations he experienced on taking a discharge from a Leyden jar through the conductor. It was also well known that a few sea creatures such as the gymnotus (a species of electric eel) and the electric ray (or torpedo fish) were able to convey a shock similar to that obtained from a Leyden jar, and they were described as sources of "animal electricity."] At this point, Galvani seems to have abandoned his careful physical investigations and to have gone off in the direction of somewhat premature physiological speculations concerning the origin of a special animal electricity in the brain and its distribution through the nerves to activate the muscles. Beyond this, he sought to find how one might exploit the new knowledge of animal electricity in the cure of disease. On publication of Galvani's work, a more physical thread of investigation was re-instituted by Alessandro Volta (1745-1827) of Pavia. Volta's interest in electricity had led him to develop delicate and highly sensitive electrometers (electroscopes provided with a scale for measuring deflection of the movable element). With these instruments, he failed to find any electric charge stored in the animal tissues and ascertained that the muscular contractions depended entirely on the presence of a bimetallic junction (direct contact between two different metals) connected externally by the frog's leg or body. In 1792, Volta referred to the effects as those of a "very feeble artificial electricity" and wrote, I am persuaded that the electric fluid is never excited and moved by the proper action of the organs, or by any vital force, or extended to be brought from one part of the animal to another, but that it is determined and constrained by virtue of an impulse which it receives in the place where the metals join. Thus Volta, eschewing vague, unphysical notions of "vital force," based the design of his investigation and experiments on the working hypothesis that he was dealing with the same phenomenon observed with electrostatic machines and Leyden jars, even though this was far from conclusively demonstrated at that time. |
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