Superphysics
Galvanism, Thom Galvani To Ohm
8 minutes • 1656 words
Until the 1790’s, electricians were occupied solely with static electricity. Their attention was then turned in a different direction.
Johann Georg Sulzer (b. 1720, d. 1779) published Recherches sur l’origine des sentiments agréables et désagréables in 1752.
He mentioned that, if 2 pieces of metal, the one of lead and the other of silver, be joined together in such a manner that their edges touch, and if they be placed on the tongue, a taste is perceived “similar to that of vitriol of iron,” although neither of these metals applied separately gives any trace of such a taste.
“It is not probable,” he says, “that this contact of the two metals causes a solution of either of them, liberating particles which might affect the tongue; and we must therefore conclude that the contact sets up a vibration in their particles, which, by affecting the nerves of the tongue, produces the taste in question."
This observation was not suspected to have any connexion with electrical phenomena, and it played no part in the inception of the next discovery, which indeed was suggested by a mere accident.
Luigi Galvani, born at Bologna in 1737, occupied from 1775 onwards a chair of Anatomy in his native city.
For many years before the event which made him famons he had been studying the susceptibility of the nerves to irritation; and, having been formerly a pupil of Beccaria, he was also interested in electrical experiments.
One day in the latter part of the year 1780 he had, as he tells us,[2] “dissected and prepared a frog, and laid it on a table, on which, at some distance from the frog, was an electric machine. It happened by chance that one of my assistants touched the inner crural nerve of the frog with the point of a scalpel; whereupon at once the muscles of the limbs were violently convulsed.
“Another of those who used to help me in electrical experiments thought he had noticed that at this instant a spark was drawn from the conductor of the machine. I myself was at the time occupied with a totally different matter; but when he drew my attention to this, I greatly desired to try it for myself, and discover its hidden principle.
So I, too, touched one or other of the crural nerves with the point of the scalpel, at the same time that one of those present drew a spark; and the same phenomenon was repeated exactly as before."[3]
After this, Galvani conceived the idea of trying whether the electricity of thunderstorms would induce muscular contractions equally well with the electricity of the machine.
Having successfully experimented with lightning, he wished[4] “to try the effect of atmospheric electricity in calm weather.
My reason for this was an observation I had made, that frogs which had been suitably prepared for these experiments and fastened, by brass hooks in the spinal marrow, to the iron lattice round a certain hanging-garden at my house, exhibited convulsions not only during thunderstorms, but sometimes even when the sky was quite serene.
I suspected these effects to be due to the changes which take place during the day in the electric state of the atmosphere. I performed experiments to test the point; and at different hours for many days I watched frogs: which I had disposed for the purpose; but could not detect any motion in their muscles.
At length, weary of waiting in vain, I pressed the brass hooks, which were driven into the spinal marrow, against the iron lattice, in order to sec whether contractions could be excited by varying the incidental circumstances of the experiment.
I observed contractions tolerably often, but they did not seem to bear any relation to the changes in the electrical state of the atmosphere.
“However, at this time, when as yet I had not tried the experiment except in the open air, I came very near to adopting a theory that the contractions are due to atmospheric electricity, which, having slowly entered the animal and accumulated in it, is suddenly discharged when the hook comes in contact with the iron lattice. For it is easy in experimenting to deceive ourselves, and to imagine we see the things we wish to see.
“But I took the animal into a closed room, and placed it on an iron-plate; and when I pressed the hook which was fixed in the spinal marrow against the plate, behold! the same spasmodic contractions as before. I tried other metals at different hours on various days, in several places, and always with the same result, except that the contractions were more violent with some metals than with others.
After this I tried various bodies which are not conductors of electricity, such as glass, guns, resins, stones, and dry wood; but nothing happened. This was somewhat surprising, and led me to suspect that electricity is inherent in the animal itself.
This suspicion was strengthened by the observation that a kind of circuit of subtle nervous fluid (resembling the electric circuit which is manifested in the Leyden jar experiment) is completed from the nerves to the muscles when the contractions are produced.
“For, while I with one hand held the prepared frog by the hook fixed in its spinal marrow, so that it stood with its feet on a silver box, and with the other hand touched the lid of the box, or its sides, with any metallic body, I was surprised to see the frog become strongly convulsed every time that I applied this artifice."[5]
Galvani thus ascertained that the limbs of the frog are convulsed whenever a connexion is made between the nerves and muscles by a metallic arc, generally formed of more than one kind of metal.
He advanced the hypothesis that the convulsions are caused by the transport of a peculiar fluid from the nerves to the muscles, the are acting as a conductor.
To this fluid the names Galvonism and Animal Electricity were soon generally applied. Galvani himself considered it to be the same as the ordinary electric fluid, and, indeed, regarded the entire phenomenon as similar to the discharge of a Leyden jar.
The publication of Galvani’s views engaged the attention of the learned world. It led to a controversy between:
- those who supported Galvani’s view
- those who believed galvanism to be a fluid distinct from ordinary electricity
- those who refused to attribute the effects to a fluid in the nervous system.
The leader of the last-named party was Alessandro Volta (b. 1745, d. 1827), Professor of Natural Philosophy in the University of Pavia. In 1792, he put forward the view[6] that the stimulus in Galvani’s experiment is derived essentially from the connexion of two different metals by a moist body.
“The metals used in the experiments, being applied to the moist bodies of animals, can by themselves, and of their proper virtue, excite and dislodge the electric fluid from its state of rest; so that the organs of the animal act only passively."
At first he inclined to combine this theory of metallic stimulus with a certain degree of belief in such a fluid as Galvani had supposed, but after the end of 1793 he denied the existence of animal electricity altogether.
From this standpoint Volta continued his experiments and worked out his theory. The following quotation from a letter[7] which he wrote later to Gren, the editor of the Neues Journal d. Physik, sets forth his view in a more developed form:—
“The contact of different conductors, particularly the metallic, including pyrites and other minerals, as well as charcoal, which I call dry conductors, or of the first class, with moist conductors, or conductors of the second class, agitates or disturbs the electric fluid, or gives it a certain impulse. Do not ask in what manner: it is enough that it is a principle, and a general principle.
This impulse, whether produced by attraction or any other force, is different or unlike, both in regard to the different metals and to the different moist conductors; so that the direction, or at least the power, with which the electric fluid is impelled or excited, is different when the conductor A is applied to the conductor B, or to another C.
In a perfect circle of conductors, where either one of the second class is placed between two different from each other of the first class, or, contrariwise, one of the first class is placed between two of the second class different from each other, an electric stream is occasioned by the predominating force either to the right or to the left-a circulation of this fluid, which ceases only when the circle is broken, and which is renewed when the circle is again rendered complete."
Another philosopher who, like Volta, denied the existence of a fluid peculiar to animals, but who took a somewhat different view of the origin of the phenomenon, was Giovanni Fabroni, of Florence (b. 1752, d. 1822), who,[8] having placed two plates of different metals in water, observed that one of them was partially oxidized when they were put in contact; from which he rightly concluded that some chemical action is inseparably connected with galvanic effects.
The feeble intensity of the phenomena of galvanism, which compared poorly with the striking displays obtained in electrostatics, was responsible for some falling off of interest in them towards the end of the eighteenth century; and the last years of their illustrious discoverer were clouded by misfortune.
Being attached to the old order which was overthrown by the armies of the French Revolution, he refused in 1798 to take the oath of allegiance to the newly constituted Cisalpine Republic, and was deposed from his professorial chair.
A profound melancholy, which had been induced by domestic bereavement, was aggravated by poverty and disgrace; and, unable to survive the loss of all he held dear, he died broken-hearted before the end of the year.[9]
