Thefrog galvanoscope was a sensitive electrical instrument used to detectvoltage[1] in the late 18th and 19th centuries. It consists of a skinned frog's leg with electrical connections to a nerve. The instrument was invented byLuigi Galvani and improved byCarlo Matteucci.
The frog galvanoscope, and other experiments with frogs, played a part in the dispute between Galvani andAlessandro Volta over the nature of electricity. The instrument is extremely sensitive and continued to be used well into the nineteenth century, even afterelectromechanical meters came into use.
Synonyms for this device includegalvanoscopic frog,frog's leg galvanoscope,frog galvanometer,rheoscopic frog, andfrog electroscope. The device is properly called agalvanoscope rather thangalvanometer since the latter implies accurate measurement whereas a galvanoscope only gives an indication.[2] In modern usage agalvanometer is a sensitive laboratory instrument for measuring current, not voltage. Everyday current meters for use in the field are calledammeters.[3] A similar distinction can be made betweenelectroscopes,electrometers, andvoltmeters for voltage measurements.
Frogs were a popular subject of experiment in the laboratories of early scientists. They were small, easily handled, and there was a ready supply.Marcello Malpighi, for instance, used frogs in his study of lungs in the seventeenth century. Frogs were particularly suitable for the study of muscle activity. Especially in the legs, the muscle contractions are readily observed and the nerves are easily dissected out. Another desirable feature for scientists was that these contractions continued after death for a considerable time. Also in the eighteenth century,Leopoldo Caldani andFelice Fontana subjected frogs to electric shocks to testAlbrecht von Haller'sirritability theory.[4]
Luigi Galvani, a lecturer at theUniversity of Bologna, was researching thenervous system of frogs from around 1780. This research included the muscular response toopiates andstatic electricity, for which experiments the spinal cord and rear legs of a frog were dissected out together and the skin removed. In 1781,[5] an observation was made while a frog was being dissected. Anelectric machine discharged just at the moment one of Galvani's assistants touched thecrural nerve of a dissected frog with a scalpel. The frog's legs twitched as the discharge happened.[6] Galvani found that he could make the prepared leg of a frog (see theConstruction section) twitch by connecting a metal circuit from a nerve to a muscle, thus inventing the first frog galvanoscope.[7] Galvani published these results in 1791 inDe viribus electricitatis.[8]
An alternative version of the story of the frog response at a distance has the frogs being prepared for a soup on the same table as a running electric machine. Galvani's wife notices the frog twitch when an assistant accidentally touches a nerve and reports the phenomenon to her husband.[9] This story originates withJean-Louis Alibert and, according to Piccolino and Bresadola, was probably invented by him.[10]
Galvani, and his nephewGiovanni Aldini, used the frog galvanoscope in their electrical experiments.Carlo Matteucci improved the instrument and brought it to wider attention.[11] Galvani used the frog galvanoscope to investigate and promote the theory ofanimal electricity, that is, that there was avital life force in living things that manifested itself as a new kind of electricity.Alessandro Volta opposed this theory, believing that the electricity that Galvani and other proponents were witnessing was due to metalcontact electrification in the circuit. Volta's motivation in inventing thevoltaic pile (the forerunner of the commonzinc–carbon battery) was largely to enable him to construct a circuit entirely with non-biological material to show that the vital force was not necessary to produce the electrical effects seen in animal experiments. Matteucci, in answer to Volta, and to show that metal contacts were not necessary, constructed a circuit entirely out of biological material, including afrog battery. Neither the animal electricity theory of Galvani nor the contact electrification theory of Volta forms part of modern electrical science.[12] However,Alan Hodgkin in the 1930s showed that there is indeed anionic current flowing in nerves.[13]
Matteucci used the frog galvanoscope to study the relationship of electricity to muscles, including in freshly amputated human limbs. Matteucci concluded from his measurements that there was an electric current continually flowing from the interior, to the exterior of all muscles.[14] Matteucci's idea was widely accepted by his contemporaries, but this is no longer believed and his results are now explained in terms ofinjury potential.[15]
An entire frog's hind leg is removed from the frog's body with thesciatic nerve still attached, and possibly also a portion of thespinal cord. The leg is skinned, and two electrical connections are made. These may be made to the nerve and the foot of the frog's leg by wrapping them with metal wire or foil,[16] but a more convenient instrument is Matteucci's arrangement shown in the image. The leg is placed in a glass tube with just the nerve protruding. Connection is made to two different points on the nerve.[17]
According to Matteucci, the instrument is most accurate if direct electrical contact with muscle is avoided. That is, connections are made only to the nerve. Matteucci also advises that the nerve should be well stripped and that contacts to it can be made with wet paper in order to avoid using sharp metal probes directly on the nerve.[18]
When the frog's leg is connected to a circuit with anelectric potential, the muscles will contract and the leg will twitch briefly. It will twitch again when the circuit is broken.[16] The instrument is capable of detecting extremely smallvoltages, and could far surpass other instruments available in the first half of the nineteenth century, including the electromagneticgalvanometer and thegold-leaf electroscope. For this reason, it remained popular long after other instruments became available. The galvanometer was made possible in 1820 by the discovery byHans Christian Ørsted that electric currents would deflect a compass needle, and the gold-leaf electroscope was even earlier (Abraham Bennet, 1786).[19] YetGolding Bird could still write in 1848 that "the irritable muscles of a frog's legs were no less than 56,000 times more delicate a test of electricity than the most sensitive condensing electrometer."[20] The wordcondenser used by Bird here means a coil, so named byJohann Poggendorff by analogy with Volta's term for acapacitor.[2]
The frog galvanoscope can be used to detect the direction ofelectric current. A frog's leg that has been somewhat desensitised is needed for this. The sensitivity of the instrument is greatest with a freshly prepared leg and then falls off with time, so an older leg is best for this. The response of the leg is greater to currents in one direction than the other and with a suitably desensitised leg it may only respond to currents in one direction. For a current going into the leg from the nerve, the leg will twitch on making the circuit. For a current passing out of the leg, it will twitch on breaking the circuit.[21]
The major drawback of the frog galvanoscope is that the frog leg frequently needs replacing.[22] The leg will continue to respond for up to 44 hours, but after that a fresh one must be prepared.[13]