It is the fundamental premise of instrument-science that a device for
detecting or measuring a physical quantity can be based on any
phenomenon associated with that physical quantity. Although the
instrumentation of electrostatics in the 18th century, for example,
relied mainly on the phenomena of attraction and repulsion and the
ubiquitous sparks and other luminosities of frictional electricity, even
the physiological sensation of electric shock was exploited
semiquantitatively by Henry Cavendish in his well-known anticipation of
Ohm's researches. Likewise, Volta in 1800[1] described at length how the
application of his pile to suitably placed electrodes on the eyelids, on
the tongue, or in the ear, caused stimulation of the senses of sight,
taste and hearing; on the other hand, he reported that electrodes in the
nose merely produced a "more or less painful" pricking feeling, with no
impression of smell. The discharges from the Leyden jars of some of the
bigger frictional machines, such as van Marum's at Leyden, were found by
1785 to magnetize pieces of iron and to melt long pieces of metal
wire.[2]
The useful instruments that emerged from all of this experience were
various deflecting "electrometers" and "electroscopes" (the words were
not carefully distinguished in use), including the important goldleaf
electroscope ascribed to Abraham Bennet in 1787.[3]
In 1786, Galvani first observed the twitching of the legs of a dissected
frog produced by discharges of a nearby electrostatic machine, thereby
revealing still another "effect" of electricity. He then discovered that
certain arrangements of metals in contact with the frog nerves produced
the same twitching, implying something electrical in the frog-metal
situation as a whole. Although Galvani and his nephew Aldini drew from
these experiments erroneous conclusions involving "animal electricity,"
which were disputed by Volta in his metal-contact theory, it is
significant from the instrumentation point of view that the frog's legs
were unquestionably by far the most sensitive detector of metal-contact
electrical effects available at the time. Without their intervention the
development of this entire subject-area, including the creation of
chemical cells, might have been delayed many years. Volta himself
realized that the crucial test between his theory and that of Galvani
required confirming the existence of metal-contact electricity by some
electrical but nonphysiological detector. He performed this test
successfully with an electroscope, using the "condensing" technique he
had invented more than a decade earlier.