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Frog galvanoscope is an instrument for detecting small voltages, made of a frog's leg.

Wikipedia notes:

The instrument is capable of detecting extremely small voltages, and could far surpass other instruments available in the first half of the nineteenth century, including the electromagnetic galvanometer and the gold-leaf electroscope.

and

Yet Golding 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."

but I couldn't find any actual values in modern units.

What was the minimum voltage in volts that could be detected by a frog galvanoscope?

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  • $\begingroup$ Similar claim: Benjamin Silliman, First Principles of Chemistry, for the Use of Colleges and Schools, 50th ed. Philadelphia: Beck & Bliss 1861, p. 141: "The irritable muscles of the frog's leg form an electroscope 56,000 times more delicate than the most delicate gold-leaf electrometer. Professor Matteucci's frog-galvanoscope (fig. 197) is therefore the most sensitive test of electricity that can be employed." $\endgroup$
    – njuffa
    Oct 1, 2023 at 10:07
  • $\begingroup$ My understanding is that a gold-leaf electrometer (also called electroscope) measures charge, and this site states: "A typical school electroscope will show a deflection for a charge as small as 0.01 pC". $\endgroup$
    – njuffa
    Oct 1, 2023 at 10:10
  • $\begingroup$ @njuffa That's a useful datapoint. Various sites note the capacitance of gold-leaf electroscope as between 1 pF and 20 pF, which would relate the 0.01 pC to a voltage of 0.5 - 10 mV. That's surprisingly low, as some other sites say minimum detectable voltage for gold-leaf electroscopes would be a few hundred volts. In any case, this seems to give a range of 10 nV to 10 mV for the answer to the question - still pretty wide uncertainty :) $\endgroup$
    – jpa
    Oct 1, 2023 at 12:11

1 Answer 1

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A frog galvanoscope does not measure voltage. Instead it detects electricity that same way that I detect my static charge after walking across a carpet in winter - by a short current flow. The current causes the frog leg to twitch and me to jump from the shock of the spark when I touch a door knob. (The analogy is reasonably close, since Galvani needed a spark gap in the circuit to make the frog leg twitch.)

In general, a galvanoscope measures current; an electrometer measures electric charge or potential. The frog-leg galvanometer a kind of threshold ammeter. It tells you that a voltage source can provide a current - at least for an instant - greater than the threshold needed to make the frog leg twitch.

The "56,000" factor comes from page 316 of Wilkinson's 1804 Elements of Galvanism, and is a comparison of charge sensitivity, not voltage. Touching a silver and a copper plate, each 0.1 inch square, to the frog leg made it twitch, but it took 20 touches connected to 6 inch diameter copper and zinc plates to make a gold leaf electroscope visibly deflect. The electric potential in both cases is about a volt. The relative sensitivity comes from the calculation: $20\,\pi\,(6/2)^2/0.1^2 = 56,549$.

I can't immediately find a source that clearly states how much current or charge is needed to make a frog leg twitch. Individual frog skeletal muscle fibres can respond to 50 nA at 50 mV for pulses longer than 5 ms, so this might be considered the ultimate sensitivity for a single-muscle-fibre galvanometer. Similar results have been observed for cockroach legs.

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