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XIV
(D)
FOURTH
LETTER
Pavia,
20 December, 1795
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SOURCES
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MANUSCRIPT |
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Cant.
Disc. P. 71
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Copy with Turin Sci. Ass. (B. XII, 10)
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Cart. Volt. J 28, J 33, L 16.
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REMARKS
TITLE:
DATE:
In the MS copy with Tur. Sci. Ass. we read: Pavia 20
December 1795. In J33 (first minute) there is: Como, 30
October
_______________________
J
28 consists in a single sheet with annotations on
experiments introducing people into the Circuit.
J33
L 16. Extremely lengthy minute.
The
MS copy at the Tor. Sci Ass. has been corrected by
comparing with the
Volt. File Manuscripts J33, L16.+
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COPY
OF ANOTHER LETTER FROM
ALESSANDRO VOLTA
To Abbé
ANTON MARIA VASSALLI
Dated, Pavia 20 December,
ON
ANIMAL
ELECTRICITY
I
have already indicated and explained in the previous letter that the simplest
and easiest way to excite convulsions and shaking in
the frog’s legs, without using any metals or first class conductors, a few
moments after preparing the frog in the best way, so that the leg may remain
united to the trunk through the sciatic
nerves, is to fold one leg or the other and put
it in contact either with the above-mentioned nerves or the dorsal muscles. In
any case, this
is not always successful nor in all
frogs, even if they are very lively and recently prepared for the experiment. Moreover,
to be successful, two conditions are required, one slightly
less than necessary, the other absolutely indispensable, viz. 1st,
the part of the leg which has to touch those nerves or the dorsal muscles
must not be fleshy substance but a part of the tendon or ligament proceeding
from the gastrocnemius muscle, which, completely bared,
extends to the extremity of the foot. 2nd,
blood or some
other glutinous or saline humour must be put in
between the contacts. Now
here is the gathering of three conductors of different kinds, but all of the
second class, as required in order to excite and set
the electric fluid in motion. That
is how the above two conditions reduce
to only one: three different conductors
forming a circle. In the experiment we are dealing with, these three conductors
are smooth, hard tendon matter on the one hand, on the other either soft nerve
or flesh
with not too smooth a
surface and glutinous or saline humour in between. The
three are so different from one another that the electric current, though still
very weak, is able sensibly
to stimulate the crural nerves of a newly prepared frog, as explained,
prepared in such a way that the current must
pass through the narrow way of those very
nerves. In the event that
one of those circumstances should be missing (either
that the frog is not prepared in the suggested way or that, after some minutes,
its excitability has
remarkably decreased, or there is no interlacing
of three different conductors or, finally, they are not different enough) the convulsions
do not occur.
Leaving
aside
what refers to the excitability of the nerves and muscles of the frog, which
must be very big for
the experiments we are speaking of to
be successful, as I have suggested more than once, let
us
consider particularly
the last circumstance, namely that we need the coincidence of three different
conductors
[1],
but they must all be very different; and let us
see which ones come out to be more or less suitable
in this regard.
Well
then, the experiment is really successful when any part of the strong, compact
tendon with a white,
bright surface, originating from the gastrocnemius muscle and
extending beyond the articulation of the foot like a pearl-coloured lamina, is
opposed to the delicate dull white sciatic
nerves and the reddish,
tender flesh of the trunk. Obviously,
they must not be in direct contact but there must be either blood or other
glutinous or saline humour between them. The experiment, I say, is very
successful, as it is
often possible to excite convulsions in the frog’s
legs even
some minutes after it has been
prepared properly. On the contrary, it gives
a very bad result if we set the fleshy parts of the leg or thigh, and
also the gastrocnemius muscle itself from the mid point upwards, against
the afore-mentioned
nerves or, worse, against any fleshy part of the trunk. Indeed the result is
so bad that you hardly
ever
get the convulsions,
even if you interpose the best of
the glutinous or saline humours, the one which makes the result easier, that is
a strong alkali.
I
say hardly ever
because it happens sometimes. Occasionally you can excite
convulsions also with the contact
of muscles
against nerves and muscles
against muscles
dirty with blood or other glutinous or saline humour (as one of these must
always be put between, which I have pointed out more
than once). Such is the experiment recalled
in previous letters and which is
due to the author of the work On the Use
and Activity of the Conducting Arc, etc., where the sciatic
nerves, either alone or with a piece of backbone tied to them, hang down out of
the thighs and you make those nerves or this piece of backbone knock against the
thighs themselves, in
this
way exciting convulsions in all the leg
muscles from top to bottom, especially
if the parts have been
previously moistened with
salt water. Such and similar are also Dr. VALLI’s
experiments
in which a person, holding a frog by its feet with one damp
hand or in the mouth, touches the sciatic
nerves or the piece of backbone hanging therefrom with
a finger of the other hand, or better with the tongue; or else the person
dipping the aforementioned
backbone with part of the nerves in water in a big
glass completes the circle by dipping a finger in the same water or by having it
dipped by another person with whom that
person forms a chain holding hands, etc. But these experiments happen quite rarely and
I’d almost say
by a miracle. This is the situation:
you will be able to stimulate muscular movements very many times in quite a lot
of frogs, though not very lively ones,
observing the conditions I have described, chiefly the one making
the sciatic nerves or any part of the bare, bloodstained trunk touch the
tendon where the gastrocnemius muscle ends.
I say you will be successful a thousand
times using such a method
and just once, in some frogs endowed with singular excitability,
if you act in other ways, even
when making the sciatic
nerves or the trunk touch the leg itself
directly
but not where
the tendon divides. In short, you can never promise yourself success without
getting a contact between some parts of this tendon and said nerves or trunk, one or other of which,
or the tendon itself, must
be dirty
with blood or some
other glutinous or saline humour. The interposing
of this third conductor different from both the conductors between which it lies
is the other
condition required.
But
finally, even when you manage to excite convulsions in those other ways the
indispensable condition I have pointed out will always be required: three or more different
conductors
interlaced; and the great principle I have stated will always occur, that is to
say that it is from the contact of different conductors that the action of
setting the electric fluid in motion is born..
As a matter of fact, also when one
muscle faces another, if there be
some even
merely superficial
difference between them in consistency,
in the humour they are bathed or simply smeared with, or else if you find
between them not one but two different humours which join in that place, then
either of such accidental differences may suffice
to give an impetus to the electric fluid or to determine very weak electric
current. This extremely weak current may go
so far as to excite convulsions in on of those extremely sensitive frogs. Here
is how also those very rare cases can be explained and how none of the
experiments of this kind can be objected to me as opposing the principle I have
stated and demonstrated by other more numerous experiments, in which you need more
evident differences between adjoining conductors in the circuit
so that convulsions may occur – differences more remarkable
than those which may perchance occur
between one muscle
and another or between a muscle and a nerve, etc.
Let us now pass on to deal with the most
indispensable condition in greater detail, viz that there must not be direct
contact of the leg, that is of the tendon coming from the gastrocnemius muscle,
with the sciatic
nerves or with the fleshy parts of the trunk, but there must be a third
conductor interposed, different in kind, but it also belonging to the second
class and also being
very different, which
we pointed out as being necessary in order
to be successful. so this
intermediate conductor has to be neither
pure water nor a humour little different from water, nor any substance soaked in
such a humour.
These watery
conductors are very little different
from this or that animal conductor, I mean from the outer surface
of the tendon or nerve which fit together, that is from the mostly watery humour
moistening
these animal parts and making them conductors ( since, when dry, they no longer
conduct). They
differ so much as to hardly move and extremely weak
current that, by chance, in once case out of a thousand, could only just move
an extremely weak electric current capable of stimulating convulsions
in some newly prepared frogs endowed with prodigious
sensitivity; but in the other 999 experiments, it will not be able
to produce the slightest muscular movement.
To give rather stronger motion
to the electric flow and to induce an efficacious current to shake even frogs
endowed with normal
excitability, provided they are perfectly and recently
prepared, it is necessary that the intermediate conductor between the leg or its
major
tendon and the sciatic
nerves or the dorsal muscles be either sticky
blood or other humour notably different from water.
That
is why the experiment succeeds not
seldom in a frog dirtied with blood seeping
from cuts after
dissection and even better when this blood, having become less watery, turns
out to be
more slimy
and sticky;
for it no longer succeeds
after the frog has been prepared and cleaned by splashing and rubbing it with
fresh water, as I have already written in the previous letter. The tests succeed again if we dry off the
water and once more soil
that part which has to be brought into contact with sticky blood or other
glutinous or saline humour. The situation is this: when, for lack of blood or other humour
very
different from water, you obtain no
convulsions in the frog however
you make one of its legs or the tendon coming from the gastrocnemic muscle touch
either
the sciatic
nerves or one or other part
of the trunk, you are successful many
times when deliberately dirtying
either the said nerves or some part of the spine
with sticky, thick blood , saliva
or better salty water and getting the contact of the folded leg in exactly the
place which is covered with the afore-mentioned humours.
It
is easier to be successful if you soil
either of those parts with soap dissolved
in a little water, somewhat
better
still
if you wet it with some drops of concentrated acid, and especially with a strong
alkaline liquid, so much so that in this manner (of course the tendon is included
in the contact) there is no frog – even weak – in which I cannot succeed in
exciting convulsions, even
many minutes after preparation. Results
from the experiments produced
up to now, and from many others I will produce later on, show
that a thin layer of such humours
suffices
between the place where the leg, or, as indicated, its big tendon, on the one hand,
meets the sciatic
nerves or any fleshy part of the trunk, on the other; only if the contact
between these two parts of the animal is not direct.
In short, it is enough if the two parts be scarcely
covered or veiled at the points where contact has to occur. But so little is
sufficient that not even a lot can be prejudicial; I mean that it is not
detrimental that either one or both parts of the animal which have to contact should
be
plentifully covered with that humour, meaning that a mass, as great as you will,
is placed between.
The condition is that one of such conductors must be interposed.
It matters not
if it is thick or thin, long or short; all the effect depends on the touching
surfaces . So
the experiment turns out equally
well, keeping the whole trunk of the frog, or part of it, immersed in a bigger or smaller
bath of blood, salt
water or half-dissolved soap, etc., but especially
saturated alkaline solution ,
and bending the leg so that some parts of the big tendon coming from the
gastrocnemius muscle (this condition is very important and must always be
respected) come to touch the liquid itself at any place.
Here
are some more, nearly equivalent ways. Lay the trunk of your frog down
on a piece of soap whose surface has been moistened so that it forms a
semi-liquid pulp. Fold
one of its legs and press
it on this pulp at any distance from the trunk, taking care, as usual, that the
contact be
in the places near the foot joint or in some other part of the well-known
tendon. In this manner you will surely succeed, as I do, in
exciting convulsions in nearly all freshly
prepared
frogs, several times in
succession, by
alternating between touching and removing contact.
Success
is obtained for longer in
the following
manner, which I often put
into practice.
I
properly
soak a piece of sponge or a strip of cardboard, cloth, leather or, better,
tinder (amadou) in
a strong alkaline solution
and lay one part of it onto the sciatic
nerves or any part of the trunk, applying it very well.
The more or less long rest of this soaked strip stands out from
the trunk so that we can make the leg touch it easily along
some part of the usual tendon. So every
time I make
this contact I excite more or less vigorous contractions and spasms in all the muscles
of the leg; and this happens also in remarkably weakened frogs and those
prepared some time before. In very lively and recently
cut frogs, convulsions are so violent, the shaking
and tossing so strong, that they are surprising. It is possible to keep on
getting them for a long time, when
a)
you would not get them not only by
means of a simple metallic
conductor used instead of an alkaline solution, or even two pieces of the same metal,
not even by means of two slightly
different metals like silver and gold, brass and iron, lead and tin, when you
apply one of them to the dorsal muscles and the other one to the thigh muscle.
I must not
fail to warn
that the convulsions one
tries
to excite by the afore-mentioned
contacts sometimes do not occur during the first moments after applying either
the alkaline liquid, the softened soap
or the salt
water, etc., to one or other
part of the frog in the way described. The convulsions occur after some time,
when the humour applied
has penetrated or
at
least has come into
more perfect contact
with that part of the animal which
it covers and soaks.
If this part is too soaked and dripping with watery
humour when the alkaline liquid or
the salty water is applied thereto,
the experiments turn out worse and results
come later
due to the fact that the watery humour dilutes the other salty or soapy liquid
too
much at the place where the two layers merge
together; the salty or soapy liquid is no longer the same, no longer is such
quantity as it should be
at
that place, unlike
the animal conductors between which it is interposed , or unlike
the humour that makes the animal substances into
conductors. On the contrary, if the quantity of salty or soapy liquid is far
more than the quantity of water wetting that
part of the leg or trunk of the frog, the result not obtained
at first comes
some time later, i.e.
when that little water is distributed throughout
the great quantity of the other liquid or soap, so that it somehow
disappears therein,
till that wet
conductor which has been interposed proves to be different enough from the
animal parts it is in contact with.
I
will also draw your attention to the fact
that when very salty
water is used, and more so with
acid and alkaline liquids, the muscles
of the frog which are touched and infiltrated
by them remarkably feel the effects of such stimuli, so that some tremors, throbbing
and jerking are to be observed.
Yet such convulsions are limited to those muscles
and fibres which the liquid irritates
locally
and do not pass to the parts far
away (from the trunk, I mean, even if it is soaked in it), to the hind
legs. These convulsions can be only
too well distinguished
from the others which are bigger and more extensive and shake all the leg
muscles, from top to bottom, and make them leap any time one of them is
bent
so that its main
tendon touches the liquid or the soaking
parts of the trunk. By such fitting together we set the electric current flowing.
On
the other hand, we can get the acrid, stimulating liquid not to penetrate or
prick
b) any muscle
of the animal, and even less its
nerves, thus avoiding
any irritation and offence that the liquid may cause. It is enough to put in
touch with the trunk of the frog a piece of fresh, juicy flesh from any animal,
a piece of curd, hard-boiled egg white, polenta, a strip of cardboard, skin or tinder soaked in water,
a bit of juicy fruit, etc., in short any
second
class conductor (but it must be a good conductor) and onto
some points of said
conductor applied to the trunk of the frog put the salty liquid we are talking
about on a bit of sponge or tinder
well soaked in it. After disposing things this way, bring the leg of the frog or
the usual tendon into contact with the liquid or the sponge soaked in
it. The convulsions will occur nearly in the same way as if the liquid itself
were in direct contact with the frog’s
trunk.
I say nearly in the same way, supposing that the body applied to the
trunk be both
a good conductor, as I have already mentioned, and
not very long, as I add now, since otherwise,
slowing down the electric current, it causes its action to be less efficient.
The
reverse
of this last experiment is to apply the
conductor we wish to add, whatever it may be, i.e.
a piece of flesh, curd, polenta, etc., no longer to the frog’s
trunk but to one of its
legs, particularly to
the tendon I have mentioned so many times, and to put on some parts of this
interposed body the alkaline liquid or a sponge drenched in it. Then put
any
part of the bare trunk in contact with this liquid.
Not
very different from that is the experiment already mentioned, when you hold the
frog in your damp hand by
its feet and you touch the dorsal muscles or other part of the trunk with one
finger of your second hand, wet with blood, salt water or dissolved soap or
better with a strong alkaline liquid..
As
I have already
pointed out, it is vary rarely, and only in some
especially excitable frogs, that we may be successful in arousing
convulsions, using this method.
This happens for both the reasons already mentioned, viz.
because the electric current is considerably slowed down,
if not held up,
by the poor conductivity
of the skin on
our hands (even
though moistened on
the surface), as well as the long distance the current has to
travel
from one hand to the other through the person’s body.
If several people
form the circuit,
namely two, three, four, hand in hand making a chain, the success of the
experiment will be much more difficult, in
proportion as the impediments increase. Nevertheless, the convulsions also occur in this way, when
the frog
is endowed with the highest excitability, as I have said.
Moreover,
experiments using two, three or more people [and] different metals again
show that the skin of the hands is a remarkable impediment to the flowing of the
current, as it is not sufficiently
soaked in humours and therefore does not conduct
electricity well.
These experiments (which
you
also
repeated more than one year ago and which I have been showing for three years)
do not obviously
produce the effect, if the interlocked hands
are
not streaming with perspiration or, failing
that,
are not moistened properly. But
since the cutis, though bathed thus on the surface, remains deficient in humours within,
that is in the layers under the first moistened
one, until
you reach the live, juicy flesh, there is still some resistance to the
electric current, obstructing it and making it a little slower; so that if this
current is already very weak, as
in the experiments we are now concerned with, when metal conductors (I mean of
the first class) are not used, and also in those experiments two or more metal
ones are used but not much different from one another, it is not astonishing
that the current be
made completely ineffective to arouse convulsions in frogs still endowed,
if
not with most remarkable and rare, with more than mediocre excitability.
In
case you should wish to introduce one of the most acrid and irritating salty
liquids between the conductors where the circuit closes, and at
the same time to avoid any part of the frog from being
penetrated – and not even lightly, fleetingly
touched by the liquid itself – you can apply a piece of the already mentioned
substances, that is flesh cut from another animal, curdled milk, or polenta,
etc. to the frog’s legs (mainly over
the big tendon) and another piece to the trunk . Then
bring these two pieces, or added conductors, into contact so that the circuit
closes and there is between them a thick or thin layer of the liquid. Also in
this way – susceptible of many variations, as can be understood
– muscle contractions and shaking of the legs occur in a lot of propitious
circumstances.
I
have purposed to describe, most
renowned Colleague, not all but the
main ways
I have varied the tests
to stimulate convulsions in the frog, thanks to fitting together only conductors
of the second class. Now one of these ways I have not dealt with yet, either
in this or
the previous letter, a way which, on the other hand, agrees with the one I have
described
above,
is to dip
one or two legs of the recently, properly prepared frog into a beaker
full of water, while the trunk, or part thereof, is immersed
in another beaker, and then to induce communication between water and water by
means of other conductors of the same second class but different from water.
Just
to give an idea of many experiments performed
by me in this manner, it is sufficient to say, in general, that when a
conducting arc was made of just one piece or of several pieces all of the same
material (e.g.
all made of one, two or more pieces of the same fresh, juicy flesh, or the same
nerve or tendon, or the whole made of pieces of hard-boiled egg-white, polenta,
cheese, etc.) or one
or more people with hands linked together, dipping a clean finger or one moistened with
simple water into each of the two beakers,
convulsions never took place. On
very few occasions, convulsions were excited
when one of the fingers or one end of the other homogeneous conducting
arc (mainly
the end which touched the water in which
the trunk of the frog was immersed)
had been soiled with sticky blood or another glutinous or saline humour,
half-dissolved soap, or better a thick alkaline liquid. I say on
very few occasions, that is
only
in the most propitious circumstances, with
frogs gifted with extraordinary excitability and during the first
attempts with the same
most recently properly prepared with alkali, have I been able to
be successful, even
a short while after and in not particularly sensitive frogs.
That
is sufficient about such experiments in which, as
in all others,
putting together simply
different conductors is necessary. Only two conductors, however different,
forming all the circuit are not enough, as the actions against each other balance out,
in two similar matching ones. As can be seen in Figure 1 attached hereto, in
which
two bodies aa,bb fit together, whatever the action on the electric fluid
may be (I mean the current that originates from such fitting together), having
to be alike from the two opposite parts, either the current tends
to pass from a to b or from b to a, from both parts,
no current can flow from right to left or from left to right, owing to the fact
that such opposite forces counter-balance.
Nothing
happens either
when the arc bb, applied
to the wet conductor aa, is metallic but made of the very same metal,
with its ends bb, alike not
only in substance but also in temper, cleanness,
etc., which I dealt with at length
in the first letter last year.
Well
then, we need three or more different bodies entering the conductor circuit in
order to determine the flow of current, since the action which pushes the
electric fluid
from left to right or from right to left prevails against the other which pushes
in the opposite direction, in virtue of the different matching.
In other words, it is necessary that the conductor aa be
placed between two, bb and cc , different from it and from each other, as in Figure
2 – and the more different the better.
If
there are more than three different conductors put together,
or any number, (Figs. 3 and 4) electric current will
be produced which is
more or less active, that is to say capable or incapable of arousing
convulsions, etc. according to and in proportion as the forces tending to move
the electric fluid in one
direction prevail over the ones tending in
the opposite direction. The flow
will be completely absent only in very rare cases, when the sum of forces on one
side properly balance the sum of the opposite forces.
But
let’s leave this subject for the moment. We will return to it at greater
length
another time. It is useful to point out here that if one of the interconnecting conductors is of
higher conductivity, e.g. a strong saline solution,
and especially a pure alkali, which is so far the most active of the second
class conductors (as zinc is among the first class ones),
and if this conductor lies between two conductors of very different kinds from
it and also remarkably different from one another, nearly all the experiment
depends on the fitting together of this
conductor with these latter
ones, so that we can say the matching
together of however many
other conductors, slightly
dissimilar from each other and not very active, counts for nothing or very
little. Such is the case of the frog partially immersed
in two beakers
of water. Water as a conductor, or rather,
according to its conductivity, is
so little different from the green,
juicy body of the frog that they can both be considered as one
single watery conductor. If you dip a slice of juicy meat, hard-boiled egg-white,
curd, polenta, etc. into the two beakers (as
in the experiment described above),
the action on the electric fluid that we get by such juxtapositioning, I mean by
the contact
of these bodies in the water, is not
something
we can rely on a lot.
Those
bodies/contacts/conductivities
decide that they generate where an either thick or thin layer of dissolved soap,
strong acid or alkaline liquid applied to the other end of such conducting arcs,
that is to say that the layer covers and wraps the other extremity of the slice
of flesh, polenta, etc., is brought into contact with the water in the other
beaker;
the fact is
that the soap or alkaline liquid is kept and pressed between the water (here)
and the conductor different from water, towards each of which it acts very
differently. In short, all or nearly all the game is played by those three most
remarkably dissimilar
bodies. So, in Figure 4, if aa is the body of water in the beaker,
cc a prepared frog, bb a slice of juicy flesh, pp a piece
of cheese or hard-boiled egg, zz a drop or a thin or thick layer of alkaline liquid, the effect
will almost entirely
depend on the network
of three bodies, aa, zz, pp, that is on the very different actions which
arise at
the dissimilar
connection points, I say, at the connecting
faces za and zp. Little or nothing will happen at the other
connections of faces ab, bc, cp, as none of the conductors aa, bb, cc,
is very different in its conductivity
c)
Anyway,
a little difference is enough to cause some movement
in the electric fluid and to produce whatever current therein, as has already
been said. This current is always weak when the circuit is made up
of second
class conductors, however different they may be in kind, as I have
pointed out several times. It is extremely weak in all cases when no conductor
in the chain is very different from the two between which it lies. Therefore, if
there is no glutinous
or alkaline conductor (which
are precisely the conductors that differ most in quality from
the others of the second class)
never or hardly ever can
we stimulate convulsions even in the liveliest and
most recently prepared of frogs. I say hardly ever because it may happen that some frogs,
most
freshly prepared and excitable to the highest degree, move even
by means of that weak electric current
which is aroused at the joining points of those conductors which are
little different from one another.
To
sum up, any time it happens that we stimulate convulsions in a frog’s legs,
connected to the trunk only by
the crural nerves, by creating
another communication between the legs and the trunk through
a conducting arc made of as many pieces as you wish, you will always find that
there are some remarkable differences between the two, or better the three such
contiguous conductors which the circuit is made of. This
includes the parts of the frog’s trunk themselves, which are also more or less
different from one another as to the property we are dealing with, individually
the sinewy
part compared with the muscular and the nerves, the acqueous and weak humours
compared with the sticky and salty
humours, as I have pointed out from
the beginning. Now when I can demonstrate – I’ll say more – when it can be merely assumed
that three or more conductors, though little different from one another, enter
the circuit, this is enough to confirm the statement, which I make
and maintain, that motion is given to the electric fluid by juxtaposing
different conductors and not by any charge or imbalance
in the animal organs or between the inner side and the outer side thereof, as GALVANI
has thought and his followers still claim.
To demonstrate the theory of this professed
animal electricity, which I declare does not exist and
which I think I have
demolished effectively and completely by
means of many experiments, replacing it
by my other principle of artificial electricity roused by an external cause, my
opponents ought to show me that convulsions occur in frogs when the circuit is
formed by conductors all of the same kind, and in
nothing differing from one another – which they will never be able to do. On
the contrary,
I can always
show, as I have already said, that
convulsions
occur when
there is
the conjunction
of three or more conductors of different kinds.
I
drafted this letter more than a month ago, as a continuation of the other,
but so far I have had no time to transcribe it. I’ll
stop here for the time being, but
others will follow.
________________________________
NOTES
BY THE BOARD OF EDITORS
AND
ADDITIONS FROM VOLTA’S MANUSCRIPTS
__________
[1]
At about this point in Ms. J33 we find the following
digression:
[…
let’s talk a little about three conductors and why
two
aren’t different enough?]
It’s
easy to understand if you think that when only two conductors form a circuit,
like A and B in the appended figure, the actions that occur at the points of
contact of A itself with B itself, at the two opposite ends, destroy each other,
being alike. Therefore no electric current can flow from right to left or from
left to right.
So,
even if A is a metal, provided that it is alike at its two ends, not only in
kind but also in temper, polish, etc., and B the body of the frog, provided that
the parts on which said ends of arc A press are similar, no motion occurs in the
frog itself, however excitable it is; since the contact of a metal with a wet
conductor, though strongly stimulating the electric fluid, does so from both
parts in opposite directions and with equal strength, and therefore cannot make
it flow.
Remark:
the first sentence, within square brackets,
can be read in the Ms., although it has been rubbed out.
Translation
by M_Luise Guerrini
Revised
and completed by John Coggan
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