The experiments Mr. K. has exhibited here, have been greatly pleasing
to all sorts of people that have seen them; and I hope, by the time
he returns to Philadelphia, his tour this way will turn to good
account. His experiments are very curious, and I think prove most
effectually your doctrine of electricity; that it is a real element,
annexed to, and diffused among all bodies we are acquainted with;
that it differs in nothing from lightning, the effects of both being
similar, and their properties, so far as they are known, the same, &c.
The remarkable effect of lightning on iron, lately discovered, in
giving it the magnetic virtue, and the same effect produced on
small needles by the electrical fire, is a further and convincing
proof that they are both the same element; but, which is very
unaccountable, Mr. K. tells me, it is necessary to produce this
effect, that the direction of the needle and the electric fire should
be north and south; from either to the other, and that just so far
as they deviate therefrom, the magnetic power in the needle is less,
till their direction being at right angles with the north and south,
the effect entirely ceases. We made at Faneuil Hall, where Mr.
K----'s apparatus is, several experiments to give some small needles
the magnetic virtue; previously examining, by putting them in water,
on which they will be supported, whether or not they had any of that
virtue; and I think we found all of them to have some small degree of
it, their points turning to the north: we had nothing to do then but
to invert the poles, which accordingly was done, by sending through
them the charge of two large glass jars; the eye of the needle
turning to the north, as the point before had done; that end of the
needle which the fire is thrown upon, Mr. K. tells me always points
to the north.
The electrical fire passing through air has the same crooked
direction as lightning[73]. This appearance I endeavour to account
for thus: Air is an electric _per se_, therefore there must be a
mutual repulsion betwixt air and the electrical fire. A column
or cylinder of air, having the diameter of its base equal to the
diameter of the electrical spark, intervenes that part of the body
which the spark is taken from, and of the body it aims at. The spark
acts upon this column, and is acted upon by it, more strongly than
any other neighbouring portion of air.
The column, being thus acted upon, becomes more dense, and, being
more dense, repels the spark more strongly; its repellency being in
proportion to its density: Having acquired, by being condensed, a
degree of repellency greater than its natural, it turns the spark out
of its strait course; the neighbouring air, which must be less dense,
and therefore has a smaller degree of repellency, giving it a more
ready passage.
The spark, having taken a new direction, must now act on, or most
strongly repel the column of air which lies in that direction, and
consequently must condense that column in the same manner as the
former, when the spark must again change its course, which course
will be thus repeatedly changed, till the spark reaches the body that
attracted it.
To this account one objection occurs; that as air is very fluid and
elastic, and so endeavours to diffuse itself equally, the supposed
accumulated air within the column aforesaid, would be immediately
diffused among the contiguous air, and circulate to fill the space
it was driven from; and consequently that the said column, on the
greater density of which the phenomenon is supposed to depend, would
not repel the spark more strongly than the neighbouring air.
This might be an objection, if the electrical fire was as sluggish
and inactive as air. Air takes a sensible time to diffuse
itself equally, as is manifest from winds which often blow for
a considerable time together from the same point, and with a
velocity even in the greatest storms, not exceeding, as it is
said, sixty miles an hour: but the electric fire seems propagated
instantaneously, taking up no perceptible time in going very great
distances. It must then be an inconceivably short time in its
progress from an electrified to an unelectrified body, which, in the
present case, can be but a few inches apart: but this small portion
of time is not sufficient for the elasticity of the air to exert
itself, and therefore the column aforesaid must be in a denser state
than its neighbouring air.
About the velocity of the electric fire more is said below, which
perhaps may more fully obviate this objection. But let us have
recourse to experiments. Experiments will obviate all objections,
or confound the hypothesis. The electric spark, if the foregoing be
true, will pass through a vacuum in a right line. To try this, let a
wire be fixed perpendicularly on the plate of an air pump, having a
leaden ball on its upper end; let another wire, passing through the
top of a receiver, have on each end a leaden ball; let the leaden
balls within the receiver, when put on the air pump, be within two
or three inches of each other: the receiver being exhausted, the
spark given from a charged phial to the upper wire will pass through
rarefied air, nearly approaching to a vacuum, to the lower wire, and
I suppose in a right line, or nearly so; the small portion of air
remaining in the receiver, which cannot be entirely exhausted, may
possibly cause it to deviate a little, but perhaps not sensibly,
from a right line. The spark also might be made to pass through air
greatly condensed, which perhaps would give a still more crooked
direction. I have not had opportunity to make any experiments of this
sort, not knowing of an air-pump nearer than Cambridge, but you can
easily make them. If these experiments answer, I think the crooked
direction of lightning will be also accounted for.
With respect to your letters on electricity, * * * * * * * * * * *
* * * *. Your hypothesis in particular for explaining the phenomena
of lightning is very ingenious. That some clouds are highly charged
with electrical fire, and that their communicating it to those that
have less, to mountains and other eminencies, makes it visible and
audible, when it is denominated lightning and thunder, is highly
probable: but that the sea, which you suppose the grand source of
it, can collect it, I think admits of a doubt: for though the sea be
composed of salt and water, an electric _per se_ and non-electric,
and though the friction of electrics _per se_ and non-electrics,
will collect that fire, yet it is only under certain circumstances,
which water will not admit. For it seems necessary, that the
electrics _per se_ and non-electrics rubbing one another, should
be of such substances as will not adhere to, or incorporate with
each other. Thus a glass or sulphur sphere turned in water, and so
a friction between them, will not collect any fire; nor, I suppose,
would a sphere of salt revolving in water; the water adhering to,
or incorporating with those electrics _per se_. But granting that
the friction between salt and water would collect the electrical
fire, that fire, being so extremely subtle and active, would be
immediately communicated, either to those lower parts of the sea
from which it was drawn, and so only perform quick revolutions; or
be communicated to the adjacent islands or continent, and so be
diffused instantaneously through the general mass of the earth. I say
instantaneously, for the greatest distances we can conceive within
the limits of our globe, even that of the two most opposite points,
it will take no sensible time in passing through: and therefore
it seems a little difficult to conceive how there can be any
accumulation of the electrical fire upon the surface of the sea or
how the vapours arising from the sea should have a greater share of
that fire than other vapours.
That the progress of the electrical fire is so amazingly swift, seems
evident from an experiment you yourself (not out of choice) made,
when two or three large glass jars were discharged through your body.
You neither heard the crack, was sensible of the stroke, nor, which
is more extraordinary, saw the light; which gave you just reason to
conclude, that it was swifter than sound, than animal sensation, and
even light itself. Now light (as astronomers have demonstrated) is
about six minutes passing from the sun to the earth; a distance, they
say, of more than eighty millions of miles. The greatest rectilinear
distance within the compass of the earth is about eight thousand
miles, equal to its diameter. Supposing then, that the velocity of
the electric fire be the same as that of light, it will go through a
space equal to the earth's diameter in about 2/60 of one second of a
minute. It seems inconceivable then, that it should be accumulated
upon the sea, in its present state, which, as it is a non-electric,
must give the fire an instantaneous passage to the neighbouring
shores, and they convey it to the general mass of the earth. But such
accumulation seems still more inconceivable when the electrical fire
has but a few feet depth of water to penetrate, to return to the
place from whence it is supposed to be collected.
Your thoughts upon these remarks I shall receive with a great deal of
pleasure. I take notice that in the printed copies of your letters
several things are wanting which are in the manuscript you sent me. I
understand by your son, that you had writ, or was writing, a paper
on the effect of the electrical fire on loadstones, needles, &c.
which I would ask the favour of a copy of, as well as of any other
papers on electricity, written since I had the manuscript, for which
I repeat my obligations to you.