"The first is the wonderful effect of pointed bodies, both in _drawing
off_ and _throwing off_ the electrical fire."
It will be observed that this statement is made in the language of the
_one_-fluid theory, of which Franklin may be regarded as the author.
This theory will be again referred to presently. Franklin electrified
a cannon-ball so that it repelled a cork. On bringing near it the
point of a bodkin, the repulsion disappeared. A blunt body had to be
brought near enough for a spark to pass in order to produce the same
effect. "To prove that the electrical fire is _drawn off_ by the
point, if you take the blade of the bodkin out of the wooden handle,
and fix it in a stick of sealing-wax, and then present it at the
distance aforesaid, or if you bring it very near, no such effect
follows; but sliding one finger along the wax till you touch the
blade, and the ball flies to the shot immediately. If you present the
point in the dark, you will see, sometimes at a foot distance or more,
a light gather upon it like that of a fire-fly or glow-worm; the less
sharp the point, the nearer you must bring it to observe the light;
and at whatever distance you see the light, you may draw off the
electrical fire, and destroy the repelling."
By laying a needle upon the shot, Franklin showed "that points will
_throw off_ as well as _draw off_ the electrical fire." A candle-flame
was found to be equally efficient with a sharp point in drawing off
the electricity from a charged conductor. The effect of the
candle-flame Franklin accounted for by supposing the particles
separated from the candle to be first "attracted and then repelled,
carrying off the electric matter with them." The effect of points is a
direct consequence of the law of electrical repulsion. When a
conductor is electrified, the density of the electricity is greatest
where the curvature is greatest. Thus, if a number of spheres are
electrified from the same source, the density of the electricity on
the different spheres will vary inversely as their diameters. The
force tending to drive the electricity off a conductor is everywhere
proportional to the density, and hence in the case of the spheres will
be greatest for the smallest sphere. On this principle, the density of
electricity on a perfectly sharp point, if such could exist, on a
charged conductor, would be infinite and the force tending to drive it
off would be infinite also. Hence a moderately sharp point is
sufficient to dissipate the electricity from a highly charged
conductor, or to neutralize it if the point is connected to earth and
brought near the conductor so as to be electrified by induction.
Public-domain text, read in full here on John Shaqi.
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