OBSERVATION.
From this experiment it seems that a greater effect in drawing off the lightning from the clouds may be expected from long pointed rods, than from short ones; I mean from such as show the greatest length, above the building they are fixed on.
EXPERIMENT III.
Instead of pinching the point between the thumb and finger, as in the last experiment, keep the thumb and finger each at near an inch distance from it, but at the same height, the point between them. In this situation, though the point is fairly exposed to the prime conductor, it has little or no effect; the electrometer rises to the height of a full charge.—But the moment the fingers are taken away, the ball falls quick to the stem.
OBSERVATION.
To explain this, it is supposed, that one reason of the sudden effect produced by a long naked pointed wire is, that (by the repulsive power of the positive charge in the prime conductor) the natural quantity of electricity contained in the pointed wire is driven down into the earth, and the point of the wire made strongly negative; whence it attracts the electricity of the prime conductor more strongly than bodies in their natural state would do; the small quantity of common matter in the point, not being able by its attractive force to retain its natural quantity of the electric fluid, against the force of that repulsion.—But the finger and thumb being substantial and blunt bodies, though as near the prime conductor, hold up better their own natural quantity against the force of that repulsion; and so, continuing nearly in the natural state, they jointly operate on the electric fluid in the point, opposing its descent, and aiding the point to retain it; contrary to the repelling power of the prime conductor, which would drive it down.—And this may also serve to explain the different powers of the point in the preceding experiment, on the slipping down the finger and thumb to different distances.
Hence is collected, that a pointed rod erected between two tall chimnies, and very little higher (an instance of which I have seen) cannot have so good an effect, as if it had been erected on one of the chimneys, its whole length above it.
EXPERIMENT IV.
If, instead of a long pointed wire, a large solid body (to represent a building without a point) be brought under and as near the prime conductor, when charged; the ball of the electrometer will fall a little; and on taking away the large body, will rise again.
OBSERVATION.