In a letter to Collinson, dated July 11, 1747, Franklin communicated to him the earliest results of his experimental use of the glass tube that Collinson had sent over to Philadelphia. The first phenomenon, which fixed his attention, was the wonderful effect of pointed bodies in drawing off the electrical fire. This was the lightning rod in its protoplasmal stage. The manner in which he described the experiment, by which this particular truth was demonstrated, is a good specimen of his remarkable faculty for simple and clear statement:
Place an iron shot of three or four inches diameter on the mouth of a clean dry glass bottle. By a fine silken thread from the ceiling, right over the mouth of the bottle, suspend a small cork ball, about the bigness of a marble; the thread of such a length, as that the cork ball may rest against the side of the shot. Electrify the shot, and the ball will be repelled to the distance of four or five inches, more or less, according to the quantity of Electricity. When in this state, if you present to the shot the point of a long slender sharp bodkin, at six or eight inches distance, the repellency is instantly destroy'd, and the cork flies to the shot. A blunt body must be brought within an inch, and draw a spark, 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, and more, a light gather upon it, like that of a firefly, or glowworm; 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 repellency. If a cork ball so suspended be repelled by the tube, and a point be presented quick to it, tho' at a considerable distance, 'tis surprizing to see how suddenly it flies back to the tube. Points of wood will do near as well as those of iron, provided the wood is not dry; but perfectly dry wood will no more conduct electricity than sealing-wax.
The repellency between the ball and the shot was likewise destroyed, Franklin stated, 1, by sifting fine sand on it; this did it gradually, 2, by breathing on it, 3, by making a smoke about it from burning wood, and 4, by candlelight, even though the candle was at a foot distance; these did it suddenly.
The same result was also produced, he found, by the light of a bright coal from a wood fire, or the light of red-hot iron; but not at so great a distance. Such was not the effect, however, he said, of smoke from dry resin dropped on hot iron. It was merely attracted by both shot and cork ball, forming proportionable atmospheres round them, making them look beautifully, somewhat like some of the figures in Burnet's or Whiston's Theory of the Earth.
Franklin also noted the fact that, unlike fire-light, sunlight, when thrown on both cork and shot, did not impair the repellency between them in the least.
In the same letter, guided by the belief that he had formed that electricity is not created by friction but, except when accumulated or depleted by special causes, is equally diffused through material substances generally, he also reached the conclusion that electrical discharges are due to circuits set up by substances that offer little resistance to the transit of the electrical current between bodies charged with more than the ordinary quantity of electrical energy and bodies not in that condition. In other words, electricity is always alert to restore its equilibrium when lost, and, if accumulated beyond its normal measure in one body, seeks with violent eagerness, as soon as a favorable medium of transmission is presented to it, to pass on its surplus of electrical energy to another body less amply supplied.
These conceptions, too, which lie at the very foundations of modern electrical science, are illustrated by Franklin with extraordinary simplicity and clearness as follows:
1. A person standing on wax, and rubbing the tube, and another person on wax drawing the fire, they will both of them, (provided they do not stand so as to touch one another) appear to be electrised, to a person standing on the floor; that is, he will perceive a spark on approaching each of them with his knuckle.
2. But, if the persons on wax touch one another during the exciting of the tube, neither of them will appear to be electrised.