By the discharge of the battery, Franklin succeeded in melting and volatilizing gold-leaf, thin strips of tinfoil, etc. His views on the nature of light are best given in his own words.
"I am not satisfied with the doctrine that supposes particles of matter called light, continually driven off from the sun's surface, with a swiftness so prodigious! Must not the smallest particle conceivable have, with such a motion, a force exceeding that of a twenty-four pounder discharged from a cannon?... Yet these particles, with this amazing motion, will not drive before them, or remove, the least and lightest dust they meet with.
"May not all the phenomena of light be more conveniently solved by supposing universal space filled with a subtile elastic fluid, which, when at rest, is not visible, but whose vibrations affect that fine sense in the eye, as those of air do the grosser organs of the ear? We do not, in the case of sound, imagine that any sonorous particles are thrown off from a bell, for instance, and fly in straight lines to the ear; why must we believe that luminous particles leave the sun and proceed to the eye? Some diamonds, if rubbed, shine in the dark without losing any part of their matter. I can make an electrical spark as big as the flame of a candle, much brighter, and therefore visible further; yet this is without fuel; and I am persuaded no part of the electrical fluid flies off in such case to distant places, but all goes directly and is to be found in the place to which I destine it. May not different degrees of the vibration of the abovementioned universal medium occasion the appearances of different colours? I think the electric fluid is always the same; yet I find that weaker and stronger sparks differ in apparent colour, some white, blue, purple, red: the strongest, white; weak ones, red. Thus different degrees of vibration given to the air produce the seven different sounds in music, analogous to the seven colours, yet the medium, air, is the same."
Mr. Kinnersley having called Franklin's attention to the fact that a sulphur globe when rubbed produced electrification of an opposite kind from that produced by a glass globe, Franklin repeated the experiment, and noticed that the discharge from the end of a wire connected with the conductor was different in the two cases, being "long, large, and much diverging when the glass globe is used, and makes a snapping (or rattling) noise; but when the sulphur one is used it is short, small, and makes a hissing noise; and just the reverse of both happens when you hold the same wire in your hand and the globes are worked alternately.... When the brush is long, large, and much diverging, the body to which it is joined seems to be throwing the fire out; and when the contrary appears it seems to be drinking in."
On October 19, 1752, Franklin wrote to Mr. Peter Collinson as follows:—
As frequent mention is made in public papers from Europe of the success of the Philadelphia experiment for drawing the electric fire from clouds by means of pointed rods of iron erected on high buildings, etc., it may be agreeable to the curious to be informed that the same experiment has succeeded in Philadelphia, though made in a different and more easy manner, which is as follows:—
Make a small cross of two light strips of cedar, the arms so long as to reach to the four corners of a large thin silk handkerchief when extended. Tie the corners of the handkerchief to the extremities of the cross, so you have the body of a kite; which, being properly accommodated with a tail, loop, and string, will rise in the air like those made of paper; but this being of silk is fitter to bear the wet and wind of a thunder-gust without tearing. To the top of the upright stick of the cross is to be fixed a very sharp-pointed wire, rising a foot or more above the wood. To the end of the twine, next the hand, is to be tied a silk ribbon, and, where the silk and twine join, a key may be fastened. This kite is to be raised when a thunder-gust appears to be coming on, and the person who holds the string must stand within a door or window, or under some cover so that the silk ribbon may not be wet, and care must be taken that the twine does not touch the frame of the door or window. As soon as any of the thunder-clouds come over the kite, the pointed wire will draw the electric fire from them, and the kite, with all the twine, will be electrified, and the loose filaments of the twine will stand out every way, and be attracted by an approaching finger. And when the rain has wetted the kite and twine so that it can conduct the electric fire freely, you will find it stream out plentifully from the key on the approach of your knuckle. At this key the phial may be charged, and from electric fire there obtained spirits may be kindled, and all the other electric experiments be performed which are usually done by the help of a rubbed glass globe or tube, and thereby the sameness of the electric matter with that of lightning completely demonstrated.
Having, in September, 1752, erected the iron rod and bells in his own house, as previously mentioned, Franklin succeeded, in April, 1753, in charging a Leyden jar from the rod, and found its charge was negative. On June 6, however, he obtained a positive charge from a cloud. The results of his observations led him to the conclusion "That the clouds of a thunder-gust are most commonly in a negative state of electricity, but sometimes in a positive state."
In order to illustrate a theory respecting the electrification of clouds, Franklin placed a silver can on a wine-glass. Inside the can was placed a considerable length of chain, which could be drawn out by means of a silk thread. He electrified the can from a Leyden jar until it would receive no more electricity. Then raising the silk thread, he gradually drew the chain out of the can, and found that the greater the length of chain drawn out the greater was the charge which the jar would give to the system, and as the chain was raised, spark after spark passed from the jar to the silver can, thus showing that the capacity of the system was increased by increasing the amount of chain exposed.
In 1755 Franklin observed the effects of induction; for, having attached to his prime conductor a tassel made of damp threads and electrified the conductor, he found that the threads repelled each other and stood out. Bringing an excited glass tube near the other end of the conductor, the threads were found to diverge more, "because the atmosphere of the prime conductor is pressed by the atmosphere of the excited tube, and driven towards the end where the threads are, by which each thread acquires more atmosphere." When the excited tube was brought near the threads, they closed a little, "because the atmosphere of the glass tube repels their atmospheres, and drives part of them back on the prime conductor." A number of other experiments illustrating electrical induction were also carried out.