The globe here referred to is the glass globe of Franklin's frictional machine of American make, which, when rotated, was electrified positively by contact with the hand or with a leather rubber. Franklin also used a sulphur ball or "brimstone" globe, and observed that the electrification produced on it differed in kind from that developed on the glass globe. (1752.)
It may here be stated that the first to use a leather cushion as a substitute for the hand in the frictional machine, was Winkler, of Leipzig (1745); the efficiency of the rubber was increased by Canton, of London, who covered it with an amalgam of tin and mercury (1762). Bose, of Wittenberg, had previously added the prime-conductor, which greatly augmented the electrical capacity and output of the machine.
In 1750 Franklin imitated the effect of lightning on the compasses of a ship by the action of a jar discharge on an unmagnetized steel needle. "By electricity," he says, "we have frequently given polarity to needles and reversed it at pleasure."
Similar experiments are made to-day in every lecture-course on static electricity; but the experimenter, when wise, does not announce beforehand which end of the needle will be north and which south, as he is just as likely to be wrong as right, the uncertainty being due to the fact that the discharge of a Leyden jar is not a current of electricity in one direction, but rather a few sudden rushes or rapid surgings of electricity to and fro; in other words, it is oscillatory in character instead of being continuous in one direction.
Franklin did not know this; although he made a very pertinent remark in 1749 when he likened the mechanical condition of the glass of a charged jar to that of a bent rod or a stretched spring. "So, a straight spring," he says, "when forcibly bent must, to restore itself, contract that side which in the bending was extended, and extend that side which was contracted." Franklin knew, of course, that the bent rod, when released, would swing to and fro a few times before settling down to its state of rest; but he failed to see the analogy between it and the strained glass of the charged Leyden jar.
It is to Joseph Henry (1799-1878), the Faraday of America, that we owe the recognition and statement of the oscillatory character of the discharge from Leyden jars and condensers generally. He discovered and published this cardinal fact in 1842. His words deserve recording. "The discharge, whatever may be its nature, is not correctly represented (employing for simplicity the theory of Franklin) by the single transfer of an imponderable fluid from one side of the jar to the other; the phenomenon requires us to admit the existence of a principal discharge in one direction and then several reflex actions backward and forward, each more feeble than the preceding, until equilibrium is attained."[8] The italics are Prof. Henry's.
It is precisely this oscillatory character of the spark-discharge that enables us to send out trains of electric waves into the all-pervading ether, and thus to communicate, by "wireless," with remote stations.
Having conclusively proved that the energy of a charged condenser resides in the dielectric, Franklin next tries to find whether "the electric matter" in the case of conductors is limited to the surface or whether it penetrates to an appreciable depth. To ascertain this, he insulates a silver fruit-can and brings a charged ball, held by a silk thread, into contact with the outer surface. On testing after removal, he found that the ball retained some of its charge, whilst it lost all if allowed to touch the bottom of the vessel. Surprised at this unexpected difference, he repeated the experiment again and again, only to find the ball every time without a trace of charge after contact with the interior of the vessel. This perplexed and puzzled him. "The fact is singular," he says, "and you require the reason? I do not know it. I find a frank acknowledgment of one's ignorance is not only the easiest way to get rid of a difficulty, but the likeliest way to obtain information, and therefore I practice it. I think it an honest policy. Those who affect to be thought to know everything, often remain long ignorant of many things that others could and would instruct them in, if they appeared less conceited."
This was in 1755. Cavendish in 1773 and Coulomb in 1788 independently attacked the same problem; and having proved by their classic experiments that a static charge is limited to the surface of conductors, it was but a step to infer that such a distribution of electricity implies that the law of force between two elements of charge, or between two point-charges, is the law of the inverse square of the distance.
It will also be remembered that Faraday, not knowing what had been accomplished eighty years before in Philadelphia, used for one of his best-known experiments an ice-pail, into which he lowered an electrified ball for the purpose of showing the exact equality of the induced and the inducing charge. The similarity of apparatus and mode of procedure are remarkable.