Newton made a considerable improvement in the electrical machine of Guericke by the substitution of a hollow globe of glass for Guericke's sulphur one. What is chiefly interesting about the improvement is the fact that Guericke's sulphur globe, of comparative weight and cumbrousness, was made by casting melted sulphur into a glass globe and then breaking off the glass. Guericke observed in the dark a peculiar luminosity of conducting surfaces when well charged by means of his machine; he compared it to the phosphorescent light observed when lump sugar is broken in the dark. It was what is now known as the brush-discharge effect.
In 1705 Francis Hawksbee discovered the peculiar phenomenon which he termed the mercurial phosphorus. It was produced by causing a stream of well-dried mercury to fall through an exhausted glass receiver. The friction of the particles of mercury against the jet piece and the glass caused an electrification which evinced itself in a phosphorescent glow. The receiver, indeed, had not to be by any means thoroughly exhausted, the phenomenon occurring at an air pressure up to about fourteen inches of the barometer.
The crackling noise and the spark accompanying electrical discharge suggested about this time the analogy of those miniature disturbances to thunder and lightning, but the identity of the two was not fully established until later.
Up to this time the fact that certain substances were capable of conducting electricity was not known, but in 1729 Stephen Gray, F. R. S., an enthusiastic investigator, made the discovery, and at the same time the cognate one that a large class of materials are nonconductors. The only source of electricity which was at the disposal of experimenters up to this time was the electrical machine, improved, as described, by Newton, which furnished intermittent currents (discharges) of infinitesimal quantity, as we should say now, but of extremely high pressure. This fact of the enormous pressure resulted in the electricity's forcing its way through very imperfect conductors, so as to cause our investigators to rank many of these latter with the metals. Thus Gray concluded that pack thread was a good conductor because it did not oppose sufficient resistance to prevent the flow of his high pressure (or, as we should now say, high voltage or tension) electricity. He tried wire as well, but did not realize it was a better conductor than the thread, although its conductivity was actually in the millions of times as great. In collaboration with his friend Wheeler he conveyed electrical discharges a distance of eight hundred and eighty-six feet, through presumably air-dry pack thread—an achievement which would almost be notable at the present time. He insulated the line by hanging it from loops of silk thread.
Gray hoped "that there may be found out a way to collect a greater quantity of electric fire, and consequently to increase the force of that power, which, si licet magnis componere parva, seems to be of the same nature with thunder and lightning."
About this time Desaguliers discovered that those materials which, upon being rubbed, develop electrical charges, are all nonconductors, and that, conversely, nonelectrics are conductors. The terms electrics and nonelectrics were applied to bodies respectively capable and incapable of excitation; the words idioelectrics and anelectrics were also used in respectively equivalent senses.
In France, Dufay discovered that the conductivity of pack thread was greatly improved by the presence of moisture, and he succeeded in conveying a discharge a distance of almost thirteen hundred feet. He suspended himself by silken cords and had himself electrified, and then observed that he could give a shock accompanied by a spark to any person standing on the ground.
He also established the fact of the two opposite kinds of electrification, and gave them the names of vitreous and resinous, from the fact that the former was developed by the excitation of glass and vitreous substances generally, and the latter from that of amber and resins. He observed that the distinguishing characteristic of the two was the fact that opposite charges attracted each other, while similar ones exerted mutual repulsion. Dufay and Gray died within three years of each other, both at the age of forty, Gray having added to the results already mentioned the discovery of the conducting powers of certain liquids and of the human body.
Experimental research now began to spread into Germany and the Netherlands. The electrical machine was greatly improved by Professor Boze, of Wittenberg, and Professor Winkler, of Leipsic, who respectively added the prime conductor and the silk rubber to that important piece of apparatus. A Scotch Benedictine monk of Erfurt—Professor Gordon—substituted a glass cylinder for the sphere, and thereby brought the instrument in its essentials practically to the form in which it exists to-day. The improvement enabled the production of very large sparks, which were caused to produce the inflammation of various combustibles. Gordon went so far as to ignite alcohol by means of a jet of electrified water.