Together with many smaller matters, Benjamin Franklin added three great discoveries to the knowledge of electricity. The first was that the electric fluid--so called for want of a better word to express the action of the mysterious force--will run its course more easily and quickly through sharply pointed metals than in any other way. This had never before been demonstrated, nor, probably, been ascertained. The second great discovery of Franklin was that of positive and negative electricity, or, as he called it for some time, _plus_ and _minus_, the latter names being really the most descriptive. Of the actual existence of these two divisions of the great and marvellous agency, now attracting and now repelling each other, much was known previous to Franklin, but he was the first to make them clearly understood, and to bring their effects within reach of calculation. To these two discoveries Benjamin Franklin added a third, the greatest of all. He established the identity between the electric force and lightning, and upon it based one of the noblest inventions of all ages, that of the lightning conductor. And perhaps there never was any invention acknowledged more deeply by mankind. The French Academy expressed it when, on Franklin’s entrance, all the members rose, and the President exclaimed ‘_Eripuit cœlo fulmen_.’
The identity of the electric force and lightning, vaguely surmised by previous inquirers, and expressed at times in hints, was not only firmly asserted by Benjamin Franklin, but at a comparatively early part of his investigations proved by him in experiments. His broad practical way of looking at facts succeeded in grasping a truth which all the learned men before him, who had busied themselves with electrical experiments, had not been able to lay hold of, simply because they lost themselves in philosophical abstractions. The professors sought the unattainable, and he confined himself strictly to what he considered within reach, and it was thus he gained his end.
The thoroughly matter-of-fact way in which Franklin went to work is strikingly exhibited in his own description as to how he came to the conclusion of the oneness of lightning and electricity. In reply to a friend and correspondent, living in South Carolina, who had asked him how he came to such an ‘out-of-the-way idea’ as that of the majestic fire from the cloud-capped firmament being exactly the same with the puny gleam from a stick of glass, rubbed with the sleeve of an old coat, Franklin wrote a highly characteristic letter. ‘I cannot answer your question better,’ he told his friend, ‘than by giving you an extract from the minutes I used to keep of the experiments I made. By this extract you will see that the thought was not so much an out-of-the-way one but that it might have occurred to any electrician. The extract, dated November 7, 1749--a date worth remembrance in the history of scientific progress--was as follows in its entirety:—‘Electrical fluid agrees with lightning, in these particulars: 1. Giving light. 2. The colour of the light. 3. In the crooked direction of the flame. 4. In the swift motion. 5. In being conducted by metals. 6. In the crack, or noise, of the explosion. 7. The subsisting in water, or ice. 8. In the rending of bodies it passes through. 9. In destroying animals. 10. In melting metals. 11. In firing inflammable substances. 12. The sulphurous smell. The electric fluid is attracted by points, and we do not know whether this property is in lightning. But since they agree in all the particulars wherein we can already compare them, is it not probable that they agree likewise in this? _Let the experiment be made._’