Makers of Electricity. James J. Walsh

Modern form of Leyden jar, with movable coatings 87 Three coated panes in series 89 " panes in parallel 89 " jars in parallel 90 " jars in cascade 90 Discharge by alternate contacts 94 Tassel of long threads or light strips of paper 101 Procopius Divisch (1696–1765) 108 The Divisch lightning conductor (1754) 111 Set of pointed rods 112 Galvani (portrait) opposite page 133 Volta " " " 162 Oersted " " " 205 The Magnetic effect of an electric current 209 Magnetic field surrounding a conductor carrying a current 212 Magnetic whirl surrounding a wire through which a current is passing 213 Ampère's molecular currents 214 The "sympathetic telegraph" from Cabeo's Philosophia Magnetica, 1629 216 The "sympathetic telegraph" from Turner's Ars Notoria, 1657 218 Ampère (portrait) opposite page 232 Faraday " " " 299 Clerk Maxwell (portrait) opposite page 334 Lord Kelvin " " " 361

MAKERS OF ELECTRICITY.

       Table of Contents

CHAPTER I.

       Peregrinus and Columbus.

       Table of Contents

      The ancients laid down the laws of literary form in prose as well as in verse, and bequeathed to posterity works which still serve as models of excellence. Their poets and historians continue to be read for the sake of the narrative and beauty of the style; their philosophers for breadth and depth of thought; and their orators for judicious analysis and impassioned eloquence.

      In the exact sciences, too, the ancients were conspicuous leaders by reason of the number and magnitude of the discoveries which they made. You have only to think of Euclid and his "Elements," of Apollonius and his Conics, of Eratosthenes and his determination of the earth's circumference, of Archimedes and his mensuration of the sphere, and of the inscription on Plato's Academy, Let none ignorant of geometry enter my door, to realize the fondness of the Greek mind for abstract truth and its suppleness and ingenuity in mathematical investigation.

      But the sciences of observation did not advance with equal pace; nor was this to be expected, as time is an essential element in experimentation and in the collection of data, both of which are necessary for the framing of theories in explanation of natural phenomena.

      The slowness of advance is well seen in the development of the twin subjects of electricity and magnetism. As to the lodestone, with which we are concerned at present, the attractive property was the only one known to ancient philosophy for a period of six hundred years, from the time of Thales to the age of the Cæsars, when Lucretius wrote on the nature of things in Latin verse.

      Lucretius records the scant magnetic knowledge of his predecessors and then proceeds to unfold a theory of his own to account for the phenomena of the wonder-working stone. Book VI. of "De Natura Rerum" contains his speculations anent the magnet, together with certain observations which show that the poet was not only a thinker, but somewhat of an experimenter as well. Thus he recognizes magnetic repulsion when he says: "It happens, too, at times that the substance of the iron recedes from the stone as if accustomed to start back from it, and by turns to follow it."

      This recognition of the repelling property of the lodestone is immediately followed by the description of an experiment which is frequently referred to in works on magnetic philosophy. It reads: "Thus have I seen raspings of iron, lying in brazen vessels, thrown into agitation and start up when the magnet was moved beneath"; or metrically,

      And oft in brazen vessels may we mark

       Ringlets of Samothrace, or fragments fine

       Struck from the valid iron bounding high

       When close below, the magnet points its powers.

       This experiment, seen and recorded by Lucretius, is of special interest to the student of magnetic history because of the use which is made of iron filings and also because it has led certain writers to credit the poet with a knowledge of what is known to-day by the various names of magnetic figures, magnetic curves, magnetic spectrum. We do not, however, share this view, because we see no adequate resemblance between the positions assumed by the bristling particles of iron in the one case, as described by the Roman poet, and the continuous symmetrical curves of our laboratories in the other. If Lucretius noticed such curves in his brazen vessels, he does not say so; nor does the meagre description of magnetic phenomena given in Book VI. warrant us in assuming that he did.

      The use of iron filings to map out the entire field of force that surrounds a magnet was unknown to classical antiquity; it was not known to Peregrinus or Roger Bacon in the thirteenth century or even to Gilbert in the sixteenth. The credit for reviving the use of filings and employing