to come down.” But she rushed to him and clung round

Undaunted by the dicta of contemporaneous science, Mr. Edison attacked the dynamo problem with his accustomed vigor and thoroughness. He chose the drum form for his armature, and experimented with different kinds of iron. Cores were made of cast iron, others of forged iron; and still others of sheets of iron of various thicknesses separated from each other by paper or paint. These cores were then allowed to run in an excited field, and after a given time their temperature was measured and noted. By such practical methods Edison found that the thin, laminated cores of sheet iron gave the least heat, and had the least amount of wasteful eddy currents. His experiments and ideas on magnetism at that period were far in advance of the time. His work and tests regarding magnetism were repeated later on by Hopkinson and Kapp, who then elucidated the whole theory mathematically by means of formulae and constants. Before this, however, Edison had attained these results by pioneer work, founded on his original reasoning, and utilized them in the construction of his dynamo, thus revolutionizing the art of building such machines.

After thorough investigation of the magnetic qualities of different kinds of iron, Edison began to make a study of winding the cores, first determining the electromotive force generated per turn of wire at various speeds in fields of different intensities. He also considered various forms and shapes for the armature, and by methodical and systematic research obtained the data and best conditions upon which he could build his generator. In the field magnets of his dynamo he constructed the cores and yoke of forged iron having a very large cross-section, which was a new thing in those days. Great attention was also paid to all the joints, which were smoothed down so as to make a perfect magnetic contact. The Edison dynamo, with its large masses of iron, was a vivid contrast to the then existing types with their meagre quantities of the ferric element. Edison also made tests on his field magnets by slowly raising the strength of the exciting current, so that he obtained figures similar to those shown by a magnetic curve, and in this way found where saturation commenced, and where it was useless to expend more current on the field. If he had asked Upton at the time to formulate the results of his work in this direction, for publication, he would have anticipated the historic work on magnetism that was executed by the two other investigators; Hopkinson and Kapp, later on.

The laboratory note-books of the period bear abundant evidence of the systematic and searching nature of these experiments and investigations, in the hundreds of pages of notes, sketches, calculations, and tables made at the time by Edison, Upton, Batchelor, Jehl, and by others who from time to time were intrusted with special experiments to elucidate some particular point. Mr. Jehl says: "The experiments on armature-winding were also very interesting. Edison had a number of small wooden cores made, at both ends of which we inserted little brass nails, and we wound the wooden cores with twine as if it were wire on an armature. In this way we studied armature-winding, and had matches where each of us had a core, while bets were made as to who would be the first to finish properly and correctly a certain kind of winding. Care had to be taken that the wound core corresponded to the direction of the current, supposing it were placed in a field and revolved. After Edison had decided this question, Upton made drawings and tables from which the real armatures were wound and connected to the commutator. To a student of to-day all this seems simple, but in those days the art of constructing dynamos was about as dark as air navigation is at present.... Edison also improved the armature by dividing it and the commutator into a far greater number of sections than up to that time had been the practice. He was also the first to use mica in insulating the commutator sections from each other."

In the mean time, during the progress of the investigations on the dynamo, word had gone out to the world that Edison expected to invent a generator of greater efficiency than any that existed at the time. Again he was assailed and ridiculed by the technical press, for had not the foremost electricians and physicists of Europe and America worked for years on the production of dynamos and arc lamps as they then existed? Even though this young man at Menlo Park had done some wonderful things for telegraphy and telephony; even if he had recorded and reproduced human speech, he had his limitations, and could not upset the settled dictum of science that the internal resistance must equal the external resistance.

Such was the trend of public opinion at the time, but "after Mr. Kruesi had finished the first practical dynamo, and after Mr. Upton had tested it thoroughly and verified his figures and results several times-- for he also was surprised--Edison was able to tell the world that he had made a generator giving an efficiency of 90 per cent." Ninety per cent. as against 40 per cent. was a mighty hit, and the world would not believe it. Criticism and argument were again at their height, while Upton, as Edison's duellist, was kept busy replying to private and public challenges of the fact.... "The tremendous progress of the world in the last quarter of a century, owing to the revolution caused by the all-conquering march of `Heavy Current Engineering,' is the outcome of Edison's work at Menlo Park that raised the efficiency of the dynamo from 40 per cent. to 90 per cent."

Mr. Upton sums it all up very precisely in his remarks upon this period: "What has now been made clear by accurate nomenclature was then very foggy in the text-books. Mr. Edison had completely grasped the effect of subdivision of circuits, and the influence of wires leading to such subdivisions, when it was most difficult to express what he knew in technical language. I remember distinctly when Mr. Edison gave me the problem of placing a motor in circuit in multiple arc with a fixed resistance; and I had to work out the problem entirely, as I could find no prior solution. There was nothing I could find bearing upon the counter electromotive force of the armature, and the effect of the resistance of the armature on the work given out by the armature. It was a wonderful experience to have problems given me out of the intuitions of a great mind, based on enormous experience in practical work, and applying to new lines of progress. One of the main impressions left upon me after knowing Mr. Edison for many years is the marvellous accuracy of his guesses. He will see the general nature of a result long before it can be reached by mathematical calculation. His greatness was always to be clearly seen when difficulties arose. They always made him cheerful, and started him thinking; and very soon would come a line of suggestions which would not end until the difficulty was met and overcome, or found insurmountable. I have often felt that Mr. Edison got himself purposely into trouble by premature publications and otherwise, so that he would have a full incentive to get himself out of the trouble."

This chapter may well end with a statement from Mr. Jehl, shrewd and observant, as a participator in all the early work of the development of the Edison lighting system: "Those who were gathered around him in the old Menlo Park laboratory enjoyed his confidence, and he theirs. Nor was this confidence ever abused. He was respected with a respect which only great men can obtain, and he never showed by any word or act that he was their employer in a sense that would hurt the feelings, as is often the case in the ordinary course of business life. He conversed, argued, and disputed with us all as if he were a colleague on the same footing. It was his winning ways and manners that attached us all so loyally to his side, and made us ever ready with a boundless devotion to execute any request or desire." Thus does a great magnet, run through a heap of sand and filings, exert its lines of force and attract irresistibly to itself the iron and steel particles that are its affinity, and having sifted them out, leaving the useless dust behind, hold them to itself with responsive tenacity.

A WORLD-HUNT FOR FILAMENT MATERIAL