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SCIENCE 



[Entered at the Posi-Offlce of New York, N.Y., as Second-Class Matter.] 



A WEEKLY NEWSPAPER OF ALL THE ARTS AND SCIENCES. 



Eighth Yeab. 

 Vol. XVI. No. 401. 



NEW YORK, October 10, 1890. 



Single Copies, Ten Cents. 

 3.50 Peb Yeab, in Advance. 



THE ELECTRO-MAGNET.' 



Introductory. 



Among the great inventions which have originated in the 

 lecture-room in which^ we are iriet are two of special interest 

 to electricians, — the application of gutta-percha to the pur- 

 pose of submarine telegraph-cables, and the electro-magnet. 

 This latter invention was first publicly described from the 

 very platform on which I stand, on May 23, 1835, by Wil- 

 liam Sturgeon, whose paper is to be found in the forty-third 

 volume of the "Transactions of the Society of Arts." For 

 this invention we may rightfully claim the very highest 

 place. Electrical engineering, the latest and most vigorous 

 offshoot of applied science, embraces many branches. The 

 dynamo for generating electric currents, the motor for 

 transforming their energy back into work, the arc-lamp, the 

 electric bell, the telephone, the recent electro-magnetic ma- 

 chinery for coal-mining, for the separation of ore, and many 

 other electro-mechanical contrivances, come witliin the pur- 

 view of the electrical engineer. In every one of these, and 

 in many more of the useful applications of electricity, the 

 central organ is an electro-magnet. By means of this simple 

 and familiar contrivance, — an iron core surrounded by a 

 copper-wire coil, — mechanical actions are produced at will, 

 at a distance, under control, by the agency of electric cur- 

 rents. These mechanical actions are known to vary with 

 the mass, form, and quality of the iron core, the quantity 

 and disposition of the copper wire wound upon it, the quan- 

 tity of electric current circulating around it, the form, 

 quality, and distance of the iron armature upon which it 

 acts. But the laws which govern the mechanical action in 

 relation to these various matters are by no means well 

 tnown ; and, indeed, several of them have long been a mat- 

 ter of dispute. Gradually, however, that which has been 

 vague and indeterminate becomes clear and precise. The 

 laws of the steady circulation of electric currents, at one 

 time altogether obscure, were cleared up by the discovery of 

 the famous law of Ohm. Their extension to the case of 

 rapidly interrupted currents, such as are used in telegraphic 

 working, was discovered by Helmholtz; while to Maxwell is 

 due their further extension to alternating, or, as they are 

 sometimes called, undulatory currents. All this was purely 

 electric work. But the law of the electro-magnet was still 

 undiscovered; the magnetic part of the pcoblem was still 

 buried in obscurity. The only exact reasoning about mag- 

 netism dealt with problems of another kind; it was couched 

 in language of a misleading character; for the practical 



' Lecture delivered Jan. 20, 1890, by Professor Silvanus P. Thompson, 

 before the Society of Arts, London. 



problems connected with the electro-magnet it was worse 

 than useless, — the doctrine of two magnetic fluids distributed 

 over the end surfaces of magnets, which, under the sanction 

 of the great names of Coulomb, of Poisson, and of Laplace, 

 had unfortunately become recognized as an accepted part of 

 science, along with the law of inverse squares. How greatly 

 the progress of electro-magnetic science has been impeded 

 and retarded by the weight of these great names, it is im- 

 possible now to gauge. We now know that for all purposes, 

 save only those whose value lies in the domain of abstract 

 mathematics, the doctrine of the two magnetic fluids is false 

 and misleading. We know that magnetism, so far from re- 

 siding on the end or surface of the magnet, is a property 

 resident throughout the mass; that the internal, not the 

 external, magnetization is the important fact to be consid- 

 ered; that the so-called free magnetism on the surface is, as 

 it were, an accidental phenomenon ; that the magnet is 

 really most highly magnetized at tliose parts where there is 

 least surface magnetization; finally, that the doctrine of 

 surface distribution of fluids is absolutely incompetent to 

 afford a basis of calculation such as is required by the elec- 

 trical engineer. He requires rules to enable him not only 

 to predict the lifting power of a given electro-magnet, but 

 also to guide him in designing and constructing electro- 

 magnets of special forms suitable for the various cases that 

 arise in his practice. He wants in one place a strong elec- 

 tro-magnet to hold on to its armature like a limpet to its 

 native rock; in another case he desires a magnet having a 

 very long range of attraction, and wants a rule to guide him 

 to the best design ; in another he wants a special form hav- 

 ing the most rapid action attainable; in yet another he must 

 sacrifice every thing else to attain maximum action with 

 minimum weight. Toward the solution of such practical 

 problems as these, the old theory of magnetism offered not 

 the slightest aid. Its array of mathematical symbols was a 

 mockery. It was as though an engiueer asking for rules to 

 enable him to design the cylinder and piston of an engine 

 were confronted with receipts how to estimate the cost of 

 painting it. 



Gradually, however, new light dawned. It became cus- 

 tomary, in spite of the mathematicians, to regard the mag- 

 netism of a magnet as something that traverses or circulates 

 around a definite path, flowing more freely through such 

 substances as iron than through other relatively non-magnetic 

 materials. Analogies between the flow of electricity in an 

 electrically conducting circuit, and the passage of magnetic 

 lines of force through circuits possessing magnetic conduc- 

 tivity, forced themselves upon the minds of experimenters, 

 and compelled a mode of thought quite other than the pre- 



