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EULOGY ON Joseph Henry. By ALFRED M. Mayer. At the meeting of this Association in 1878 a committee, composed of Professors Baird, Newcomb and myself, was appointed to prepare a culogy on our revered and lamented colleague and former president, Joseph IIENTY.

This, I will not say labor, but duty of affection, has devolved on me alone. I would that the other members of tlis committee had laid before you their tributes to his memory, because for years they had been closely associated with him in his social and professional life in Washington. Yet, while Professor Henry hail been the friend of their manhood, lie was the friend of my boyhood; and during twenty-five years he ever regarded me - as was liis wont to say-with a "paternal interest." To his disinterested kindness and wise counsels is due much, very much, of whatever usefulness there is in me. Hence, I have said that it is a duty of affection for me to speak to you about one who was my beloved friend.

I shall not, however, attempt a biography of Joseph IIenry, nor shall I speak of his administrative life as Director of the Smithsonian Institution, for this is known and valued by the whole world. His best eulogy is an account of his discoveries, for a man of science, as such, lives in what he has done, and not in what he has said ; nor will he be remembered in what he proposed to do. I will, therefore, with your permission, confine myself chiefly to Henry as the Discoverer; and I do this the more willingly because I am familiar with his researches, and also because Professor Henry, from time to time, took pleasure in giving me accounts of those mental conceptions which preceded his work, leid liim to it, and guided him in it.

To appreciate a discoverer rightly we should not look at his work from our time, but go back and regard it from his time; we should not judge his work in the fulness of the light of present knowledge, but in the dim twilight which alone illuminated him to then unknown — but now well-known — facts and laws. I will, therefore, endeavor first to present you with a clear, but necessarily very concise, view of the state of our knowledge of electricity when llenry began his original researches in that branch of science, A. A. A. S., VOL. XXIX.



and then point out the value of his discoveries by showing wliat they added to knowledge, and how they instigated and influenced tlie discoveries and inventions of other men.

Henry began his electrical researches at the age of twenty-eight, in the year 1827, while he was professor of mathematics and natural philosophy in the Albany Academy. At these he continuously worked till 1832, when, at the age of thirty-three, he moved to the College of New Jersey (Princeton). After a year's break in his work, caused by the preparation of his course of lectures for the college, he is again at original research, and continues his contributions to electrical discoveries till 1842. Thus, during fourteen years, while between the ages of twenty-eight and forty-three, he was a constant and fertile worker. What he did in these years will be given after a review of what had been already discovered up to the time he began his original experiments.

Through the labors of Gilbert, Boyle, Otto von Guericke, Newton, Wall, Gray, Franklin, Æpinus, and Volta, it had been discovered that all matter could be electrically excited, and that bodies differed greatly in permitting the diffusion of electricity over their surfaces; the facts of electric attraction and repulsion, of electric induction, the action of points, and the identity of lightning and electricity had been discovered; and these facts had been explained and bound together in a body of doctrine by the hypothesis of Dufay or by that of Franklin ; while Coulomb and Poisson, in a series of beautiful experimental and mathematical labors, had given us the knowledge of the laws of the actions at a distance of electric attraction and repulsion, and had shown in what manner electricity diffuses itself over conductors of various forms.

About 1820, men of science spoke of electrical knowledge as almost complete. The mathematical consequences of the laws discovered by Coulomb and others liaving been, they thought, fully developedl; electricity was liardly to be regarded as an experimental science, but henceforth might be grouped with me. chanics. Such opinion was so general that Faraday (in 1831), when he began his ever remarkable series of discoveries, was influenced by this prevailing feeling to style his papers Experimental Researches in Electricity."

It seemed almost impossible that any discovery could again give an impulse to electrical studies equal to that produced by the

brilliant and most fertile researches of Volta ; yet to the universal surprise of the scientific world this happened. In the winter of 1819, Oersted announced that lie had at last discovered a correlation of actions between clectricity and magnetism in his celebrated experiment of the deflection of a magnet athwart the conjunctive wire of a battery when the latter was laid parallel to the direction of the magnet.

During the month of July, 1820, the news of Oersted's discovo ery reached Paris. It at once profoundly excited the ever active and versatile mind of Ampère. This man, already celebrated as a mathematician, was now destined to show greater genius as an esperimenter. lle at once began a series of researches in the field opened by the discovery of Oersted; and with astonishing rapidity reached results of such importance that they gained him the title of the Newton of electro-clynamics; and justly, for he did for this branch of science even more than Coulomb had previously done for electro-statics.

On the 18th of September, 1820, Ampère read his first paper on electro-dynamics, before the Academy of Sciences of Paris. In this he shows that the battery exerts an electro-magnetic action as well as its conjunctive wire, and he gives a rule by which one can readily predict the direction in which a magnet will be deflected by a voltaic current. He supposes a current to flow from the copper to the zinc plate of the battery; then, says he, if you imagine yourself at full length and facing the wire, the current entering your heels and passing out at your head, the north pole of the magnet is always deflected toward your left hand. In the same paper, he says that he will soon experiment with spirals and helices of wire which, he predicts, will have the same properties as magnets as long as a current of electricity flows througlı them. He then gives his well-known hypothesis of the nature of a magnet. Ile says that if we assume a magnet to consist of an assemblage of minute currents of electricity whirling all with the same direction of rotation around the steel molecules and in planes at right angles to the axis of the bar, we shall have an hypothesis which will account for all the known properties of a magnet. Ampère constructed his spirals and helices, and to the astonishment of the scientific world made magnets formed only of spools of copper wire traversed by electric currents. We can readily

imagine the intense interest awakened by this discovery ; a discovery which caused Arago to exclaim : “What would Newton, Halley, Dufay, Apinus, Franklin, and Coulomb bave said if one had told them that the day would come when a navigator would be able to lay the course of his vessel without a magnetic needle and solely by means of electric currents?”

" For several weeks physicists of France and from abroad crowded Ampère's humble study in Rue Fossée Saint Victor, to see with astonishment a suspended loop of wire, in the circuit of a battery, take a definite position through the directive magnetic action of the earth.”

This hypothesis of Ampère had a powerful hold on Henry's mind; and, as I know that he used it as a guiding light in his researches, it may here be well to give Arago's account of how Ampère was led to its conception :

“Thanks to the profound researches of Ampère, the law which governs celestial movements, the law extended by Coulomb to the phenomena of electricity at rest or in tension, and then, though with less certainty, to magnetic phenomena, becomes one of the characteristic features of the powers exercised by electricity in motion. The general formula which gives the value of the mutual actions of the infinitely small elements of currents once understood, the determination of the combined actions of limited currents of different forms becomes a simple problem of integral analysis. Ampère did not fail to follow out these applications of his discoveries. Ile first tried to discover how a rectilinear current acts on a system of cireular closed currents, contained in planes perpendicular to the rectilinear current. The result of the calculation, confirmed by experiment, was that the planes of the circular currents would, supposing them movable, arrange themselves parallel to the rectilinear current. If like transverse currents pass over the whole length of a magnetic needle, the cross direction which, in the experiment of Oersted, seemed an inesplicable anomaly, would become a natural and necessary fact. Is it not then evident to all how memorable would that discovery be which should rigorously establish the fact that to magnetize a needle is to excite, to put in motion around each molecule of the steel, a small circular, electrical vortex? Ampère fully realized the wide reach of the ingenious generalization that had taken possession

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