of his mind; and he hastened to submit it to experimental proofs and numerical verifications, which, in our day, are the only processes considered entirely demonstrative."

About this time Arago found that the conjunctive wire of the battery had the property of causing iron filings to arrange themselves around it in concentric rings. Guided by Ampère's discovery that a helix conducting a voltaic current had properties similar to those of a magnet, Arago inferred that these properties could be given to iron and steel by placing wires or bars of these substances in the interior of one of Ampère's helices. Experiment showed that his inference was correct. The same effects he obtained by passing electrical discharges from an ordinary frictional electrical machine, or from a Leyden jar, through a helix inclosing a steel needle.

In subsequent memoirs, exhibiting great philosophic acumen. and marked ability in the application of mathematical analysis to the elucidation of physical phenomena, Ampère developed the consequences of the general laws he had previously discovered.

In 1821, six years before Henry began his work, Faraday-then thirty years of age, and as yet an assistant of Davy-published his first paper on electrical research. In this he shows that a wire conveying an electrical current can be made to rotate around the pole of a magnet. He then reverses the action, and holding the wire at rest makes the magnetic pole rotate around the wire. These phenomena were shown by Ampère to be entirely conformable to his hypothesis of the electro-dynamic nature of a magnet. While Ampère, in 1820, was pursuing his researches, Schweigger, of Halle, invented his galvanometer. This he formed by inclosing a suspended magnetic needle in several turns and layers of an insulated wire. This instrument excited a powerful influence in electrical researches, and the contemplation of its action led Henry to make his first trials as an original experimenter.

The history of another research is now in order as bearing directly on one of Henry's investigations-and one which he ever regarded with considerable pride. In 1827, Savary began experiments on the magnetizing actions of the discharge of the Leyden jar on steel needles. These needles, of various lengths, diameters, and degrees of hardness, were placed at right angles to the wire conveying the electric discharge. They were also put in the interior of Ampère's helices, after the manner of Arago's original experi

ments. The phenomena thus observed were found to be of the most complex characters. It was found that the direction of the polarity in the needle and the intensity of its magnetization depended on its distance from the wire, on the diameter of the needle, on the potentiality of the discharge, and on the resistance of the wire through which the discharge took place. Similar phenomena were observed when the needles were placed in one of Ampère's helices, through which the discharge was thrown. After a long and tedious research Savary concluded that these facts could only be explained by the supposition that the discharge of a Leyden jar was not continuous, but consisted of a series of rebounds or reflections to and from the two coatings of the jar. In 1842, Henry, apparently ignorant of this research of Savary, went over the same ground, and arrived independently at the same inference which Savary had formed fifteen years before-an inference directly confirmed by the experiments of Feddersen, who, in 1862, got the life history of the electric spark of the Leyden jar by photographing its image reflected from a concave mirror revolving 800 to 1,000 times in a second.

Two years previous to Savary's work, i. e., in 1825, William Sturgeon, of Woolwich, England, improved on Arago's experiment of magnetizing steel and iron with the voltaic current. Sturgeon's improvement consisted in bending the straight rods used by Arago into U-shaped pieces, and then, coating them with shellac varnish, he wound them with uncovered copper wire. The coils of the wire were separated, so that the current flowed through the wire around the surface of the iron. This magnet, in proportion to its weight, was the most powerful made up to this date. It certainly did not require great mental effort or acumen on the part of Sturgeon to bend a straight bar magnet into the then common U form of the permanent steel magnet known as the horse-shoe magnet; yet his experiments with this magnet mark an important point of departure in electric science, and evidently led Henry to his first and most important scientific research.

I have now given as much of the history of electrical research as is requisite to the understanding of Henry's position as a discoverer in this branch of knowledge when, in 1827, he began to make original experiments in electricity.

As with many other men of originality, Henry's first essays were in the direction of improving the means of illustrating well

His first paper, of

established scientific facts and principles. October, 1827, is interesting because it was his first. In it he improves on the usual apparatus which had been used by Ampère and others to show electro-dynamic actions, by employing several turns of insulated wire, instead of one, as had previously been the practice. Thus, for example, to show the directive action of the earth's magnetism on a freely-moving closed circuit, Henry covered copper wire with silk and then made out of it a ring about twenty inches in diameter, formed of several turns of the wire. The extremities of this wire were soldered to zinc and copper plates. The coil was then suspended by silk filaments. On plunging the metal plates into a glass of dilute acid the ring rotated around its point of suspension till its plane took a permanent position at right angles to the magnetic meridian. By a similar arrangement of two concentric coils, one suspended within the other, he neatly showed the mutual actions of voltaic currents flowing in the same or in opposite directions; which facts are the foundations of Ampère's celebrated law.

We now reach a period when Henry appears as a discoverer, and truly one of no mean order. As I remember his narration to me in the year 1859, it was as follows: He said that one evening he was sitting in his study in Albany with a friend, when, after a few moments of reverie, he arose and exclaimed, "Tomorrow I shall make a famous experiment!" For several months he had been brooding over Ampère's electro-dynamic theory of magnetism, and he was then deeply interested in the phenomena of the development of magnetism in soft iron as shown in the experiments of Arago and Sturgeon. At the moment he had arisen from his chair it had occurred to him that the requirements of the theory of Ampère were not fulfilled in the electro-magnets of Arago and of Sturgeon, but that he could get those conditions which the theory required by covering the enveloping wire with a non-conductor like silk, and then wrapping it closely around the soft iron bar in several layers; for the successive layers of wire coiling first in one direction and then in the other would tend to produce a resultant action of the current at right angles to the axis of the bar; and furthermore, the great number of convolutions thus obtained would act on a greater number of molecules of the bar and therefore exalt its magnetism. "When this conception," said Henry, "came into my brain I was so pleased with

it that I could not help rising to my feet and giving it my hearty approbation."

Henry did go to work the next day, and to his great delight and encouragement discoveries of the highest interest and importance revealed themselves to him week after week. When he had finished his newly conceived magnet he found that it supported several times more weight than did Sturgeon's magnet of equal size and weight. This was his first original discovery.

I will now give, as far as possible, Henry's own words in narrating the subsequent investigations of these very interesting phenomena:

"The maximum effect, however, with this arrangement and a single battery was not yet obtained. After a certain length of wire had been coiled upon the iron the power diminished with a further increase of the number of turns. This was due to the increased resistance which the longer wire offered to the conduction of electricity. Two methods of improvement, therefore, suggested themselves. The first consisted not in increasing the length of coil, but in using a number of separate coils on the same piece of iron. By this arrangement the resistance to the conduction of the electricity was diminished and a greater quantity made to circulate around the iron from the same battery. The second method of producing a similar result consisted in increasing the number of elements of the battery, or, in other words, the projectile force of the electricity, which enabled it to pass through an increased number of turns of wire, and thus, by increasing the length of the wire, to develop the maximum power of the iron.

"To test these principles on a larger scale, an experimental magnet was constructed. In this a number of compound helices were placed on the same bar, their ends left projecting, and so numbered that they could be all united into one long helix, or variously combined in sets of lesser length.

"From a series of experiments with this and other magnets it was proved that, in order to produce the greatest amount of magnetism from a battery of a single cup, a number of helices are required; but when a compound battery is used, then one long wire must be employed, making many turns around the iron; the length of wire, and consequently the number of turns, being commensurate with the projectile power of the battery.

"In describing the results of my experiments the terms intensity

and quantity magnets were introduced to avoid circumlocution, and were intended to be used merely in a technical sense. By the intensity magnet I designated a piece of soft iron so surrounded with wire that its magnetic power could be called into operation by an intensity battery; and by a quantity magnet a piece of iron so surrounded by a number of separate coils that its magnetism could be fully developed by a quantity battery.

"I was," says Henry, "the first to point out this connection of the two kinds of the battery with the two forms of the magnet, in my paper in Silliman's Journal, January, 1831, and clearly to state that when magnetism was to be developed by means of a compound battery one long coil was to be employed, and when the maximum effect was to be produced by a single battery, a number of strands were to be used."

Here is Henry's description of one of his quantity magnets: "A bar of iron 21 inches long and 2 inches square with rounded corners was bent into a U form, having legs about nine inches long. This bar weighed twenty-one pounds. Its armature was formed of a piece of a similar bar and weighed seven pounds. Nine coils of copper bell-wire, each sixty feet in length, were wrapped in sections on the iron. These coils were not continued around the whole length of the bar, but cach strand of wire, according to the principle before mentioned, occupied about two inches, and was coiled several times backward and forward over itself; the several ends of the wire were left projecting and all numbered, so that the first and last end of each strand might be readily distinguished. In this manner was formed an experimental magnet on a larger scale, with which several combinations of wire could be made by merely uniting the different projecting ends. Thus, if the second end of the first wire be soldered to the first end of the second wire, and so on through all the series, the whole will form a continued coil of one long wire. By soldering different ends the whole may be formed into a double coil of half the length, or into a triple coil of one-third the length, etc. The horse-shoe was suspended in a rectangular wooden frame 3 feet 9 inches high and 20 inches wide.

"In order to ascertain the effect of a very small galvanic element on this large quantity of iron, a pair of plates, exactly one square inch, was attached to all the wires: the weight lifted was eighty-five pounds. To find out the greatest supporting power of the magnet, with all of its nine coils in a circuit, a small battery