His delicate constitution seemed unable to bear the continued strain of study and teaching, and at twenty-six, through the friendship of an influential judge, Henry received the appointment of engineer in the survey of a road between the Hudson River and Lake Erie, a distance of about three hundred miles. This gave him out-of-door life, which he needed, and, though much of his work was done in winter, in deep snow, making his way through dense forests, he entirely regained his health, and gave such excellent satisfaction that he was asked to construct a canal in Ohio, and assist in a mining enterprise in Mexico. Both of these he refused, accepting the chair of Mathematics and Natural Philosophy in the Albany Academy, at the urgent solicitation of his friend, Dr. Beck.
Elected in the spring, and not entering upon his work till autumn, he spent the intervening months in geological exploration in New York State. Every hour was occupied. He had commenced solid study in earnest, as he had told the members of the "Rostrum" he should do.
Having entered upon his profession, he taught mathematics seven hours daily. But he found time to make experiments in natural philosophy. The first paper which he brought before the Albany Institute was, "On the Chemical and Mechanical Effects of Steam: with Experiments designed to illustrate the Great Reduction of Temperature in Steam of High Elasticity when suddenly expanded."
His next published scientific paper was, "On the Production of Cold by the Rarefaction of Air: accompanied by Experiments." "One of these experiments most strikingly illustrated the great reduction of temperature which takes place on the sudden rarefaction of condensed air. Half a pint of water was poured into a strong copper vessel of a globular form, and having a capacity of five gallons; a tube of one-fourth of an inch caliber, with a number of holes near the lower end, and a stop-cock attached to the other extremity, was firmly screwed into the neck of the vessel; the lower end of the tube dipped into the water, but a number of holes were above the surface of the liquid, so that a jet of air mingled with water might be thrown from the fountain.
"The apparatus was then charged with condensed air, by means of a powerful condensing pump, until the pressure was estimated at nine atmospheres. During the condensation, the vessel became sensibly warm. After suffering the apparatus to cool down to the temperature of the room, the stop-cock was opened: the air rushed out with great violence, carrying with it a quantity of water, which was instantly converted into snow. After a few seconds, the tube became filled with ice, which almost entirely stopped the current of air. The neck of the vessel was then partially unscrewed, so as to allow the condensed air to rush out around the sides of the screw; in this state the temperature of the whole interior atmosphere was so much reduced as to freeze the remaining water in the vessel."
Other pamphlets followed this publication, but in 1831 a notable paper in the "American Journal of Science and the Arts" brought Henry's name to the front line of discoverers in electro-magnetism. Sturgeon made the first electro-magnet; Henry made the electro-magnet what it is.
Says W. B. Taylor, in an address before the "Philosophical Society of Washington:" "The electro-magnet figured and described by Sturgeon consisted of a small bar or stout iron wire bent into a
or horse-shoe form, having a copper wire wound loosely around it in eighteen turns, with the ends of the wire dipping into mercury-cups connected with the respective poles of a battery having one hundred and thirty square inches of active surface."
Henry improved upon this in 1828, but in March of 1829 he exhibited before the Institute a somewhat larger magnet. "A round piece of iron about one-quarter of an inch in diameter was bent into the usual form of a horse-shoe, and, instead of loosely coiling around it a few feet of wire as is usually described, it was tightly wound with thirty-five feet of wire covered with silk, so as to form about four hundred turns; a pair of small galvanic plates, which could be dipped into a tumbler of diluted acid, was soldered to the ends of the wire, and the whole mounted on a stand. With these small plates, the horse-shoe became much more powerfully magnetic than another of the same size and wound in the usual manner, by the application of a battery composed of twenty-eight plates of copper and zinc each eight inches square."