Professor Henry And His Philosophical Toys


Henry’s “trough,” and the way it was connected to the circuit, accounted for the success of this experiment. The trough consisted of a large number of zinc and copper plates in a dilute acid solution. This “intensity” battery produced high voltage or, as Henry put it, would give current enough “projectile force” to send it through a long wire. In addition Henry realized that the battery and the circuit could be arranged in different ways to produce different effects. Thus while the intensity circuit (which we now call a “series” circuit) was necessary to allow a signal to pass through a long wire, Henry used the “quantity” (or “parallel”) circuit to achieve the greatest lifting capacity in his electromagnets. In effect he had anticipated Ohm’s law by matching the resistance of the battery to that of the circuit.

The discovery that electricity could be sent through a long wire was perhaps the most important finding leading to long-distance telegraphy. As Francis O. J. (“Fog”) Smith, one of Morse’s original partners, put it: “Barlow used the quantity battery and Morse used the quantity battery and neither could succeed.” In 1848, in the case Morse v. O’Reilly (O’Reilly was another “inventor” of the telegraph), Morse admitted that Henry had shown that electricity could be transmitted over long distances, but said that he had always “assumed this truth.” This assumption, however, got him at the time no further than Barlow, who had reached the opposite conclusion.

In any event, Henry wasted no time in putting his discovery to dramatic use. In September of 1831 he strung more than a mile of wire around a third-story classroom. At one end he connected the wire to an intensity battery; at the other end the wire was wrapped around an iron bar to form an electromagnet. Close by, set on a pivot, was a permanent magnet. When a surge of current passed through the line activating the electromagnet, it repulsed one end of the permanent magnet. It swung away and clanged a bell.

This was the world’s first electromagnetic telegraph. Moreover, it had an audible signal—later European electromagnetic telegraphs used the old needle-deflection method of signalling. Admittedly it was more of a laboratory toy than a working system, but it was nevertheless in operation a dozen years before Morse demonstrated his Washington-Baltimore line in 1844. Henry made no attempt to patent it as an “invention.” Many years later, however, in one of those rare moments when he indicated that the American dream of fame and fortune had not left him totally untouched, he noted: “In this, I was perhaps too fastidious.”

Thus in a few years of part-time research Henry had invented the modern electromagnet, built the first electric motor, and assembled the first electromagnetic telegraph. He does not usually receive credit for the last two, nor does he for his next and greatest discovery, that of electromagnetic induction, which is generally attributed to Faraday.

Between 1823 and 1831 Faraday several times carried out experiments designed to produce electricity from magnetism. All failed—largely because of the faulty arrangement of his laboratory apparatus. (Faraday was more liable to distrust his own intuition than to doubt the efficiency of the Royal Society’s laboratory equipment.) But finally, in August of 1831, he succeeded.

The experiment was simple enough. Faraday wound two separate coils of wire on an iron ring. One was connected to a battery, the other to a galvanometer, a device for measuring current. Faraday noticed that when—and only when—the battery was being connected or disconnected with the coil, there was a surge of current in the other coil. He had finally found the secret, that a change in a magnetic field produces a current. He subsequently showed, in an even simpler experiment, that a current can be produced simply by thrusting a magnet in and out of a coil of wire. “Distinct conversion of magnetism into electricity,” Faraday wrote triumphantly in his diary. On November 24 he read a paper before the Royal Society, “On the Evolution of Electricity from Magnetism.” And then, after building a working model of an electric generator, the English scientist dropped the search for any practical uses to which this knowledge could be put. “I have rather,” he wrote, “been desirous of discovering new facts … on magneto-electric induction, than of exalting the force of those already obtained.” This scientific credo, an echo of Henry’s approach to applied science, postponed the full development of the dynamo until the i88o’s. Like Henry, however, Faraday was not unaware of the implications of his discovery. When Mr. Gladstone, the Prime Minister, once asked him whether his research had any practical value Faraday replied, “Why sir, you will soon be able to tax it.”

Henry did not hear of Faraday’s achievement until the following April. He made the best of things, dashing off an article for the American Journal of Science and Arts full of excuses for his delay. “I was … accidentally interrupted,” he wrote. “Before having any knowledge of the method given in the above account [Faraday’s], I had succeeded in producing electrical effects in the following manner …”