After giving up the subject of electrical discharges in gases I looked around for another problem of research which I could manage with my meagre laboratory facilities. Rowland had found distortions in an alternating current when that current was magnetizing iron in electrical power apparatus. This distortion consisted of the addition of higher harmonics to the normal harmonic changes in the current. This reminded me of harmonics in musical instruments and in the human voice. Helmholtz was the first to analyze the vowels in human speech by studying the harmonics which they contained. The vowel o, for instance, sung at a given pitch, contains in addition to its fundamental pitch—say one hundred vibrations per second—other vibrations the frequencies of which are integral multiples of one hundred, that is two, three, four, ... hundred vibrations per second. These higher vibrations are called harmonics of the fundamental. Helmholtz detected these harmonics by the employment of acoustical resonators; it was an epoch-making research. I proceeded to search for a similar procedure for the analysis of Rowland’s distorted alternating currents, and I found it. I constructed electrical resonators based upon dynamical principles similar to those in the acoustical resonators employed by Helmholtz. These electrical resonators play a most important part in the radio art of to-day, and a few words regarding their operation seem desirable. In fact, there is to-day a cry from the Atlantic to the Pacific on the part of millions of people who wish to know what they are really doing when they are turning a knob on their radio-receiving sets in order to find the correct wave length for a certain broadcasting station. I am responsible for the operation, and I owe them an explanation of it.
The mass and form of an elastic body, say a tuning-fork, and its stiffness determine the pitch, the so-called frequency of vibration. When a periodically varying force, say a wave of sound, acts upon the tuning-fork, the maximum motion of the prongs will be produced when the pitch or frequency of the moving force is equal to the frequency of the tuning-fork. The two are said then to be in resonance, that is, the motion of the fork resonates to or synchronizes with the action of the force. Every elastic structure has a frequency of its own. The column of air in an organ-pipe has a frequency of its own; so has the string of a piano. One can excite the motion of each by singing a note of the same frequency; a note of a considerably different frequency excites practically no motion at all. Acoustical resonance phenomena are too well known to need here any further comment. There are also electrical resonance phenomena very similar to those of acoustical resonance. If you understand one of them there is no difficulty in understanding the other.
If an electrical conductor, say a copper wire, is coiled up so as to form a coil of many turns, and its terminals are connected to a condenser, that is to conducting plates which are separated from each other by insulating material, then the motion of electricity in that conducting circuit is subject to the same laws as the motion of the prongs of a tuning-fork. Every motion, whether of electricity or of matter, is determined completely by the force which produces the motion, and by the forces with which the moving object reacts against the motion. If the law of action of these several forces is the same in the case of moving matter as in the case of moving electricity, then their motions also will be the same. The moving forces are called the action and the opposing forces are called the reaction, and Newton’s third law of motion says: Action is equal to the opposing reaction. I always considered this the most fundamental law in all physical sciences. It is applicable to all motions no matter what the thing is which moves, whether ponderable matter or imponderable electricity. Twenty-six years ago a student of mine, Albert R. Gallatin, brother of the present park commissioner of New York, presented a large induction coil to the electrical laboratory at Columbia College in recognition of my services to him, because, he said, this formulation of the fundamental law in the electrical science, which I have just given, made everything very clear to him. This was most encouraging to a young professor, and it goes without saying that ever since that time he and I have been warm friends. He is a banker and I am still a professor, but the interest in the fundamental principles in physical sciences are a strong bond of union between us.
The electrical force which moves the electricity in the circuit, just described, experiences two principal reactions. One reaction is due to the lines of electrical force which, attached to the electrical charge on the condenser plates, are crowded into the insulating space between these plates. This reaction corresponds to the elastic reaction of the prongs of the tuning-fork, and follows the same law. In the case of the tuning-fork the elastic reaction is proportional to the displacement of the prongs from their normal position; in the electrical case the reacting force is proportional to the electrical charges which have been pulled apart, the negative from the positive, and driven to the plates of the condenser. Call this separation electrical displacement, and the law can be given the same form as above, namely: The reacting force is proportional to the electrical displacement. The greater the distance between the plates, and the smaller their surface, the greater is the reaction for a given electrical displacement. By varying these two quantities we can vary the electrical yielding, the so-called capacity, of the electrical condenser. This is what you do when you turn the knob and vary the capacity of the condenser in your receiving set.
The moving prongs have a momentum, and a change in the momentum opposes a reacting force, the so-called inertia reaction, which is equal to the rate of this change. This was discovered by Galileo over three hundred years ago. We experience the operation of this law every time we bump against a moving object. The Irish sailor who, after describing the accident which made him fall from the mast, assured his friends that it was not the fall which hurt him but the sudden stop, appreciated fully the reacting force due to a rapid change of momentum. Every boy and girl in the public schools should know Galileo’s fundamental law, and they would know it if by a few simple experiments it were taught to them. But how many teachers really teach it? How many of my readers really know that law? Just think of it, what an impeachment it is of our modern system of education to have so many intelligent men and women, boys and girls, ignorant of so fundamental a law as that which Galileo discovered so long ago!
The moving electricity has a momentum. The magnetic lines of force produced by this motion are a measure of this momentum. Their change is opposed by a reacting force equal to the rate of this change. This was discovered by Faraday nearly a hundred years ago. The larger the number of turns in the coil of wire the larger will be the momentum for a given electrical motion, that is, for a given electrical current. But how can anybody understand very clearly this beautiful law, discovered by Faraday, who does not understand Galileo’s simpler discovery? The fact that electricity, just like matter, has inertia, and that both obey the same law of inertia, is one of the most beautiful discoveries in science. Whenever I thought that so many intelligent and cultured people knew nothing about it I rebelled against the educational system of modern civilization.
The motion of electricity in the conductor described above overcomes reacting forces which follow the same laws as the motion of the elastic prongs of the tuning-fork. The motion of one has, therefore, an analogy in the motion of the other. In an electrical circuit having a coil and a condenser the moving electricity has a definite inertia and a definite electrical stiffness; hence it will have a definite pitch or frequency for its vibratory motion, just like a tuning-fork; it will act as a resonator. It is obvious, therefore, that an electrical resonator, the pitch of which can be adjusted by adjusting its coil or its condenser or both, is a perfect parallel to the acoustical resonator. By means of an electrical resonator of this kind, having an adjustable coil and an adjustable condenser, I succeeded in detecting every one of the harmonics in Rowland’s distorted alternating currents, in the same manner in which Helmholtz detected the harmonics in the vowel sounds, but with much greater ease, because the pitch of an electrical resonator can be very easily and accurately changed by adjusting its coil and condenser. There are millions of people to-day who are doing that very thing when they are turning the knobs on their radio receiving sets, adjusting them to the wave-length of the transmitting station. The expression, “adjusting them to the pitch or frequency of the transmitting station,” is much better, because it reminds the operator of the analogy existing between acoustical and electrical resonance. The procedure was inaugurated thirty years ago in the “cowshed” of old Columbia College. I called it “electrical tuning” and the name has stuck to it down to the present time. The word “tuning” was suggested by the operation which the Serbian bagpiper performs when he tunes up his bagpipes, which I watched with a lively interest in my boyhood days. Those early impressions had made acoustical and electrical resonance appear to me later as obvious things.
The results of this research were published in the American Journal of Science and also in the Transactions of the American Institute of Electrical Engineers for 1894. They, I was told, had never been anticipated; and they confirmed fully Rowland’s views concerning the magnetic reaction of iron when subjected to the magnetic action of an alternating current. When Helmholtz visited this country in 1893, I showed him my electrical resonators and the research which I was conducting with their assistance. He was quite impressed by the striking similarity between his acoustical resonance analysis and my electrical resonance analysis, and urged me to push on the work and repeat his early experiments in acoustical resonance, because my electrical method was much more convenient than his acoustical method.
Helmholtz was always interested in the analysis as well as in the synthesis of vibrations corresponding to articulate speech. The telephone and the phonograph were two inventions which always enjoyed his admiring attention. During his visit in America he looked forward with much pleasure to meeting Graham Bell and Edison. The simplicity of their inventions astonished him, because one would hardly have expected that a simple disk could vibrate so as to reproduce faithfully all the complex vibrations which are necessary for articulation. He spent a Sunday afternoon as my guest at Monmouth Beach and in the course of conversation I told him what impression the telephone had made upon me when I first listened through it. It happened during the period when I was serving my apprenticeship as greenhorn, and when I was trying hard to master the articulation of the English language. The telephone plate repeated perfectly everything spoken at the other end, and I said to myself: “These Americans are too clever for me; they can make a plain steel plate articulate much better than I can ever expect to do it with all my speaking organs. I had better return to Idvor and become a herdsman again.” Helmholtz laughed heartily and assured me that the articulating telephone plate had made a similar impression upon him, although he had spent several years of his life studying the theory of articulation. “The phonograph disk is just as clever as the telephone disk,” said Helmholtz, “perhaps even more so, because it has to dig hard while it is busily talking.”
My scientific friends in New York saw in the construction of my electrical resonator and in its employment for selective detection of alternating currents of definite frequency a very suitable means for practising harmonic telegraphy, first suggested by Graham Bell, the inventor of the telephone. They finally persuaded me to apply for a patent and I did so. I often regretted it, because it involved me in a most expensive and otherwise annoying legal contest. Two other inventors had applied for a patent on the same invention. One of them was an American, and the other a French inventor, and each of them was backed by a powerful industrial corporation. A college professor with a salary of two thousand five hundred dollars per annum cannot stand a long legal contest when opposed by two powerful corporations; but it is a curious psychological fact that when one’s claim to an invention is disputed he will fight for it just as a tigress would fight for her cub. The fight lasted nearly eight years and I won it. I was declared to be the inventor, and the patent was granted to me. But a patent is a piece of paper worth nothing until somebody needs the invention. I waited a long time before that somebody came, and when he finally showed up I had almost forgotten that I had made the invention. In the meantime I had nothing but a piece of paper for all my pains, which nearly wrecked me financially.