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.