The simplest form of the Vibrator is shown in the Jew’s Harp, as it is popularly called, though it is not a harp, and has nothing to do with Jews.
The word is really a mistaken pronunciation of “jaw’s harp,” because the instrument is held against the teeth, while its tongue is vibrated by strokes of the finger. These vibrations affect the air within the mouth, and, by expanding or contracting the mouth, the sound is lowered or raised according to the laws of Acoustics. Of course, the range of notes is very small, being limited to those of the common chord, and even they being attainable only by a practised performer. Very good effects, however, have been produced by means of a series of Jew’s Harps, set to different tones by loading the end of the tongue with sealing-wax or similar substances.
An apparatus constructed on the same principle is to be found in the vocal organs of the male Cicada. If one of these insects be examined on the lower surface, two curious and nearly circular flaps will be seen, just at the junction of the thorax with the abdomen. It is by the action of these two little vibrators that the insect is able to produce a sound so loud, that in calm weather it may be heard at the distance of a mile.
The accompanying illustration is, in fact, a sort of chart as to the vibration of sound.
On the right is shown the Æolian Harp, with its upper lid raised, so as to show the structure of the strings. These are all tuned to the same note, the present D being generally accepted as being most free from false tuning, and less liable for the errors of “temperament.” Several of the strings are an octave lower than the others, but the tonic is always the same.
The instrument is placed in a current of air, generally in a window, with the sash let down upon it, and the air-currents set the strings vibrating in a most wonderful manner.
There is no need for human fingers to touch them, but they automatically divide themselves into the component parts of the common chord, and produce octaves, fifths, and thirds ad infinitum.
On the left hand of the same illustration is exhibited a string of the same length and tension, vibrating in two different ways. The upper figure shows it divided into three portions, each of which gives the fifth above the tonic, and all of which, when sounding simultaneously, give a fulness and richness to the tone which could only be attained otherwise by three distinct instruments. All players of stringed instruments know how invaluable are these harmonics, without which many passages of well-known music could not be played, and which are produced by “damping,” and not pressing the strings.
So, if the string be lightly touched, or damped at the crossing portion at either end, the result will be that the string divides itself into three portions, and all three resound simultaneously.