Note 18 — APPLICATIONS OF THE TELEPHONE.

Helmholtz’s resonator is based upon the principle that a volume of air contained in an open vase emits a certain note when placed in vibration, and that the height of the note depends on the size of the vase and of its opening. Helmholtz makes use of a globe with a large opening on one side and a small one on the other, and the small one is applied to the ear. If a series of musical notes take place in the air, the one which is in harmony with the fundamental note of the globe is intensified, and can be distinguished from the rest. The same effect takes place when, on singing to a piano accompaniment, some strings are heard to vibrate more strongly than others, namely, those which vibrate in unison with the sounds emitted. The resonators are made in various ways; those most generally used are cases of different lengths which also serve as sounding-boxes.