The tone variator is a closed-tube sound producing instrument. It consists of a cylindrical brass bottle, the bottom of which is a piston controlled by a delicate cog-wheel mechanism. By means of this the depth of the bottle may be readily changed, thus varying the pitch. The sound is produced by a stream of air directed across the mouth of the bottle through a tube set at the proper angle. A pointer attached to the piston’s setting gear indicates the pitch in terms of vibrations per second
