The tonograph is a device consisting of a hollow brass cylindrical section closed at one end and equipped with a hole. The cylinder is in all respects similar to that of the organs. Through bellows, operated through one's feet, the air flows through the cylindrical tube making a sound. Inside the cylinder there is a piston whose position is regulated by a thin rod and, as the position of the piston changes, the length of the cylinder also changes. As the plunger (and therefore the rod) changes, the instrument will generate a different sound. A graduated scale makes it possible to "measure" the intonation and inflection of human voice, by matching one's voice with the sound made by the device.