As is known, tones arise from rarefactions and condensations of the air following quickly after one another. If these motions of the air, known as waves, strike upon the thin membrane, they press it against the little plate of platinum with which it is in contact, and immediately let it vibrate back again into the hollow cube (or so-called artificial ear): they act so that the membrane now takes a form hollowed toward the cube, now bulged toward the outside. The little plate of platinum touching it thereby acquires a vibrating motion, so that it now is pressed against the spike of the second [platinum plate], now leaves the same.
If now one little plate of platinum be united by a wire with one pole of a voltaic battery, and the electricity be led, by a wire fastened to the other pole of the battery, to any desired distance; there carried through a spiral, about six inches long, made of a six-fold winding of very thin covered copper wire; thence led back to the second platinum strip on the wooden cube through a second insulated wire; then at every vibration of the membrane an interruption in the current of electricity takes place because the platinum point no longer touches the other little strip of platinum. Through the hollow of the wire-spiral there is stuck a thin iron wire (a strong knitting-needle), which is ten inches long, and which rests upon two bridges of a sounding-board by its ends which project on both sides about two inches out of the spiral.
It is known[21] that if an electric current be led through a spiral which surrounds an iron rod in the manner described, at every interruption of the same a tone is audible arising from the vibration of the rod. If the closings and interruptions of the circuit follow one another relatively slowly, then there is produced by the changes of position of the molecules of the rod, evoked by the electricity, a tone,--the so-called longitudinal tone of the rod,--which is dependent upon the length and stoutness of the rod. But if the closings and interruptions of the electric current in the spiral follow one another more rapidly than the vibrations of the smallest particles of the iron rod,[22] which vibrations are determined by its elasticity, then these particles cannot complete their paths, receive new impacts, their vibrations become smaller, but quicker, and follow one another as frequently as the interruptions. The iron rod then no longer gives its longitudinal tone, but a tone, which is higher according as the interruptions are more frequent in the given time, or lower, as they are less frequent. It is known that the height and depth of tones depends only on the number of air-waves which follow one another in a second. We have seen above that by this is determined the number of interruptions of the electric current of our apparatus by means of the membrane and the platinum strip. The iron wire must therefore give out the tone in the same height or depth as that which struck the membrane. Now since a very far leading of the electricity makes it suffer scarcely any weakening in proper apparatus, it is intelligible that one can make the tone which acts on the membrane at one place audible, by means of the iron rod, at any desired distance.