Philipp Reis: Inventor of the Telephone / A Biographical Sketch

by Silvanus P. Thompson

Two decades ago we had not yet gone beyond the first attempts to give signals at a great distance by the aid of electricity. Since then telegraphy has attained such a completeness, and the telegraph wire has reached such a universal extension, that there seems little left for even the boldest wish to desire.

Now there crops up a first serious research to reproduce tones at any desired distance by the aid of electricity. This first experiment which has been crowned with some success, has been made by the teacher of Natural Science at Friedrichsdorf, not far from Frankfort-on-the-Main, Herr Ph. Reis, and has been repeated in the Auditorium of the Physical Society in Frankfort, before numerous assembled members on the 26th of October, 1861. He caused melodies to be sung not very loudly into one part of his apparatus, which was placed in a building (the Bürger-Hospital), about 300 feet distant, with closed windows and doors. These same melodies were audible to the members in the meeting-hall by means of the second part of the apparatus. These wonderful results were attained with the following simple pieces of apparatus. A little light box, a sort of hollow cube of wood, has a large opening at its front side, and a small one at the back on the opposite side. The latter is closed with a very fine membrane (of pig’s smaller-intestine) which is strained stiff. A narrow springy strip of platinum foil, fixed at its outer part to the wood, touches the membrane at its middle; a second platinum strip is fastened by one of its ends to the wood at another spot, and bears at its other end a fine horizontal spike, which touches the other little platinum strip where it lies upon the membrane.

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.

That the tone is made audible at a distance by the electric agitations, and not by direct conduction of the sound-waves through the wires is proved in the most evident way of all, because one instantly hears no more the tone through the spiral when a good short circuit is made, as, for example, by laying upon the two wires which conduct the electricity a strip of sheet metal right in front of the spiral.

The reproduced tones are, of course, somewhat weaker than the original ones, but the number of vibrations is similar. If thus the reproduction [of tones] in exactly similar height and depth is easily attained, it is however difficult for our ear, amidst the always smaller vibrations, to which the diminished strength of the tone is due, to evaluate exactly the magnitude of the vibrations. But the character of the tone depends upon the number of variations of amplitude (Anschwellungen), that is to say, depends upon whether, for example, in the tones which have similar pitch and therefore a similar number of waves per second, the fourth, sixth, eighth, tenth, or sixteenth wave is stronger than the others. For physicists have shown that an elastic spring is set in vibration by the thrust of the teeth of a cog-wheel; the first vibration is the greatest, all those that follow being less. If there comes, before the spring comes to rest, a fresh thrust from a cog, then the next vibration is again equal to the greatest first vibration without the spring making any more vibrations on that account; and by this means vowel-tones may be artificially produced.