This experiment, then, was the germ of the microphone. For the metal ends of the wire in contact, he substituted carbon points, and obtained a much more sensitive arrangement. When one of the carbon pencils was lightly pressed against the other in a stable position, he found that the joint was sensitive to the slightest jar, and could transmit the voice when spoken to direct. Pursuing his researches further, he found that a loose and somewhat crazy metal structure, such as a pile of gold-chain or a framework of French nails, acted in a similar way, though not so powerfully as carbon. This material was found so sensitive, that a fly walking on the board supporting the microphone could be distinctly heard in the telephone, and each tap of its trunk upon the wood was said by one observer to resemble the ‘tramp of an elephant.’
The marvels of the microphone were published to the world in the early summer of the next year; and many useful applications followed. The most obvious was its use as a telephone transmitter; and as Professor Hughes had made a public gift of his invention, a great many telephone transmitters were based upon it. Edison, who had invented a carbon transmitter which bore some resemblance to the microphone, laid claim to having anticipated the invention; but the merit of the discovery remains with Professor Hughes.
It is through the help of the microphone that telephony has become so practical and so extensively adopted. The Blake transmitter, the Ader, and many others by which music and speech are now conveyed so many miles, are all varieties of the carbon microphone. In some churches, microphone transmitters are now applied to the pulpit, so that the sermon can be transmitted by telephone to invalid members who cannot leave home. At the Electrical Exhibitions of Paris, Vienna, and the Crystal Palace, the music of an entire opera was transmitted from the stage by wire to other buildings where great numbers of persons sat and listened to it. The transport of music and other sounds in no way directly connected with the wire, is frequently effected by what is termed induction or leading-in. Over and over again, persons listening into telephones for the purpose of hearing what a friend is saying, have heard the strains of this music—aside, communicated by induction from some neighbouring line to theirs. Not long ago, a telegraph clerk in Chicago was listening in a telephone early one morning, and to his surprise heard the croaking of frogs and the whistling of birds. The explanation of the phenomenon is, that a loose joint in the telephone wire where it passed through a wood, acted as a microphone, and transmitted the woodland chorus to his ears. Messages in process of transmission are sometimes drowned by the rumbling noise of street-traffic induced by the wire.
The microphone is not only useful as a transmitter of sounds, but also as a relay of sounds received on a telephone. Professors Houston and Thomson of America were perhaps the first to construct a telephonic relay. They mounted a carbon microphone on the vibrating plate of a telephone in such a way that the vibrations of the plate due to the received speech would react on the microphone, and be transmitted in this way over another line to another receiving telephone at a distance. Thus the speech would be relayed, just as a telegraph message is relayed, when it is weak, and sent further on its way. Curiously enough, the microphone acts as a relay to itself, if placed on the same table with the telephone with which it is in circuit. The jar of placing the microphone on the table causes the telephone to emit a sound; this sound in turn is transmitted by the microphone to the telephone, which again repeats it. The microphone re-transmits it as before, the telephone utters it, and so the process of repetition goes on ad infinitum.
Since the microphone can, as it were, magnify small sounds, and in this respect has some resemblance to the microscope, which magnifies minute objects, it might be thought that it would prove useful for deaf persons. But though the microphone enables a person with good ears to hear mechanical vibrations which otherwise would be inaudible, the sounds that are heard are not in themselves very loud, and hence a dull aural nerve might fail to appreciate them. M. Bert, the well-known French physicist, constructed a microphone for deaf persons; but its success was doubtful. Professor Hughes, however, has succeeded in making deaf persons hear the ticking of a watch by means of the microphone. In this case the telephone was placed against the bones in the head, and the vibrations communicated in this way to the aural nerve. The ‘audiphone,’ a curved plate held between the teeth, and vibrated by the sound-waves, also acts in this way; and it is probable that we hear ourselves speak not through our ears, but through the bones of the head as set in vibration by the voice.
Its power of interpreting small sounds has caused the microphone to be applied to many other purposes. Professor Rossi, for example, uses it to detect the earth-tremors preceding earthquakes and volcanic eruptions. It has been employed in Austria to detect the trickling of underground water; and its use has also been suggested for hearing the signal-taps of entombed miners and the noise of approaching torpedo boats. It is not, however, quite possible to realise all that has been claimed for it. Thus the Danbury News jestingly remarks that ‘with a microphone a farmer can hear a potato-bug coming down the road a quarter of a mile away, and can go out with an axe and head it off.’
In 1876, a year before the microphone was invented, a writer named Antoinette Brown Blackwell foretold the use of such an apparatus. ‘It remains,’ she said, ‘to invent some instrument which can so retard the too rapid vibrations of molecules as to bring them within the time adapted to human ears; then we might comfortably hear plant movements carrying on the many processes of growth, and possibly we might catch the crystal music of atoms vibrating in unison with the sunbeam.’ Without calling in question the writer’s theory, which does not apply to the microphone, we may mention that Professor Chandler Roberts attached a microphone to a thin porous septum, and on allowing hydrogen gas to diffuse through the latter, he heard a rushing sound, as of a wind, which became silent when the rapid diffusion ceased. The jar of the atoms on the pores of the septum was probably the source of this molecular sound. Again, Professor Graham Bell has found a metal microphone joint sensitive to the impact of a beam of intermittent light; and it is highly probable that a microphone with selenium contacts would be still more sensitive to the sound of light falling upon it.
In medicine, the microphone has been usefully applied to enable a physician to read the pulse better and auscultate the heart.
Numerous experiments have been made recently with the microphone by Messrs Stroh, Bidwell, and others. Not long after the original invention of the apparatus, Professor Blyth found that the microphone would act as a receiver as well as a transmitter of sounds in an electric circuit. Thus, with two boxes of coke cinders (hard carbon) connected together through a wire and battery, Professor Blyth found that if words were spoken into one of the boxes, he could faintly hear them by listening in the other. Mr Bidwell has constructed a receiving microphone, composed of a pile of carbon cylinders resting on a mica diaphragm, and this gives out distinct effects when a strong battery is employed. On speaking to the transmitting microphone in circuit, the words can be distinctly heard in the receiving one.
By the use of the microscope, Mr Stroh has observed that the carbon points of the microphone which were supposed to be in contact, are not really so during the action of the instrument, but are separated by a minute distance. It would appear, then, that there is a repulsion between the points, and this repulsion accounts for the action of the microphone as a receiver. Metal microphones are also reversible in their action, and give out feeble sounds when used as receivers. The probability is that the contacts vibrate rapidly on each other, either in direct or very close contact, against a certain repulsive action of the current, which operates like a cushion or re-acting spring.