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The Microphone

THE extreme sensitiveness of the telephone to little sounds was not suspected until the invention of the marvellous little instrument called the microphone. The discovery of this detector of minute sounds is due to Professor Hughes of London. Surmising that most of my readers have, by the directions already given, succeeded in making the former instrument, I shall now explain how to make a serviceable microphone by which many experiments can be made which will prove both interesting and entertaining.

It will be found that, if a telephone be joined in circuit with a battery cell of any description, a peculiar rasping sound will be heard in the instrument directly the final contact is made to complete the circuit. Professor Hughes was the first to experiment in this direction. He included in the circuit a thin wire to which was attached a weight. This weight he increased until the wire broke, but he found that this rupture was always preceded by a peculiar noise in the telephone, due to the gradual disruption of the particles of metal which formed the wire. He next joined the two broken wires loosely together, and found to his surprise that the imperfect connection responded to every noise made in its neighbourhood. It was in fact a detector of the most minute sonorous vibrations. Carrying his experiments further, he finally produced the microphone, by which not only the most delicate noises are heard in the telephone, but by which the articulations of the human voice can be carried by means of wires to immense distances.

The form of the microphone has undergone many modifications and changes since it first left the hand of its gifted discoverer. The most sensitive form of microphone which he produced is shown below. It consists of a little pencil of carbon hung upon a brass pin so that one of its ends just rests upon a little block of the same material. The little instrument is mounted upon a piece of board with blocks of indiarubber below to prevent the access of any accidental vibrations. At one end of the board is a little box (in the original instrument this was a common match-box with a window cut in one side filled in with net) in which a fly can be imprisoned. The trampings of this little being as he seeks an exit from his dungeon, although quite inaudible to the unassisted ear, are plainly heard in the telephone as distinct and loud footfalls. In fact, the instrument serves the same purpose to the ear as does the microscope to the eye. Sounds hitherto, on account of their extreme minuteness, unsuspected to exist are magnified into noises which can be plainly heard and appreciated.

The microphone is a terrible eavesdropper, for if it is placed hidden away in some corner of a room, and wires be connected with it to a telephone at some distant place, any conversation carried on in its vicinity is plainly heard at the distant station. To show its capabilities in this respect, I quote the following from the Halifax Guardian, and from experiments that I have made with the instrument I feel sure that there is no reason to doubt the accuracy of the statement. "Last Sunday a microphone was placed in the pulpit of a chapel in this town, and connected by a private telegraph line running by with the residence of a gentleman over a mile distant. Every part of the service was distinctly heard at the gentleman's house, with the exception of a few words rendered indistinct by the preacher becoming a little excited, and shaking the microphone, the presence of which he never dreamt of, or he might have been somewhat nervous at the idea that his sermon was being conveyed away he knew not whither. So faithfully did the microphone do its work that the chapel-keeper was heard to close the doors after service, walk up the aisle, and up the pulpit steps, in conversation with some one else. During the week experiments have been made in the schoolroom of another chapel, and the singing of the scholars was transmitted and retransmitted over a number of telegraph lines with remarkable clearness. The idea is about to be put to practical use, the gentleman already referred to having given instructions that his house should be connected with that of a gentleman near, in order that an invalid may hear the service from one of the churches in the town." The extremely simple character of the microphone as first designed by Professor Hughes is one of its most remarkable features. So long as the substances employed took the form of a loose connection in the electrical circuit, the necessary conditions were secured. Two French nails connected with the battery and telephone, with another nail loosely resting upon them, forming together the letter H, produced an instrument capable of detecting the most feeble sounds. Another more convenient form of microphone is made as shown in the above cut. Two little blocks of gas-carbon, each half an inch square, are cemented by sealing-wax to an upright board fixed at the end of a cigar-box. Each cube has a little pit for the reception of a pencil of carbon pointed at each end like a cigar. The carbon blocks have wires fixed to them as shown in cut, and these wires are joined up with a battery cell and a telephone. With this tiny instrument--it need be but two inches long--articulate sounds are clearly transmitted. Another form of microphone (see next cut), and one of the most efficient that I have seen, is made by Mr. Browning, of the Strand. The pressure between the points of contact can be regulated with the greatest nicety, a matter of great importance in experiments where sometimes it is desired to transmit loud sounds, and sometimes delicate articulation.

A microphonic arrangement, which is now in common use with the telephone, is patented by Mr. L. J. Crossley, of Halifax. It consists of four pencils of carbon fitted loosely into four carbon blocks as shown in the cut below; the two opposite blocks A and B being connected with the circuit wires. Words spoken several feet away from a microphone of this description are plainly heard at a distant place by means of the telephone.

It will be thus seen that the microphone, simple as it is in construction, is no mere toy to be invented and forgotten, but is likely to have an extended use in the service of mankind. It is not only used for business purposes as a help to the telephone in the transmission of sounds, but is also in growing demand for other fields of usefulness. In the surgeon's hands it can be used with a probe for the detection of foreign bodies in wounds. So long as the probe only touches the soft tissues, no sound is apparent in the telephone, but directly it grates against a fragment of bone, the hard touch is instantly magnified into a loud rasping sound which is unmistakable in its character. In the same way a bullet buried in the flesh could readily be detected with the hope of its speedy removal.

Another instrument which is the outcome of the microphone is also due to Professor Hughes. It is called the audiometer. This is used to test the power of hearing in those who are partially deaf, and is likely to be of great benefit to those afflicted in this way. The source of sound in the instrument is a microphone.

One more contrivance, which is also a valuable acquisition to medical science, is the sphygmophone, by which the sound of a patient's pulse is made audible in a large room. Dr. B. W. Richardson constantly uses this instrument in his daily practice, and finds it a great help in diagnosis. The branch of scientific inquiry opened up by the microphone, telephone, and kindred instruments is but in its infancy, and we may look upon them as the precursors of still greater wonders in the future.