RECENT SCIENCE.

(PROFESSOR II UXLEY has kindly recd, and aided the Ellitor and compilers with his

adrice upon, the following article.)

a

WITH
Wiru the invention of the phonograph following hard upon that of

telephone, it might be fairly assumed that acoustical science had spent itself, and that it could afford for a while to lie fallow. So far, however, from this being actually the case, it continues to show abundant signs of fertility, and has, indeed, quite recently given birth to an instrument which in some respects is more marvellous than either of its predecessors. Wonderful as it is to transmit articulate speech by means of the telephone, it is perhaps more wonderful to register that speech and reproduce it at will by the phonograph ; but surely the most wonderful thing of all is this, that we should be able to magnify feeble sounds, which are absolutely inaudible by the unaided human ear, until they may be heard with almost painful intensity at the receiving station miles away. Yet this is actually accomplished though the medium of the recently invented microphone. In fact, it is hardly too much to say that as the microscope is to the eye, so will the microphone become to the ear.

It is notable that most of the recent advances in practical electricity have been effected in America. It is to the United States that we are indebted, for example, for the introduction, if not for the original invention, of quadruplex telegraphy and of telephony. The new method of transmitting and magnifying sounds by means of electricity is due to Professor D. E. Hughes, of London, the inventor of the well-known type-printing telegraph, who is, however, an American citizen long settled in this country. It would be an error to assume that the discovery resulted from a happy accident or from a sudden inspiration. It has, on the contrary, been gradually evolved, like the telephone, from the careful study of scientific principles, and is the legitimate outcome of patient work.

Knowing that the resistance which a body offers to the passage of an electrical current is invariably affected by changes of temperature, and even in some cases, as in that of selenium, by variations in the intensity of light, it occurred to Professor Hughes that this physical property might also be influenced by the vibrations

which constitute sound. In point of fact, Sir W. Thomson and some other physicists had already shown that the electrical resistance of a wire is affected by its being thrown into a state of strain. Now when a sound-wave travels along a wire, there must be rapid variations in the strains at different points; and hence the plausible basis of Mr. Hughes's conjecture. To determine whether this conjecture could be verified or not, he introduced a stretched wire into a closed circuit, and by the action of the voice threw it into a state of vibration. In this experiment, as in his subsequent researches, the current employed was that of three Minotto's cells—a convenient modification of Daniell's constant battery. An ordinary telephone of Professor Bell's construction was introduced into another part of the circuit for the purpose of detecting sound, or as a phonoscope. If the resistance of the wire were affected by the sonorous vibrations, the strength of the electric current would vary; thereupon the telephone-plate would be thrown into a state of vibration, and the sound would consequently be reproduced. On making the experiment, however, nothing was heard until the tension of the wire reached the breaking strain, and then, just at the moment of rupture, a peculiar sound became audible. But on placing the two broken ends together and applying a small pressure, so as to procure electric continuity, Professor Hughes had the satisfaction of finding that sounds produced in the neighbourhood of the wire were heard in the telephone.

To understand how this may come about, let it be imagined that the wire is made up of molecules arranged in file, and that a soundwave passes along this line of particles. The transmission of the pulse implies alternate compression and dilatation in the conveying medium-a squeezing together of the particles in one place, and their pulling asunder in another. As long as the conducting wire remains homogeneous throughout, the condensations and rarefactions exactly neutralise each other. But let the wire be broken, and then at the points of rupture the unbalanced effects become sensible. When the particles at one of the ends are under the influence of dilatation they swell out, and, by thus increasing the pressure at the points of contact, they also increase the electrical resistance. Conversely the resistance is diminished when the particles are compressed, and the pressure at the point of contact is relieved. These variations in the electric resistance affect the telephone, and the passage of sodorous vibrations along the wire thus gives rise to corresponding sounds emitted by this instrument. Such, at least, is Professor Hughes's interpretation of the phenomena.

Two ends of a broken conductor gently pressed together form in this way a crude apparatus for transmitting sound. The discoverer, however, was not slow in improving upon this primitive arrange

ment. Instead of bringing the two ends of the wire close together, he slightly separated them, and bridged over the interval by a metallic conductor, such as a steel chain, or a piece of watch spring, or even a common nail. Under these circumstances the transmission of sound was facilitated ; and the effect was further improved by building up the nails, log-hut fashion, into a square configuration, using ten or twenty nails.' The improved transmission appears to be due to the numerous points of contact which the nails offer, for as the number is multiplied the effect is increased. Small clean shot may be used, but metal filings are still better, while excellent results are obtained by use of the metallic powder sold under the name of white bronze.' Finely-divided quicksilver is, however, the medium which Professor Hughes has hitherto found to give the best results. A piece of willow charcoal, such as that used by artists for sketching, is strongly heated, and then quickly plunged into mercury. The charcoal, being extremely porous, becomes permeated with innumerable globules of mercury, and thus forms an excellent conductor of electricity. It is also possible to impregnate the charcoal with other metals, such as zinc, tin, and iron. A few small pieces of this metallised charcoal are placed end to end in a glass tube, about 2 inches in length and #inch in diameter. This, and nothing more, forms the transmitting instrument, which is introduced by wires into the circuit, and the arrangement is then complete. When this little tube of mercurial charcoal is talked at, the words are faithfully taken up and conveyed along the conductor, it may be for many miles, to the telephone at the receiving station, where they are reproduced with precision.

The extreme simplicity of this instrument is only equalled by its extreme delicacy. It is indeed so sensitive that the merest whisper is audibly recorded. Moreover, sounds which are too weak to be ordinarily heard are capable of affecting the instrument; and this is unquestionably the most extraordinary part of the discovery. If, for example, the soft part of a feather merely touch the sounding-board on which the instrument is mounted, the noise will be heard at the receiving station. To magnify feeble sounds, Professor Hughes employs a special form of apparatus which he appropriately calls a microphone.

It is well-known that the carbonaceous deposit in gas-retorts is a good conductor of electricity, and it is indeed between two pencils of this material that the electric light is ordinarily produced. The microphone consists simply of a diamond-shaped piece of gas-carbon (about one inch long, inch wide at the centre, and } inch thick) placed vertically between two pieces of similar carbon. The lower end forms a pivot which turns in a small cup hollowed out in one of the carbon supports, while the upper end plays freely in a hollow in the carbon above. These two carbon blocks are connected with the

а

circuit, and the lozenge thus forms a moveable conductor between them. To improve the conductivity of the carbon, it may be saturated with mercury

By means of this extremely simple apparatus, the most marvellous effects are produced. •The beating of a pulse, the tick of a watch, the tramp of a fly, can thus be heard at least a hundred miles distant from the source of sound.' But, startling as these results are, it must be remembered that the apparatus is only yet in its infancy, and its future can at present be simply conjecture. It should be remarked, in justice to Professor Hughes, that he does not intend to secure patent rights, as he regards his researches and their results as belonging to the domain rather of discovery than of invention. The field is therefore open to inventors, and we may rest assured that numerous applications, and probably improved forms of the apparatus, will speedily be suggested. To the man of science it is of peculiar interest, since it promises to open up a new line of inquiry with reference to some of the recondite problems connected with the molecular constitution of matter.

Among the many experimentalists who have lately been patiently at work with the view of improving our means of transmitting sound over great distances, no one has been more successful than Mr. T. A. Edison, of New Jersey. In a specification which was filed in our Patent Office at the beginning of this year, he describes a large number of improvements which he has introduced into the nerborn art of Telephony. In addition, however, to these methods of transmitting and receiving sounds by means of electricity, he has astonished every one by constructing a mechanical apparatus which, without the aid of either electricity or magnetism, registers and reproduces the complex sounds of the human voice. This marvellous instrument is becoming well-known under the name of the Phonograph or Talking Machine.3

It is true that long before the phonograph had been heard of various attempts had been made to imitate the human voice, and some of these attempts had been marked by a fair measure of success. All these talking-machines, however, have been complicated imitations of our vocal organs. Mr. Edison, proceeding on an entirely different track, makes no attempt to imitate the structure of the larynx and other organs which aid the production of speech; but he causes the vibrations of the voice to imprint themselves upon a metallic surface from which the original sounds are capable of being reproduced.

Nothing, in short, can be simpler than the mechanism of this instrument. It consists of a cylinder mounted on a horizontal axle, and capable of rotation by means of a handle, or preferably, as uniformity of speed is essential, by means of clockwork. The cylinder is not only capable of rotation, but has also a gentle lateral movement, which is effected by a screw cut on part of the shaft, and working in a nut. A screw-thread is likewise cut on the cylinder,

and the cylinder itself is in most cases coated with tin-foil. This foil is gently pressed by a metal pin, or style, which is attached to a thin disc of iron furnished with a funnel-shaped mouthpiece of vulcanite. When words are spoken into this mouthpiece the vibrations of the air are communicated to the metal diaphragm, and the pin which it carries is thus thrown into agitation. As the cylinder slowly travels along it is constantly pressed by this style ; and if the pressure continued uniform a spiral furrow, everywhere of equal depth, would be traced around the barrel. But when the voice agitates the iron plate the pin is caused to press unequally upon the cylinder, and the metal surface is therefore indented to an unequal extent in different parts of the winding line. While the foil readily yields to pressure, and thus offers but little opposition to indentation, its lack of elasticity prevents it from springing back, and hence the impressions once made are permanently retained. To reproduce the sounds which have thus been impressed upon the metal, the cylinder has to be brought back to its original position. It is then rotated beneath the pin, which is jerked up and down as the elevations and depressions pass beneath it. These movements of the pin are faithfully followed by the metal diaphragm, which throws the air into vibration, and thus produces sounds exactly corresponding with those by which the indentations were originally produced. As the pitch of the sound is altered by varying the velocity of rotation, it is evident that an exact reproduction of the voice can only be effected by causing the cylinder to revolve at precisely the same rate as that wbich it possessed when it originally received the sounds that it is seeking to emit. This precision of movement may be effected by means of clockwork. In some modifications of the instrument, it has been found that copper and even iron may be indented by the movement of the pin.

It should be borne in mind that the phonograph, although an extraordinary instrument, is far from being perfect in its action. It appears, indeed, to be incapable of producing certain sounds. Even in its present imperfect form, however, it promises to yield results of great interest to the student of acoustics. An ingenious application has been suggested by the President of the American Philological Society, who proposes to apply it to the preservation of the languages of some of the tribes of North American Indians who are fast dying out, and whose language will soon be lost unless preserved by phonographic registration.