APPLICATIONS OF THE MICROPHONE.
The applications of the microphone increase in number every day, and in addition to those of which we have just spoken, there are others of really scientific and even of practical interest. Among the number is the use which can be made of it as a system of relays for telegraphy, in science for the study of vibrations imperceptible to our senses, in medicine and surgery, and even in manufactures.
Its application to Scientific Research.—We have seen that several physicists, including Messrs. Spottiswoode, Warwick, Rossetti, Canestrelli, Wiesendanger, Lloyd, Millar, Buchin, and Blyth, have been able to hear what is said in a telephone which has no iron diaphragm, but it was so difficult to establish the fact that it has been often disputed. More certain evidence was desirable, and the microphone is an opportune agent for affording it.
The ‘Telegraphic Journal’ of September 1, 1878, observes that M. du Moncel, in order to claim the victory in his controversy with Colonel Navez, had still to show that the sounds which appeared to be inarticulate in telephones without a diaphragm might become intelligible if they were intensified. This has been done for him by the use of Mr. Hughes’s microphone, and the following experiments were made for the purpose.
1. If a magnetising coil, surrounding a bar of soft iron, is inserted in the circuit of a microphone, with a battery of three cells, the ticking of a watch and other sounds of the same kind may be heard on approaching the ear to the electro-magnet which has been thus constituted. It is true that these sounds are very faint when they are not amplified, but if the electro-magnet is fastened to a board, and a second microphone is fixed to the same board, the sounds produced by the electro-magnet are magnified, and become distinctly audible in the telephone which is placed in connection with this second microphone.
2. These sounds may be further amplified by resting one of the extremities of the core of the electro-magnet on one of the poles of a permanent magnet, which is fixed upon the board. Articulate speech may then be heard in the telephone which is placed in connection with the microphone resting on the board, and the point at issue between MM. Navez and Du Moncel is completely decided in this way: for the auxiliary microphone can only propagate and amplify the vibration of articulate sounds, which are communicated by the bar magnet of the coil to the board on which the two instruments are placed. In this way it would be possible to render articulate sounds perceptible to M. Navez, when transmitted by the bar magnet of a telephone without a diaphragm.
3. When a second bar magnet rests on the free pole of the electro-magnet, so as to present to it a pole of the same nature as the one with which it is already in communication—in a word, if a bar is placed between the two poles of a horseshoe electro-magnet, the effects are still more marked, and hence it may be assumed that the bar reacts as an armature, by concentrating the lines of magnetic force in the vicinity of the helix.
4. When the two poles of a horseshoe magnet are inserted together inside a coil, their effects are equally energetic, although by this arrangement one of the poles might be expected to neutralise the effect of the other: but the most important effects have been obtained by placing an armature of soft iron across the poles of the magnet which has been already inserted in the coil. Under these conditions articulate sounds are distinctly heard.
These experiments were confirmed by Mr. F. Varley, in a letter published in the ‘Telegraphic Journal’ of September 15, 1878, and among the fresh experiments mentioned by him, we will quote those which he made with an iron tube inserted in a helix, in which the two opposite poles of two bar magnets are introduced. These poles are only separated from each other by the interval of an inch, so that the centre of the iron tube may be strongly magnetised.
Mr. Varley says that this last arrangement reproduces the articulate sounds which issue from a sending microphone, and this experiment is more decisive than that of Professor Hughes, in which case it might be supposed that the bar magnet, resting on the polar end of an electro-magnetic bar, was only a modification of the disk in the Bell telephone, set in vibration by the alternate currents passing through the helix, and that these vibrations were communicated to the board, and became sensible when enlarged by the microphone. But such an objection cannot be alleged in the case of the arrangement described above, for since the sound is produced between the current passing into the helix and the magnetic current of the bar, it can only be the result of a vibration produced by a disturbance of the reciprocal relations subsisting between these two elements. Mr. Varley adds that these experiments confirm M. du Moncel’s researches, which have thrown considerable light upon the causes which are at work in the action of the speaking telephone, and with which we have hitherto been imperfectly acquainted.
Its application to Telephonic Relays.—In February 1878, I first began to consider the mode of forming telephonic relays, but I was checked by the discovery that there was no vibration in the receiving telephone, and I made the following communication on the subject to the Académie des Sciences on February 25:—‘If the vibrations of the disk in the receiving telephone were the same as those of the sending telephone, it is easy to see that if a telephone with a local battery, acting both as sender and receiver, were substituted for the receiving telephone, it might, by the intervention of the induction coil, act as a relay, and might therefore not only amplify the sound, but also transmit it to any distance. It is, however, doubtful whether the vibrations of the two corresponding disks are of the same nature, and if the sound be due to molecular contractions and expansions, the solution of the problem becomes much more difficult. Here is therefore a field for experiments.’ These experiments have been successfully made by Mr. Hughes, who acquainted me with them early in June 1878, and they led to the discovery of a most interesting system of microphonic relays.
On a wooden board of moderate size, such as a drawing board, he placed a microphone with a carbon brought to a fine point at each end, and fixed in a vertical position. One or more telephones were placed in the circuit, with their membranes facing the board, and a continuous sound was heard, sometimes resembling a musical note, sometimes the singing of boiling water in an oven; and the sound, which could be heard at a distance, went on indefinitely, as long as the electric force was exerted. Mr. Hughes explains this phenomenon in the following way.
The slightest shock which affects the microphone has the effect of sending currents, more or less broken, through the telephones, which transform them into sound vibrations, and since these are mechanically transmitted by the board to the microphone, they maintain and even amplify its action, and produce fresh vibrations on the telephones. Thus a fresh action is exerted on the microphone, and so on indefinitely. Again, if a second microphone, in connection with another telephonic circuit, be placed upon the same board, we have an instrument which acts as a telephonic relay, that is, it transmits to a distance the sounds communicated to the board, and these sounds may serve either as a call, or as the elements of a message in the Morse code, if a Morse manipulator is placed in the circuit of the first microphone. Mr. Hughes adds that he has made several very successful experiments with this system of instruments, although he only employed a Daniell battery of six cells without any induction coil. By fastening a pasteboard tube, 40 centimètres in length, to the receiving telephone, he was able to hear in all parts of a large room the continuous sound of the relay, the ticking of a watch, and the scratching of a pen upon paper. He did not try to transmit speech, since it would not have been reproduced with sufficient distinctness under such conditions.
Since this first attempt, Mr. Hughes has arranged another and still more curious system of microphonic relays, for which two microphones with vertical carbons are required. He places two microphones of this description on a board, and connects one of them with a third microphone, which acts as a sender, while the second is in communication with a telephone and a second battery: in this way the words uttered before the sender are heard in the telephone, without employing any electro-magnetic organ for the telephonic relay.
In August 1878, Messrs. Houston and Thomson likewise arranged a system of telephonic relays which only differs from that of Mr. Hughes in the particular of having the microphone fixed on the diaphragm of the telephone, and not on the board beside it. The system consists of three vertical microphones, which can be combined for tension or quantity, according to the conditions for which they are required. The model of this instrument was represented in the ‘Telegraphic Journal’ of August 15, 1878, to which we must refer our readers, if they wish for further information on the subject.
Its application to Medicine and Surgery.—The extreme sensitiveness of the microphone suggested its use for the observation of sounds produced within the human body, so that it might serve as a stethoscope for listening to the action of the lungs and heart. Dr. Richardson and Mr. Hughes are now busy in the attempt to carry out this idea, but so far the result is not very satisfactory, although they still hope to succeed. Meanwhile, M. Ducretet has made a very sensitive stethoscopic microphone, which we represent in fig. 51. It consists of a carbon microphone C P, with a simple contact, of which the lower carbon P is fitted to one of M. Marais’ tambourines with a vibrating membrane T. This tambourine is connected with another T′, by a caoutchouc tube, which is to be applied to the different parts of the body which demand auscultation, and which is therefore termed the tambour explorateur. The sensitiveness of the instrument is regulated by means of a counterpoise P O, which is screwed upon the arm of a bent lever, and to this the second carbon C is fixed. The extreme sensitiveness of M. Marais’ tambourines in transmitting vibrations is well known, and since their sensitiveness is further increased by the microphone, the instrument becomes almost too impressionable, since it reveals all sorts of sounds, which it is difficult to distinguish from each other. Such an instrument can only be of use when entrusted to experienced hands, and a special education of the organ of hearing is needful, in order to turn it to account.
In a work lately published by M. Giboux on the application of the microphone to medicine, this stethoscopic system is rather severely criticised, and not without reason if, as M. Giboux asserts, it is only sensitive to the movements which take place on the surface of the body, and those which are internal are either lost or altogether changed in character. But without pronouncing on the improvements which may ultimately be made in the instrument, M. Giboux thinks that its most important use in medical practice consists in its allowing a certain number of students to observe with the professor the different sounds of the body, to study them with him in their different phases, and thus to profit more readily by his teaching. A microphonic circuit might bifurcate between several telephones, so that each person might hear for himself what is heard by others.
The most important application of the instrument to surgical purposes has lately been made by Sir Henry Thompson, aided by Mr. Hughes, for the examination of the bladder in cases of stone. It enables him to ascertain the presence and precise position of calculi, however small they may be. For the purpose of research, he uses a sound, made of a Maillechort rod, a little bent at the end, and placed in communication with a sensitive carbon microphone. When the sound is moved about in the bladder, the rod comes in contact with stony particles, even if they are no larger than a pin’s head, and friction ensues, producing in the telephone vibrations which can be easily distinguished from those caused by the simple friction of the rod on the soft tissues of the sides of the bladder. The arrangement of the instrument is shown in fig. 52. The microphone is placed in the handle which contains the sound, and is the same as that given in fig. 42, but of smaller size, and the two conducting wires e which lead to the telephone, issue from the handle by the end a opposite to that bb to which the sound dd is screwed. As this instrument is not intended to reproduce speech, retort carbons instead of wood carbons may be used.
Some deaf people, whose sense of hearing is not completely destroyed, have been able to hear by an expedient based upon the principle of the microphone. For this purpose two telephones, connected by a metallic crown, which is placed on the temples, are applied to the ears of the deaf person, and the telephones are placed in communication with a battery microphone, which hangs to the end of a double conducting wire. The deaf man keeps the microphone in his pocket, and presents it as an acoustic tube to the person who wishes to converse with him. Mr. Hughes’s speaker, represented fig. 42, is the one used.
Various Applications.—The microphone may be used in many other ways, some of which are suggested in the ‘English Mechanic’ of June 21, 1878. The article states that by means of this instrument, engineers will be able to estimate the effects of the vibrations caused on old and new buildings by the passage of heavy loads; a soldier will be able to discover the enemy’s approach when he is several miles off, and may even ascertain whether he has to do with artillery or cavalry; the approach of ships to the neighbourhood of torpedoes may be automatically heralded on the coast by this means, so that an explosion may be produced at the right moment.
It has also been proposed to use the microphone to give notice of an escape of gas in coal-mines. The gas, in escaping from between the seams of coal, makes a whistling noise, which might, with the aid of the microphone and telephone, be heard at the top of the shaft. Again, it has been suggested that the microphone might be used as a seismograph to reveal the subterranean noises which generally precede earthquakes and volcanic eruptions, and which would be much intensified by this instrument. It might even be of use to Signor Palmieri for his observations in the Vesuvius Observatory.
The microphone has also been used by Mr. Chandler Roberts to render the diffusion of gaseous molecules through a porous membrane sensible to the ear.
As might have been expected, the acclamation with which Mr. Hughes’s invention was received led to the assertion of other claims to priority, and in addition to that of Mr. Edison, on which we have already given our opinion, there are several others, showing that if some microphonic effects were discovered at different times before the date of Mr. Hughes’s discovery, they could not have been considered important, since they were not even announced. Among the number was that of Mr. Wentworth Lascelles Scott, specified in the ‘Electrician’ of May 25, 1878, and that of M. Weyher, presented to the Société de Physique, Paris, in June 1878. Another, made by M. Dutertre, is of somewhat greater importance, for his experiments were reported in the Rouen papers in February of the same year: yet there is no just ground for such claims, since the earliest date of his experiments is subsequent to the experiments first made by Mr. Hughes. These began early in December 1877, and in January 1878 they were exhibited to officials of the Submarine Telegraph Company, as Mr. Preece declared in a letter addressed to the several scientific men.