Part 10

The action of the currents sent through the helix of a telephone can be easily explained. Whatever may be the magnetic conditions of the bar, the induced currents of different intensity which act upon it produce modifications in its magnetic state, and hence the molecular vibrations follow from contraction and expansion. These vibrations are likewise produced in the armature under the influence of the magnetisations and demagnetisations which are produced by the magnetic action of the core, and they contribute to the vibrations of the core itself, while at the same time the modifications in the magnetic condition of the system are increased by the reaction of the two magnetic parts upon each other.

When the bar is made of soft iron, the induced currents act by creating magnetisations of greater or less energy, followed by demagnetisations which are the more prompt since inverse currents always succeed to those which have been active, and this causes the alternations of magnetisation and demagnetisation to be more distinct and rapid. When the bar is magnetised, the action is differential, and may be exerted in either direction, according as the induced currents corresponding to the vibrations which are effected pass through the receiving coil in the same or opposite direction as the magnetic current of the bar. If these currents are in the same direction, the action is strengthening, and the modifications are effected as if a magnetisation had taken place. If these currents are of opposite direction, the inverse effect is produced; but, whatever the effects may be, the molecular vibrations maintain the same reciprocal relations and the same height in the scale of musical sounds. If the question is considered from the mathematical point of view, we find the presence of a constant, corresponding with the intensity of the current, which does not exist in mechanical vibrations, and which may possibly be the cause of the peculiar tone of speech reproduced by the telephone, a tone which has been compared to the voice of Punch. M. Dubois Raymond has published an interesting paper on this theory, which appeared in ‘Les Mondes,’ February 21, 1878, but we do not reproduce it here, since his remarks are too scientific for the readers for whom this work is intended. We will only add that Mr. C. W. Cunningham asserts that the vibrations produced in a telephone cannot be manifested under precisely the same conditions as those which affect the tympanum of the ear, because the latter has a peculiar funnel-shaped form, which excludes every fundamental note, specially adapted to it, and this is not the case with the bars and magnetic plates which possess fundamental notes capable of greatly altering the half-tones of the voice. He considers the alteration of the voice observed in the telephone must be ascribed to these fundamental notes.

_M. Wiesendanger’s Thermophone._--M. Wiesendanger, in an article inserted in the ‘English Mechanic and World of Science,’ September 13, 1878, ascribes the reproduction of speech in certain telephones to vibratory movements resulting from molecular expansions and contractions produced by variations of temperature, and these variations would follow from the currents of varying intensity which are transmitted through the telephonic circuits. He was conscious of one objection to this theory, namely, that the movements of expansion and contraction due to heat are slowly produced, and consequently are not capable of substantial action, rapid enough to produce vibrations; but he considers that molecular effects need not take place under the same conditions as those which are displayed in the case of material substances.

M. Wiesendanger believes that this hypothesis will explain the reproduction of speech in the receiving microphones of Mr. Hughes, and that it may even be applied to the theory of the electro-magnetic telephone, if we consider that a magnetising helix, as well as a magnetic core, round which an electric current circulates, is more or less heated, according to the intensity of the current which traverses it, especially when the wire of the helix and the core are bad conductors of electricity and of magnetism. Pursuing this idea, M. Wiesendanger has sought to construct telephones in which calorific effects are more fully developed, and with this object he used very fine wire of German silver and platinum to make the coils. He ascertained that these coils could produce sounds themselves, and, to increase their intensity, he put them between disks of iron, or on tin tubes, placed on resonant surfaces close to the disks. In this way he says that he was able to make a good receiving telephone without employing magnets. He afterwards arranged the instrument in different ways, of which the two following are the most noteworthy.

In the first, the electro-magnetic system was simply formed by a magnetic disk with a helix wound round it, of which the wire was in connection with the circuit of a microphone, and which was fastened to the centre of the parchment membrane of an ordinary string telephone; the disk consisted of two iron plates separated by a carbon disk of smaller diameter, and the whole was so compressed as to form a solid mass.

In the second, the helix was wound on a tin tube, six inches long and five-eighths of an inch in diameter, which was soldered by merely a point to the centre of the diaphragm of an ordinary telephone.

The inventor asserts that the tube and diaphragm only act as resonators, and that the sounds produced by this instrument are nearly the same as those obtained from the ordinary string telephone: the tunes of a musical box were heard, and the reproduction of speech was perfect, both in intensity and in distinctness of sound; it even appeared that telephonic sounds were audible with the tin tube alone, surrounded by the helix. M. Wiesendanger says that ‘these different receiving telephones show clearly that the diaphragm and magnet are not essential, but merely accessory, parts of a telephone.’

VARIOUS EXPERIMENTS MADE WITH THE TELEPHONE.

We must now consider a series of experiments which demonstrate the wonderful properties of the telephone, and which may also give some indication of the importance of the influences by which it is liable to be affected.

_Experiments by M. d’Arsonval._--We have seen that the telephone is an extremely sensitive instrument, but its sensitiveness could scarcely be appreciated by ordinary means. In order to gauge it, M. d’Arsonval has compared it to the nerve of a frog, which has hitherto been regarded as the most perfect of all galvanoscopes, and it appears from his experiments that the sensitiveness of the telephone is two hundred times greater than that of the frog’s nerve. M. d’Arsonval has given the following account of his researches in the records of the Académie des Sciences, April 1, 1878:

‘I prepared a frog in Galvani’s manner. I took Siemens’ instrument of induction, used in physiology under the name of the chariot instrument. I excited with the ordinary pincers the sciatic nerve, and I withdrew the induced coil until the nerve no longer responded to the electric excitement. I then substituted the telephone for the nerve, and the induced current, which had ceased to excite the latter, made the instrument vibrate strongly. I withdrew the induced coil, and the telephone continued to vibrate.

‘In the stillness of night I could hear the vibration of the telephone when the induced coil was at a distance fifteen times greater than the minimum at which the excitement of the nerve took place; consequently, if the same law of inverse squares applies to induction and to distance, it is evident that the sensitiveness of the telephone is two hundred times greater than that of the nerve.

‘The sensitiveness of the telephone is indeed exquisite. We see how much it exceeds that of the galvanoscopic frog’s leg, and I have thought of employing it as a galvanoscope. It is very difficult to study the muscular and nervous currents with a galvanometer of 30,000 turns, because the instrument is deficient in instantaneous action, and the needle, on account of its inertia, cannot display the rapid succession of electric variations, such as are effected, for example, in a muscle thrown into electric convulsion. The telephone is free from this inconvenience, and it responds by vibration to each electric change, however rapid it may be. The instrument is therefore well adapted for the study of electric tetanus in the muscle. It is certain that the muscular current will excite the telephone, since this current excites the nerve, which is less sensitive than the telephone. But for this purpose some special arrangement of the instrument is required.

‘It is true that the telephone can only reveal the variations of an electric current, however faint they may be; but I have been able, by the use of a very simple expedient, to reveal by its means the presence of a continuous current, also of extreme faintness. I send the current in question into the telephone, and, to obtain its variations, I break this current mechanically with a tuning-fork. If no current is traversing the telephone, it remains silent. If, on the other hand, the faintest current exists, the telephone vibrates in unison with the tuning-fork.’

Professor Eick, of Wurzburg, has also used the telephone for physiological researches, but in a direction precisely opposite to that explored by M. d’Arsonval. He ascertained that when the nerves of a frog were placed in connection with a telephone, they were forcibly contracted when anyone was speaking into the instrument, and the force of the contractions chiefly depended on the words pronounced. For instance, the vowels _a_, _e_, _i_ produced hardly any effect, while _o_ and especially _u_ caused a very strong contraction. The words _Liege still_, pronounced in a loud voice, only produced a faint movement, while the word _Tucker_, even when spoken in a low voice, strongly agitated the frog. These experiments, reminding us of those by Galvani, were necessarily based on the effects produced by the induced currents developed in the telephone, and they show that if this instrument is a more sensitive galvanoscope than the nerve of a frog, the latter is more susceptible than the most perfect galvanometer.

_Experiments by M. Demoget._--In order that he might compare the intensity of the sounds transmitted by the telephone with the intensity of original sounds, M. Demoget placed two telephones in an open space. He held the first to his ear, while his assistant withdrew to a distance, constantly repeating the same syllable with the same intensity of tone in the second instrument. He first heard the sound transmitted by the telephone, and then the sound which reached him directly, so that comparison was easy, and he obtained the following results.

At a distance of 93 yards the original and the transmitted sounds were received with equal intensity, while the vibrating disk was about 5 centimètres from the ear. At this moment, therefore, the relative intensity was as 25 to 81,000,000. In other words, the sound transmitted by the telephone was only 1/3000000 of the emitted sound. ‘But,’ said M. Demoget, ‘since the stations at which we worked could not be regarded as two points freely vibrating in space, the ratio may be reduced by one half on account of the influence of the earth, and the sound transmitted by the telephone may be supposed to be 1,500,000 times weaker than that emitted by the voice.

‘Again, since we know that the intensity of the two sounds is in proportion to the square of the range of vibrations, it may be concluded that the vibrations of the two telephone disks were in direct proportion to the distance, that is, as 5 to 9,000, or that the vibrations of the sending telephone were eighteen hundred times greater than those of the receiving telephone. These latter may therefore be compared to molecular vibrations, since the range of those of the sending telephone was extremely small.

‘Without in any degree detracting from the merit of Bell’s remarkable invention,’ continues M. Demoget, ‘it follows from what I have said above that the telephone, considered as a sending instrument, leaves much to be desired, since it only transmits the 18/100 part of the original power; and if it has produced such unexpected results, this is due to the perfection of the organ of hearing, rather than to the perfection of the instrument itself.’

M. Demoget considers this loss of power which takes place in the telephone to be chiefly owing to the eight transformations in succession to which sound is subjected before reaching the ear, setting aside the loss due to the electric resistance of the line, which might in itself suffice to absorb the whole force.

In order to estimate the force of the induced currents which act upon a telephone, M. Demoget has attempted to compare them with currents of which the intensity is known, and which produce vibrations of like nature and force: for this purpose he has made use of two telephones, A and B, communicating through a line 22 yards in length. He placed a small file in slight contact with the vibrating disk of the telephone A, and caused friction between the file and a metallic plate: the sound thus produced was necessarily transmitted by the telephone B, with an intensity which could be estimated. He then substituted a battery for the telephone A, and the file was introduced into the circuit by connecting it with one of the poles. The current could only be closed by the friction of the file with the plate, which had a spring, and was in communication with the other end of the circuit. In this way broken currents were obtained, which caused vibration in the telephone B, and produced a sound of which the intensity varied with the strength of the battery current. In this way M. Demoget endeavoured to find the electric intensity capable of producing a sound similar to that of the telephone A, and he ascertained that it corresponded in intensity to that produced in a small thermo-electric battery formed of an iron and a copper wire, two millimètres in diameter, flattened at the end, and soldered to the tin: the faint current produced by this battery only caused a short wire galvanometer to deviate two degrees.

This estimate does not appear to us to unite so many conditions of accuracy as to enable us to deduce from it the degree of sensitiveness possessed by a telephone, a sensitiveness which the experiments of Messrs. Warren de la Rue, Brough, and Peirce show to be much greater. Mr. Warren de la Rue, as we have seen, used Thomson’s galvanometer, and compared the deviation produced on the scale of this galvanometer with that caused by a Daniell cell traversing a circle completed by a rheostat: he ascertained that the currents discharged by an ordinary Bell telephone are equivalent to those of a Daniell cell traversing 100 megohms of resistance, that is, 6,200,000 miles of telegraphic wire. Mr. Brough, the Director of Indian Telegraphs, considers that the strongest current which at any given moment causes a Bell telephone to work does not exceed 1/1000000 of the unit of current, that is, one Weber, and the current transmitted to the stations on the Indian telegraphic line is 400,000 times as strong. Finally, Professor Peirce, of Boston, compares the effects of the telephonic current with those which would be produced by an electric source of which the electro-motive force should be 1/200000 part of a volt, or one Daniell cell. Mr. Peirce justly remarks that it is difficult to estimate the real value of these kinds of currents at any precise sum, since it essentially varies according to the intensity of the sounds produced on the transmitting telephone; but it may be affirmed that it is less than the 1/1000000 part of the current usually employed to work the instruments on telegraphic lines.

Signor Galileo Ferraris, who has recently published an interesting treatise on this question in the ‘Atti della Reale Accademia delle Scienze di Torino’ (June 13, 1878), states that the intensity of the currents produced by the ordinary Bell telephone varies with the pitch of the sound emitted.

_Experiments by M. Hellesen, of Copenhagen._--In order to estimate the reciprocal effects of different parts of a telephone, M. Hellesen has made telephones of the same size with three different arrangements which act inversely to each other. The first was of the ordinary form, the second like that of Bell’s first system, that is, with a membrane supporting a small iron armature on its centre, instead of a vibrating disk, and the third telephone consisted of a hollow cylindrical magnet, with the vibrating disk fixed to one of its poles, and the disk was adapted to move before a flat, snail-shaped spiral, of which the number of spirals equalled those of the two other helices. In this last arrangement, the induced currents resulting from the vibrations of the voice might be assimilated to those which follow from the approximation and withdrawal of the two parallel spirals, one of which should be traversed by a current. It is this last arrangement which Mr. Bell has adopted as producing the best effects, and it is rare in the history of discoveries that an inventor hits at once on the best arrangement of his instrument.

_Experiments by M. Zetsche._--There are always a few perverse minds, impelled by a spirit of contradiction to deny evidence, and thus they attempt to depreciate a discovery of which the glory irritates them. The telephone and the phonograph have been the objects of such unworthy criticism. It has been said that electric action had nothing to do with the effects produced in the telephone, and that it only acted under the influence of mechanical vibrations transmitted by the conducting wire, just as in a string telephone. It was in vain to demonstrate to these obstinate minds that no sound is produced when the circuit is broken, and in order to convince them M. Zetsche has made some experiments to show, from the mode in which sound is propagated, that it is absurd to ascribe the sound produced in a telephone to mechanical vibration. He wrote to this effect in an article inserted in the ‘Journal Télégraphique,’ Berne, January 25, 1878:

‘The correspondence by telephone between Leipzig and Dresden affords another proof that the sounds which reproduce words at the receiving station are due to electric currents, and not to mechanical vibrations. The velocity with which sound is transmitted by vibrations on the wire, in the case of longitudinal undulations, may be estimated at three miles one furlong a second, so that the sound ought to traverse the distance from Leipzig to Dresden in 25 seconds. The same time ought to elapse before receiving the answer. Consequently there should be an interval of more than three-quarters of a minute allowed for each exchange of communication, which is by no means the case.’

_Experiments which may be made by anyone._--We will conclude this chapter, devoted to the account of the different experiments made with the telephone, by the mention of a singular experiment, which, although easily performed, has only been suggested a few months ago by a Pennsylvanian newspaper. It consists in the transmission of speech by a telephone simply laid on some part of the human body adjacent to the chest. It has been asserted that any part of the body will produce this effect, but according to my experience, I could only succeed when the telephone was firmly applied to my chest. Under such conditions, and even through my clothes, I could make myself heard when speaking in a very loud voice, from which it appears that the whole of the human body takes part in the vibrations produced by the voice. In this case, the vibrations are mechanically transmitted to the diaphragm of the sending telephone, not by the air, but by the body itself acting on the outside of the telephone.

THE MICROPHONE.

The microphone is in fact only the sender of a battery telephone, but with such distinctive characteristics that it may be regarded as an original invention which is entitled to a special name. The invention has lately given rise to an unfortunate controversy between its inventor, Mr. Hughes, and Mr. Edison, the inventor of the carbon telephone and the phonograph--a controversy which has been embittered by the newspapers, and for which there were no grounds. For although the scientific principle of the microphone may appear to be the same as that of Mr. Edison’s carbon sender, its arrangement is totally different, its mode of action is not the same, and the effect required of it is of quite another kind. Less than this is needed to constitute a new invention. Besides, a thorough examination of the very principle of the instrument must make us wonder at Mr. Edison’s claim to priority. He cannot in fact regard as his exclusive possession the discovery of the property possessed by some substances of moderate conductivity of having this power modified by pressure. In 1856, and often subsequently, as for example in 1864, 1872, 1874, and 1875, I made numerous experiments on this point, which are described in the first volume of the second edition of my ‘Exposé des applications de l’Electricité,’ and also in several papers presented to the Académie des Sciences, and inserted in their _Comptes rendus_. M. Clarac again, in 1865, employed a tube made of plumbago, and provided with a moveable electrode, to produce variable resistances in a telegraphic circuit. Besides, in Mr. Edison’s telephonic sender, the carbon disk, as we have seen, must be subjected to a certain initial pressure, in order that the current may not be broken by the vibrations of the plate on which it rests, and consequently the modifications of resistance in the circuit which produce articulate sounds are only caused by greater or less increase and diminution of pressure, that is, by differential actions. We shall presently see that this is not the case with the microphone. In the first place, the carbon contact is effected in the latter instrument on other carbons and not with platinum disks, and these contacts are multiple. In the second place, the pressure exerted on all the points of contact is excessively slight, so that the resistances can be varied in an infinitely greater ratio than in Mr. Edison’s system; and for this very reason it is possible to magnify the sounds. In the third place, a microphone can be made of other substances besides carbon. Finally, no vibrating disk is needed to make the microphone act; the simple medium of air is enough, so that it is possible to work the instrument from some little distance.

We do not therefore see the grounds for Mr. Edison’s assertions, and especially for the way in which he has spoken of Messrs. Hughes and Preece, who are well known in science and are in all respects honourable men. I repeat my regret that Mr. Edison should have made this ill-judged attack on them, since it must injure himself, and is unworthy of an inventor of such distinction. If we look at the question from another point of view, we must ask Mr. Edison why, if he invented the microphone, he did not make us acquainted with its properties and results. These results are indeed startling, since the microphone has in so short a time attracted general attention; and it is evident that the clear-sighted genius of this celebrated American inventor would have made the most of the discovery if it were really his. The only justification for Mr. Edison’s claim consists in his ignorance of the purely scientific discoveries made in Europe, so that he supposed the invention of the microphone to be wholly involved in the principle which he regards as his peculiar discovery.