EXTERNAL INFLUENCE ON TELEPHONIC TRANSMISSIONS.
The obstacles which occur in telephonic transmissions proceed from three causes: 1. The intensity of sound is diminished by the loss of current in transmission—a loss which is much greater in the case of induced currents than in those received from a battery. 2. Confusion is caused by the influence of adjacent currents. 3. The induction from one wire to another. This last influence is much greater than is usually supposed. If two perfectly insulated wires are placed side by side, one in communication with the circuit of an electric bell, and the other with the circuit of a telephone, the latter will repeat the sounds of the bell with an intensity often great enough to act as a call without applying the instrument to the ear. MM. Pollard and Garnier, in their interesting experiments with the induced currents of the Ruhmkorff coil, have ascertained that in this way not merely sounds may be obtained which correspond with the induced currents resulting from the action of the primary current, but also those which result from the action of the secondary current on other helices, which are termed currents of the second order. These different reactions frequently cause the telephonic transmissions made on telegraphic lines to be disturbed by irregular sounds, arising from the electric transmissions on adjoining lines; but it does not appear that these influences altogether neutralise each other, so that conversation held in the ordinary way and a message sent in the Morse code may be heard simultaneously.
At the Artillery School, Clermont, a telephonic communication has been established, for the sake of experiments, between the school and the butts, which are at a distance of about eight miles. Another communication of the same kind has been established between the Clermont Observatory and the one at Puy-de-Dôme, which is nearly nine miles from the former. These two lines are carried on the same posts for a course of six miles, together with an ordinary telegraphic wire, and for a distance of 330 yards there are seven other such wires. The two telephonic wires are separated from each other by a space of 85 centimètres. The following facts have been observed under these conditions.
1. The school telephone is perfectly able to read off from their sound the Morse messages which pass through the two adjacent telegraph wires, and the ticking of the instrument does not at all interfere with the vocal communication of the telephone, nor render it inaudible.
2. The two adjacent telegraphic lines, although not in contact, confuse their messages together, and it has sometimes been possible to hear messages from Puy-de-Dôme at the school through the wire which runs to the butts, although the distance between the two lines is nowhere less than 85 centimètres.
These inconveniences have been in some degree remedied by inserting strong resistances in the circuit, or by putting the current to earth at some distance from the telephonic stations.
M. Izarn, Professor of Physics at the Lycée, Clermont, holds that telephonic electric currents may readily be turned aside by the earth, especially if in the course of their passage they encounter metallic conductors, such as gas or water pipes. He writes as follows on the subject, in a paper addressed to the Académie des Sciences, on May 13, 1878:—‘I set up a telephone in the Clermont Lycée with a single wire, more than 50 yards in length, which crosses the court-yard of the Lycée, and goes from the laboratory, where it is suspended to a gas-burner, to a room near the porter’s lodge, where it is suspended to another gas-burner. When I applied my ear to the telephone, I could distinctly hear the telegraphic signals, Morse or otherwise, which came either from the telegraph office at Clermont, or from the telephone office which was at work between the School of Artillery and the butts below Puy-de-Dôme, a distance of eight miles. I could overhear words, and especially the military orders issued at the butts for the purpose of being heard at the school. Yet my wire is perfectly independent of those used for signalling, and is even very remote from them; but as the wires of the telegraph office and of the School of Artillery go to earth at a little distance from the gas-pipes, it is probable that this phenomenon is caused by a diversion of the current produced in my wire, by means of the earth and the network of metal pipes.’
Mr. Preece made the same remark in his notice of ‘some physical points connected with the telephone.’ Again, we read in the ‘Telegraphic Journal’ of June 15, 1878, that in a telephonic concert transmitted from Buffalo to New York, the singers at Buffalo were heard in an office placed outside the telegraphic circuit in which the transmission was effected. On enquiry, it was ascertained that the wire through which the telephonic transmission took place, was at one point in its course close to the one which directly transmitted the musical sounds, but the distance between the two wires was not less than ten feet.
When the circuits are altogether metallic, there is much less risk of confusion, and M. Zetzche declares that sounds proceeding from other wires are in this case little heard, and then only momentarily, so that it is much more easy to hear with this arrangement than with the one in ordinary use. ‘It is not,’ he says, ‘the resistances of the wire, but rather the diversions of the current near the posts, which interfere with telephonic correspondence on long lines above ground. This was proved by the following experiments:—I connected the telegraphic line from Dresden to Chemnitz with a line from Chemnitz to Leipzig (54 miles), which made a circuit of 103 miles, going to earth at its two extremities. There was no communication between Dresden and Leipzig, but Leipzig and Dresden could communicate with ease, in spite of the greater extent of line. I broke the connection with earth, first at Leipzig, then simultaneously at Leipzig and Dresden, and I observed the following effects. When insulation took place at Leipzig only, the telephone could be heard at the stations of Dresden, Riesa, and Wurzen; when the line was insulated at both ends, the communication was good between the two latter stations, but it was observed that at the intermediate station the words spoken at Wurzen were more distinctly heard than the words spoken at Riesa were heard at Wurzen. Since the distance from Wurzen to Leipzig is little more than half that from Riesa to Dresden, there are consequently nearly twice as many posts on the latter line, which carry the currents to earth, and hence I conclude that these diversions of current explain the possibility of conversing on an insulated line, and also why sounds are more distinctly heard at the Riesa station in consequence of the greater intensity of current still remaining on the line.’
Some vibrations also result from the action of currents of air on telegraphic wires, which produce the humming sound so well known on some lines, and these may also react on the telephone; but they are in this case generally mechanically transmitted, and they may be distinguished from the others, if the sounds which ensue are heard after the telephone is excluded from the circuit by a break with a short circuit and after the communication to earth established behind the telephone has been broken.
The induced reactions caused by the line wires on each other are not the only ones which may be observed on a telephonic circuit: every manifestation of electricity near a telephone may produce sounds of greater or less force. Of this we have already given a proof in M. d’Arsonval’s experiments, and others by M. Demoget demonstrate the fact still more clearly. In fact, if a small bar magnet provided with a vibrator be placed before one of the telephones of a telephonic circuit, and the vibrating plate of the telephone be removed, in order to draw away the sound produced by the vibrator, its humming noise may be distinctly heard on the second telephone of the circuit; a noise which attains its maximum when the two extremities of the electro-magnet are at their nearest point to the telephone without a diaphragm, and it is at its minimum when this electro-magnet is presented to it along its neutral line. M. Demoget supposes that the action which is exerted in this instance is that of a magnet exerting two inducing actions which are opposite and symmetrical, with a field limited by a double paraboloid and with an axis, according to his experiments, which extended 55 centimètres beyond the magnetic core, and a vertical diameter of 60 centimètres. He believes that in this way it would be easy to telegraph on the Morse system, and that, in order to do so, it would only be necessary to apply a key to the inducing electro-magnet.
Mr. Preece points out three ways of overcoming the difficulty presented by the induced reactions caused by the wires on each other.
1. By increasing the intensity of the transmitted currents, so as to make them decidedly stronger than the induced currents, and to reduce the sensitiveness of the receiving telephone.
2. To place the telephonic wire beyond the range of induction.
3. To neutralise the effects of induction.
The first mode may be effected by Edison’s battery system, and we have seen that it is very successful.
In order to put the second mode in practice, Mr. Preece says that it would be necessary to study the two kinds of induction which are developed on telegraphic lines: electro-static induction, analogous to that produced on submarine cables, and electro-dynamic induction, resulting from electricity in motion. In the former case, Mr. Preece proposes to interpose between the telephone wire and the other wires a conducting body in communication with the earth, capable of becoming a screen to the induction by itself absorbing the electro-static effects. He says that this might be accomplished by surrounding the telegraphic wires adjacent to the telephonic wire with a metallic envelope, and then plunging them in water. He adds that the effects of static induction are not completely destroyed in this way, since the substance used is a bad conductor, but they are considerably reduced, as he has proved by experiments between Dublin, Holyhead, Manchester, and Liverpool. In the second case, Mr. Preece admits that an iron envelope might paralyse the electro-dynamic effects produced by absorbing them, so that if insulated wires were employed, covered with an iron case, and communicating with the earth, the two induced reactions would be annulled. We will not follow Mr. Preece in his theory as to these effects—a theory which seems to us open to question, but we content ourselves with pointing out his proposed mode of attenuation.
In order to carry out the third expedient, it might be thought that it would be enough to employ a return wire instead of going to earth, for under such conditions the currents induced on one of the wires would be neutralised by those resulting from the same induction on the second wire, which would then act in an opposite direction; but this mode would only be successful when there is a very small interval between the two telephone wires, and they are at a considerable distance from the other wires. When this is not the case, and they are all close together, as in submarine or subterranean cables, consisting of several wires, this mode is quite inefficient. A small cable, including two conductors, insulated with gutta-percha, may be successfully carried through the air.
The use of two conductors has the further advantage of avoiding the inconvenience of stray currents on the line and through the earth, which, when the communications to earth are imperfect, permit the line current to pass more or less easily into the telephonic line.
In addition to the disturbing causes in telephonic transmission we have just mentioned, there are others which are also very appreciable, and among them are the accidental currents which are continually produced on telegraphic lines. These currents may proceed from several causes, at one time from atmospheric electricity, at another from terrestrial magnetism, at another from thermo-electric effects produced upon the lines, at another from the hydro-electric reactions produced on the wires and disks in communication with the earth. These currents are always very unstable, and consequently they are likely, by reacting on the transmitted currents, to modify them so as to produce sounds upon the telephone. Mr. Preece asserts that the sound proceeding from earth currents somewhat resembles that of falling water. The discharges of atmospheric electricity, even when the storm is remote, produce a sound which varies with the nature of the discharge. When it is diffused and the clap takes place near at hand, Dr. Channing, of Providence, U.S., says that the sound resembles that produced by a drop of fused metal when it falls into water, or, still more, that of a rocket discharged at a distance: in this case it might seem that the sound would be heard before the appearance of the flash, which clearly shows that the electric discharges of the atmosphere only take place in consequence of an electric disturbance in the air. Mr. Preece adds that a wailing sound is sometimes heard, which has been compared to that of a young bird, and which must proceed from the induced currents which terrestrial magnetism produces in the metallic wires when placed in vibration by currents of air.
M. Gressier, in a communication made to the Académie des Sciences on May 6, 1878, has spoken of some of these sounds, but he is totally mistaken in the source to which he ascribes them.
‘In addition to the crackling sound caused by the working of telegraph instruments on the adjacent lines, a confused murmur takes place in the telephone, a friction so intense that it might sometimes be thought that the vibrating disk was splitting. This murmur is heard more by night than by day, and is sometimes intolerable, since it becomes impossible to understand the telephone, although nothing is going on in the office to disturb the sound. The same noise is heard when only one telephone is used. A good galvanometer inserted in the circuit reveals the presence of sensible currents, sometimes in one direction, sometimes in another.’
I studied these currents for a long time with the galvanometer, and made them the subject of four papers which were laid before the Académie des Sciences in 1872, and I am convinced that they have in general nothing to do with atmospheric electricity, but result either from thermo-electric or hydro-electric influence. They take place constantly and in all weathers on telegraph lines, whether these lines are insulated at one end, or in contact with the earth at both ends. In the first case, the polar electrodes of the couple are formed by the telegraph wire and the earth plate, generally of the same nature, and the intermediate conducting medium is represented by the posts which support the wire and the earth which completes the circuit. In the second case, the couple is formed in almost the same way, but the difference in the chemical composition of the ground at the two points where the earth plates are buried, and sometimes their different temperature, exert a strong influence. If only the first case be considered, it generally happens that on fine summer days the currents produced during the day are inverse to those which are produced by night, and vary with the surrounding temperature in one or the other direction. The presence or absence of the sun, the passage of clouds, the currents of air involve abrupt and strongly marked variations, which may be easily followed on the galvanometer, and which cause more or less distinct sounds in the telephone.
During the day, the currents are directed from the telegraph line to the earth plate, because the heat of the wire is greater than that of the plate, and these currents are then thermo-electric. During the night, on the other hand, the wire is cooled by the dew, which causes a greater oxidation on the wire than that which takes place on the plate, and the currents then become hydro-electric.
I say more about these currents because, in consequence of a mistaken belief as to their origin, it has been supposed that the telephone might serve for the study of the variations of the atmospheric electricity generally diffused through the air. Such an application of the telephone would, under these conditions, be not only useless, but also misleading, by inducing the study of very complex phenomena, which could lead to nothing more than I have already stated in my different papers on the subject.
Certain local influences will also produce sounds in the telephone. Thus the distension of the diaphragm by the moist heat of the breath, when the instrument is held before the mouth in speaking, causes a perceptible murmur.
From the electro-static reactions, so strongly produced on the submarine cables, in consequence of electric transmissions, it might be supposed that it would not be easy to hold telephonic correspondence through this kind of conductor, and, to ascertain the fact, an experiment was made on the cable between Guernsey and Dartmouth, a distance of sixty miles. Articulate speech, only a little indistinct, was, however, perfectly transmitted. Other experiments, made by Messrs. Preece and Wilmot, on an artificial submarine cable, placed in conditions analogous to those of the Atlantic cable, showed that a telephonic correspondence might be kept up at a distance of a hundred miles, although the effects of induction were apparent. At the distance of 150 miles, it was somewhat difficult to hear, and the sounds were very faint, as if some one were speaking through a thick partition. The sound diminished rapidly until the distance of 200 miles was reached, and after that it became perfectly indistinct, although singing could still be heard. It was even possible to hear through the whole length of the cable, that is, for 3,000 miles, but Mr. Preece believed this to be due to the induction of the condenser on itself: he holds, however, that singing may be heard at a much greater distance than speech, owing to the more regular succession of electric waves.
Mr. Preece also made experiments on the subterranean telegraphs between Manchester and Liverpool, a distance of 30 miles, and found no difficulty in exchanging correspondence; and it was the same with the cable from Dublin to Holyhead, a distance of 67 miles. This cable had seven conducting wires, and when the telephone was connected with one of them, the sound was repeated through all the others, but in a fainter degree. When the currents of the telegraphic instruments passed through the wires, the induction was apparent, but not so great as to prevent telephonic communication.