Clément Ader's experiments with distant telephone listening used dual lines connected to microphones placed on both sides of the stage, with the then-remarkable result that the sounds were heard in stereo, including the novelty that "aural impressions change with the relative positions of the singers, and their movements can in this way be followed".
 
Scientific American, December 31, 1881, pages 422-423:

THE  TELEPHONE  AT  THE  PARIS  OPERA.
Figs. 1 & 2     One of the most popular attractions at the Paris Electrical Exhibition is the nightly demonstration of the marvelous powers of the Ader telephone, by its transmission of the singing on the stage and the music in the orchestra of the Grand Opera at Paris, to a suite of four rooms reserved for the purpose in one of the galleries of the Palais de l'Industrie. This demonstration is given nightly between eight and eleven o'clock, and the enormous number of people who crowd the entrance to the building before the doors are open to the evening visitors rapidly resolve themselves into patient queues as soon as they can obtain access to the gallery adjoining the telephone rooms. There they patiently await their time for admission, and the privilege of hearing for a few minutes whatever may be going on at the opera--solo, chorus, instrumental music, or possibly all three, until the allotted time has expired, and the listeners have to give way for a fresh installment from the outside. In this way eighty telephones are constantly at work at the same time, at short intervals the communication being shifted to another set of eighty similar instruments in two other rooms. It may be remarked in passing that this distant audience of the performance at the opera enjoy their allotted moments of actual transmission and that interludes do not count. Certainly nothing has ever been done before so effectually to popularize science, and to render the masses familiar with the effect, however ignorant they may be of the cause, of this marvelous invention, the first feeble voice of which was heard in the Centennial Exhibition of 1876. Our contemporary, L'Electricien, publishes this week an excellent description of the installation at the opera and in the Exhibition, and from this we gather our information and illustrations on the subject. Fig. 4
    The transmitters are microphones on the Ader system, placed in front of the opera stage, close to the footlights and behind them. Figs. 1 and 2 are a plan and longitudinal section of one of these transmitters. Each consists of ten small carbon pencils, A A, arranged in two series of five each, and supported by three cross pieces, B C D, fixed to a small pine board, which receives the vibration and serves as a cover to the instrument This board rests, as shown, in a massive block of lead, P, which in its turn is supported on four blocks of soft rubber. This arrangement is found to prevent any vibrations of the stage from being transmitted to the microphones, and the only movements taken up by the instrument are the sonorous vibrations of the air. The microphone is in connection with a Leclanché battery, and the wire of a small induction coil without any condenser. The line, laid in double wire, is connected on the one hand with the induction coil, and on the other with a series of telephone receivers placed in the rooms at the Palais de l'Industrie. There are eight receivers thus coupled to each transmitter. The undulatory induction currents developed in the fine wire of the induction coil by the variation in intensity of the current traversing the induction wire, react on the receiver. There are ten such installations as we have just described on the stage of the opera, each with its own battery and induction coil, and double line to the Exhibition. As the batteries become rapidly polarized, two sets are provided for each transmitter, and the batteries are shifted every fifteen minutes by a commutator. Fig. 3 is a diagram showing the arrangement, the transmitters being numbered one to ten; the batteries are shown at P, the induction coil at B, and the receivers in connection are marked A to H. Only two complete circuits are shown to avoid confusion.
    The Ader receiving telephone, shown in Fig. 4, is well known; it is a magneto-electric instrument, the magnet of which is formed into a ring so as to serve as a handle (see A, Fig. 4). The two cores, B B, are attached to the poles, and have wires coiled round them; a soft iron ring, F F, is placed over the poles, and in front of the diaphragm. The object of this ring is to serve as a supplementary excitor, and its object is to give to the lines of magnetic force a direction perpendicular instead of divergent to the diaphragm; by this arrangement the variations produced in the magnet by the induction currents of the coils have a maximum effect on the diaphragm; it is to this arrangement that the clearness of definition of the Ader telephone is due. Fig. 3
    M. Hospitaller, in the article from which we are drawing our information, refers to a peculiar property of the Ader telephone which we cannot do better than deal with in his own words: "We will now consider the new acoustic effect which Mr. Ader has discovered, and applied for the first time in the telephonic transmission at the Electrical Exhibition. Every one who has been fortunate enough to hear the telephones at the Palais de l'Industrie has remarked that, in listening with both ears at the two telephones, the sound takes a special character of relief and localization which a single receiver cannot produce. It is a common experience that, in listening at a telephone, it is practically impossible to have even a vague idea of the distance at which the person at the other end of the line appears to be. To some listeners this distance seems to be only a few yards. To others the voice apparently proceeds out of a great depth of the earth. In this case there is nothing of the kind. As soon as the experiment commences the singers place themselves, in the mind of the listener, at a fixed distance, some to the right and others to the left. It is easy to follow their movements, and to indicate exactly, each time that they change their position, the imaginary distance at which they appear to be. This phenomenon is very curious, it approximates to the theory of binauriclar auduition, and has never been applied, we believe, before to produce this remarkable illusion to which may almost be given the name of auditive perspective. Having explained this phenomenon, we may consider its cause, which is a very simple one. In order to realize it, we may recall the stereoscope, which allows us to see objects in their natural relief. A similar effect is produced to the ear, and may be explained by referring to Fig. 3. Each person is placed in front of a transmitter with two telephones, which receive the impression from two distinct transmitters, placed a certain distance apart. These transmitters are grouped in pairs, l and 6, 2 and 7, 3 and 8, 4 and 9, and 5 and 10. Fig. 3 shows the complete arrangement for group 1 and 6. This group supplies sixteen telephones adapted for eight listeners, but the transmitter 1 serves the eight telephones on the left, and the transmitter 6 the eight telephones on the right of the eight listeners, A, B, C, to H. When the singer is at the point A, the transmitter 1 is more strongly influenced than the transmitter 6; the left ear is, therefore, more deeply impressed than the right ear, and the singer appears to be on the left to the eight listeners of the group. When the singer is at A, the transmitter 6 is more affected than the transmitter 1, and the singer appears to the right of the audience; these aural impressions change with the relative positions of the singers, and their movements can in this way be followed."
    The use of the double conducting wire has been necessary to obviate the effect of induction, and in this respect it has been entirely successful, although of course it increases the cost of installation.
    It may be interesting to note that experiments have been made to connect the Théatre Français with the Exhibition, but up to the present time these have not been successful, chiefly owing to the fact that the footlights create a powerful upward current and interfere with the vibrations to the transmitters. At the opera the footlights are closed at the top, and are burnt with a powerful down draught.--Engineering.


One of the telephone listening rooms at the 1881 Paris Electrical Exhibition. Listeners who held telephone receivers to both ears heard the theater program in stereo:
Telephone Listening Room
[Illustration from "The International Exhibition and Congress of Electricitiy at Paris", Nature, October 20, 1881, page 587.]