Scientific American Supplement, March 16, 1895, pages 16009-16010:
Figure 1
    Following the introduction of the telephone, the idea occurred to devise an arrangement by means of which theatrical plays and opera music could be heard at home. It was in 1881, during the Exposition of Electricity, that the first public experiments were made. Since then the expositions of Munich, Vienna, Frankfort, and of Paris in 1881, have had their musical telephonic auditions in turn.
    The success of these first experiments is easily explained, and the more so in that the difficulty of solving the problem was not very great, at least in principle, since it was a question simply of going from one point to another through a special wire connecting the improved apparatus that were already at one's disposal.
    The following were the arrangements adopted at the Paris Exposition of 1881:
    Two halls, in the arrangement of which precautions were taken for deadening external noises, were, each of them, capable of accommodating twenty auditors, who had at their disposal twenty pairs of Ader receivers. The hearing lasted about five minutes. The transmitters were also of the Ader system. They were placed in the auditorium of the opera house, on each side of the prompter's box in front of the footlights, and to the number of ten on each side. Each of these microphone transmitters, supported by a leaden foot and separated from the woodwork by rubber, was actuated by a battery. An induction coil was in the circuit of a double wire line that ended at the receivers. In order to compensate for the difference of intensity of the sound produced by the presence of the singers upon the right or left of the stage, the receivers of a same pair corresponded to two microphones, one to the left and the other to the right of the prompter's box. In this way there was obtained a nearly constant resultant intensity, the right ear being more strongly influenced when the left ear was less so, and vice versa. Figure 2
    The question becomes complicated when it concerns the possibility of putting a series of theaters at the disposal of all the subscribers of the telephone system, and even of those who are not subscribers. Nevertheless, the thing has now been accomplished and the success is complete; and one can hear what is going on in a theater from any point whatever of France or from foreign countries connected with Paris by telephone. Such result is due to the initiative of Messrs. Marinovitch & Szarvady, who have succeeded in forming a company which, since its organization in 1882, has been constantly developing, and has always operated with the greatest regularity.
    This company, for a certain amount of money, installs apparatus at the houses of the subscribers of the Paris telephone system that permit them to hear at home the plays produced at the different theaters. Each series of auditions gives rise to the collection of a fixed sum, whatever be the number of the auditors, the subscribers, moreover, being able to choose from among all the theaters connected with the telephone company, and having the faculty, too, of changing theater during the course of the same evening as many times as they wish. Hearings have likewise been provided for in a certain number of public establishments, such as hotels, restaurants, cafes, etc., by the aid of a special system of lines connecting these various centers in small groups with the central office of the theatrophone company.
    There exists, then, at Paris, for the public establishments, a permanent service of auditions, by the aid of apparatus called theatrophones. For subscribers, hearings are afforded only upon their request. They may book themselves for a definite date, or else, what is most frequently the case, request an audition during the course of an evening.
    Fig. 1, without faithfully reproducing the establishment of the lines just mentioned, nevertheless gives a sufficient idea of it to allow the installation of the system to be understood.
    The central office of the Theatrophone Company is situated at No. 23 Louis-le-Grand Street, in a basement at which end the cables through which all the communications are made. These various cables are distributed between three categories of very distinct lines: (1) Those connecting this central station with the microphones placed in the theaters; (2) those connecting this station with the central office of the administration of telephones, where a communication may be established with all the subscribers of the system in France or foreign countries; and (3) those serving the audition apparatus placed in public establishments. Figure 3
    In each theater, microphone transmitters are arranged in front or behind of or even between the footlights. These apparatus are connected with a special room located near the stage, occupied by an employe of the Theatrophone Company and containing commutators and the microphones that receive the current of from six to eight Leclanche or Lalande and Chaperon elements.
    From thence, according to the size of the theater and the probable number of requests for hearings, start a certain number of lines that at the central office of the company end in a distributing board (Fig. 2) where the different cables that we have before mentioned are arranged in order. We find here the lines that connect the office of the theatrophone with the theaters, the lines designed for the service of public auditions, the subscribers' lines, and, finally, what is called the city line, and which connects the company's office with the central state office for the service communications.
    Starting from the distributing board, the different lines continue through paraffined wires and end at a switchboard comprising annunciators and conjunctors (Fig. 3). This board is completed by a crank commutator connected with repeating telegraphs in number equal to that of the lines of the theatrophones. Let us add that each hearing apparatus is provided with a dial that reproduces the same indications as those of the repeating telegraph connected with the line with which the said apparatus are connected.
    The current utilized for setting these repeaters in operation, as well as the indications of the theatrophone system, is the same as that which supplies the incandescent lamps that serve for lighting. So the posterior face of the switchboard is provided with circuit breakers designed to prevent accidents. This current is derived from the general system of lines of the quarter.
    Two employes are at present necessary for assuring the transmission of the auditions. One of them receives the communications of the subscribers, while the other, seated in front of the board, in proximity to the crank commutator, holds In her hand, as it were, the entire system of the theatrophones. It is her duty to control at a distance the service of the auditions at the houses of private subscribers, and also to change periodically the programme of the public auditions. As the repeating telegraphs reproduce the indications of the dials of the system of lines, the telephonist has merely to glance at her board in order to know what is being heard in such or such a place. Besides, she can control the proper working of the telephones by placing her apparatus (a Berthon-Ader) not directly in the circuit (which would uselessly increase the resistance of the line), but in proximity with it. The play of a lever provided with keys suffices to effect this operation. The telephonist then hears, through induction, feebly, but very clearly, all that is going on upon the line.
    A measuring apparatus completes the installation of the central office and permits a special employe, exclusively charged with this service, to verify at every instant the state of the lines and to make sure that no derivation is intervening to weaken the normal intensity of the auditions. Figure 4
    In the public establishments a portable apparatus called a theatrophone can be installed upon collectors distributed through different halls, and so arranged as to give satisfaction to the demands of the patrons. The apparatus is placed in the circuit by means of a jackknife that enters a jaw provided with metallic contacts and attached to one of the extremities of a flexible cord whose other extremity is fixed to the wall. It is provided with telephone receivers, but remains mute until a coin has been introduced into a slot provided for the purpose.
    In each establishment there is a dial telegraph similar to those that exist above the switchboard of the central office, and these automatic apparatus repeat the indication of the telegraph. As all the dials of the same line of theatrophones are set in operation simultaneously through the maneuver of the crank commutator, they indicate through the position of their needle the nature of the audition of the moment. It is therefore always possible to know what one is going to hear, and as the communications are frequently changed during the course of the same evening, one can wait for the moment in which the theater that he desires to hear is announced. The coin introduced into the apparatus, upon sliding into the money box, automatically establishes a connection between the line and the receiver, and, at the end of a certain time, another ungearing, by causing the coin to slide to the bottom of the box, interrupts the hearing, which the introduction of another coin renews.
    During the moments (which are quite short and rare) in which all the theaters of the line are at the period of between acts a pianist makes himself heard in a hall in the vicinity of the company's central office, and all the lines of the theatrophone being then branched upon this music hall, all the dials give the indication of the piano. There can therefore be no surprise nor interruptions in the auditions.
    The home service of the subscribers is performed very simply, the company being connected with the central telephone office by a large number of lines. It is by the employes of this office that the communication is established between the lines of the subscriber and one of the lines of the theatrophone company. Connection with the theater is made at the office of the company, upon the switchboard of which we have spoken.
    The subscribers can listen to the performances either by means of the receivers of their telephones, or by means of special auditory apparatus that permit several persons to hear simultaneously without the distinctness of the sounds diminishing.
    When a subscriber of the telephone line is connected with the theatrophone company for an audition, the employes of the central offices cut off the communications that are habitually left upon the lines.
    The induction that would greatly decrease the distinctness of the hearing is thus avoided. An Ader relay is interposed in the line at theatrophone company's central office.
    This relay, in consequence of its feeble resistance and the absence of a core of soft iron, in nowise interferes with the telephone transmission. On another hand, it operates with so feeble intensities of current that the subscribers can, by means of the ordinary battery of their line apparatus, directly call Louis-le-Grand Street, whatever be the distance therefrom that they are situated. Figure 5
    The subscriber, thus connected with the theater, has therefore no longer any relation with the line, but, since he can always communicate with the central office of the theatrophone, it is easy for him to obtain thereby any communication that suits him. The telephonist of the theatrophone, moreover, can always have recourse to the intervention of the central telephone office, which she calls up by the city line.
    In order to render it possible to measure at any instant the resistance of the line of a subscriber who is listening, and so rapidly that he may not be aware of it, there is adopted an arrangement that consists in the use of a play of double direction commutators, A1, A2, A3, etc., B, C, D, and a Wheatstone bridge with mirror galvanometer and scale, the arrangement of which is shown in Fig. 4.
    The employe whose business it is to make the measurements has a list of the subscribers, opposite the name of each of which is inscribed the normal resistance of the line, according as it is connected with such or such a theater.
    Let us suppose, for example, that he wishes to verify the state of the line of the subscriber who is listening upon line No. 8. He begins by regulating the variable resistance of his Wheatstone bridge, so as to obtain an equilibrium by the resistance that should be presented by the line of the subscriber who is connected, say this evening, with line No. 8. This done, he places the commutator A3 upon the "open" indication, and then, with his eyes fixed upon the luminous index of the scale in front of him, he connects the line with the Wheatstone bridge for an instant by causing the commutator, D, to play. If everything is in order, the index does not budge during this maneuver. The commutator, C, permits of interposing a pair of receivers in the circuit of the line of the subscriber. As for the commutator, B1, that serves for connecting the Wheatstone bridge with a flexible double conductor cord ending in a springjack. By means of the latter, anyone of the cables ending at the switchboard is connected with the bridge, and all the local measurements that may seem necessary are easily made.
    Let us say, too, a few words concerning the needle telegraphs (Fig. 5) that are used by the theatrophone company and the mechanism of which is both very simple and very ingenious. Upon an axis, a, which carries the needle, is keyed a toothed wheel, b, having a number of teeth equal to that of the divisions of the dial upon which the needle moves. Alongside of this toothed wheel, and mounted loosely upon the same axis with it, there is a soft iron armature, c, which revolves freely, with a very feeble play, between the polar pieces, P P' (hollowed out in semicircular form) of an electromagnet, E. A spiral spring, R, keeps the armature pressed against the fixed contact, m, and brings it back to the position shown in the figure as soon as it is left to itself.
    When a current is made to pass into the electromagnet, E, the armature, C, owing to the initial keying, displaces itself in a direction opposite that of the hands of a watch, so as to embrace as large a number as possible of lines of force of the magnetic field created between the polar pieces, P and P'. In this motion it carries along with it, through the intermedium of the click, d, the toothed wheel, b, and consequently the needle keyed upon the same axis. The amplitude of the angular displacement, which thus affects the entire system, is regulated by the stop, f, which arrests the armature and which, at the same time, chocks the click, d.--Revue Industrielle.