ESTABLISHMENT OF A TELEPHONIC STATION.
Although the telephonic system of telegraphy is very simple, yet certain accessory arrangements are indispensable for its use. Thus, for example, an alarum call is necessary, in order to know when the exchange of correspondence is to take place, and information that the call has been heard is likewise necessary. An electric bell is therefore an indispensable addition to the telephone, and since the same circuit may be employed for both systems, if a commutator is used, it was necessary to find a mode of making the commutator act automatically, so as to maintain the simple action of the system which constitutes its principal merit.
MM. Pollard and Garnier’s System.—With this object, MM. Pollard and Garnier devised a very successful arrangement last March, which employs the weight of the instrument to act upon the commutator.
For this purpose, they suspended the instrument to the end of a spring plate, fastened between the two contacts of the commutator. The circuit wire corresponds with this plate, and the two contacts correspond, the one with the telephone, the other with the bell. When the telephone hangs below the spring-support, that is, when it is not at work, its weight lowers the spring plate on the lower contact, and the communication of the line with the bell is established: when, on the other hand, the telephone is raised for use, the spring plate touches the higher contact, and communication is established between the line and telephone. In order to make the bell sound, it is only necessary to establish, on the wire which connects the line with the bell contact of the commutator, a breaker which can both join and break the current, and which communicates on one side with the contact of the bell, and on the other with its battery. The ordinary push of an electric bell will be sufficient, if it is supplied with a second contact, but MM. Pollard and Garnier wished to make this action also automatic, and consequently they devised the arrangement represented in fig. 53.
In this system, as well as in those which have since been devised, two telephones are employed, one of which is constantly applied to the ear, and the other to the mouth, so as to make it possible to speak while listening. The telephones are supported by three wires, two of which contain flexible conductors, while the third only acts as a support.
Two of the four wires of the two telephones are connected with each other, and the other two are connected with the two binding screws of the commutator t, t′: the wires without conductors are suspended to the extremities of the two flexible plates l, l′, which correspond with earth and line.
When at rest, the weight of the telephones presses the two plates l, l′, on the lower contacts S, S′, but when the instruments are taken up these plates press against the higher contacts.
The two bell wires terminate on the lower contacts, those of the telephones on the higher contacts, and one of the poles of the battery is connected with the lower contact on the left S′, the other with the higher contact on the right T.
When at rest, the system is applied to the electric bell, and the current sent from the opposite station will follow the circuit L l S S′ S′ l′ T, so that the call will be made. On taking up the two telephones, the circuit of the bell system is broken, and that of the telephones is established, so that the current follows the course L l T t t′ T′ l′ T. If only one telephone is held at a time, the current is sent into the bell system of the opposite station, and follows the route + P S l L T l′ T′ t P —. In this way the three actions necessary for calling, corresponding, and enabling the corresponding instrument to give a call, are almost involuntarily made.
System by MM. Bréguet and Roosevelt.—In the system established by the Paris agents of the Bell company, the arrangement resembles the one just described, except that there is only one spring commutator, and the call is made with the push of an ordinary electric bell. A mahogany board is suspended from the wall, and on it are arranged, first, the ordinary electric bell system, with a sending push fixed below it; second, two forks supporting two telephones, one of which is fastened to the bar of a commutator, arranged as a Morse key. The two telephones are connected by two conducting wires, so arranged as to be capable of extension, and two of their four binding screws are in immediate connection with each other, and the other two with the earth, line, and battery, by means of the commutator, the sending push, and the bell system. The arrangement is shown in fig. 54.
The commutator A consists of a metallic bar a c, bearing the suspension fork of one of the telephones F′ below its point of articulation: it ends in two pins a and c, below which the two contacts of the commutator are fixed, and a spring compresses the lower arm of the bar, so as to cause the other arm to rest constantly on the higher contact. For greater security a steel tongue a b is fastened to the lower end of the bar, and rubs against the small shaft b, which is provided with two insulated contacts, corresponding to those of the board. The bar is in communication with the line wire by means of the call-push, and the upper of the two contacts we have just described corresponds with one of the telephone wires which is inserted in the same circuit, while the other corresponds with the bell system S, which is in communication with earth. It follows from this arrangement, that when the right telephone presses its whole weight on the support, the bar of the commutator is inclined on the lower contact, and consequently the line is in direct communication with the bell, so that the call can be made. When, on the other hand, the telephone is removed from its support, the bar rests on the higher contact, and the telephones are connected with the line.
Pressure on the sending push serves to call the corresponding station: the connection of the line with the telephones is then broken, and it is established with the battery of the sending station, which sends its current through the bell of the corresponding station. In order to obtain this double effect, the contact spring of the sending push generally rests upon a contact fastened to a piece of wood shaped like a joiner’s rule, which covers it in front, and below this spring there is a second contact, which communicates with the positive pole of the station battery. The other contact corresponds with the line wire, and a connection takes place between the earth wire and the negative pole of the station battery, so that the earth wire is common to three circuits:
1st. To the telephone circuit. 2nd. To that of the bell system. 3rd. To that of the local battery.
The second fork, which supports the telephone on the right, is fixed to the board, and is independent of any electric current.
It is clear that this arrangement may be varied in a thousand ways, but the model we have just described is the most practical.
Edison’s System.—The problem becomes more complex in the case of battery telephones, since the battery must be common to both systems, and the induction coil must be inserted in two distinct circuits. Fig. 55 represents the model adopted in Mr. Edison’s telephone.
In this arrangement, there is a small stand C on the mahogany board on which the bases of the two telephones rest. The bell system S is worked by an electro-magnetic speaker P, which serves, when a Morse key is added to the system, for exchange of correspondence in the Morse code, if there should be any defect in the telephones, or to put them in working order. Above the speaker there is a commutator with a stopper D to adapt the line for sending or receiving, with or without the bell; and below the stand C the induction coil, destined to transform the voltaic currents into induced currents, is arranged in a small closed box E.
When the commutator is at reception, the line is in immediate correspondence either with the speaker or with the receiving telephone, according to the hole in which the stopper is inserted; when, on the other hand, it is at sending, the line corresponds to the secondary circuit of the induction coil. Under these conditions the action is no longer automatic; but since this kind of telephone can only be usefully employed for telegraphy, in which case those who work it are acquainted with electric apparatus, there is no inconvenience in this complication.