FIG. 58.—LONGITUDINAL SECTION OF BELL TELEPHONE.
Although the Bell telephone is both a transmitter and receiver, in practice a more sensitive and better form of transmitter has taken its place. That most generally used and best known is the “Blake transmitter,” which was brought out about 1880. This employs two important elements. The first is the carbon microphone, which is a means for producing the undulations in the current by the variations in pressure on carbon contacts, and the second is an induction coil operated by a local battery, whose primary circuit passes through the contacts of the carbon microphone, and whose secondary circuit passes over the line. These fundamental elements of the Blake transmitter were the inventions of Berliner and Edison, and were made in 1877. The broad idea of producing electric undulations by varying the pressure between electrodes by vocal vibrations, was a large bone of contention in the Patent Office between various inventors. An application for a patent for the same was filed in the Patent Office by Emile Berliner, June 4, 1877, which was contested in an interference by Gray, Edison, Richmond, Dolbear, Holcombe, Prof. Bell, and others. After fourteen years of litigation the patent was finally awarded to Berliner. The patent granted to him November 17, 1891, No. 463,569, is a valuable one, and has become the property of the American Bell Telephone Company. The application of a low resistance conductor (carbon) in a microphone was invented by Edison as early as 1877, but his patent, No. 474,230, did not issue until May 3, 1892, on account of the interference with Berliner on the broader principle.
FIG. 59.—BLAKE TRANSMITTER.
FIG. 60.—DIAGRAM OF CIRCUITS IN BLAKE TRANSMITTER.
The Blake transmitter takes its name from the inventor of its mechanical features, who has assembled in it the fundamental principles of Berliner and Edison in a sensitive and practical mechanical construction, covered by minor patents, dated November 29, 1881. It is the little box in the middle of the familiar telephone outfit into which the talking is done. Its internal construction is shown in [Fig. 59]. To the rear of the door is secured the cast iron circular ring A, inside of which lies the Russia iron diaphragm B, cushioned at its edges with a rubber band. A circular seat a little larger than the diaphragm is formed in the iron ring, and on this seat the diaphragm rests. A short, thin metal plate attached to the ring A on the right hand side clamps the diaphragm in position by resting squarely on the rubber edge of the diaphragm. Its function is like that of a hinge, which allows the diaphragm to freely swing inward. A steel damping spring is secured to the ring at the opposite edge of the diaphragm, and has its free end provided with a rubber glove on which is cemented a thin piece of fluffy woolen material. The padded end of the damping spring rests against the diaphragm and prevents excessive vibration. The iron ring A has at its bottom a projection holding an adjusting screw, and to a similar top projection is attached by screws a brass spring, from which depends another casting C, supporting the microphone apparatus, which is best shown in the diagram, [Fig. 60]. In this diagram A is one terminal of the battery connected by wire S to the hinge H of the box. From the other leaf of the hinge the wire M passes to K, where it is soldered to the upper end of a German silver spring I. At K this spring is clamped and insulated from the iron work by two pieces of hard rubber. On the lower end of the spring I is soldered a short piece of thick platinum wire, whose ends are rounded into heads, one of which bears against the diaphragm N, and the other against the carbon button J. This button is attached to a small brass weight, and is supported by a spring R, clamped at its upper end to the metal support T. This spring is surrounded its entire length by rubber tubing to deaden vibration. The transmitter is adjusted by screw O, which, acting upon casting T, brings the carbon button, the platinum heads, and also the diaphragm N, against each other with a regulated pressure. The current passes from the part K to the spring I, the platinum head, carbon button J, and its supporting spring R, to metal casting T, and ring V, thence by wire L to the lower hinge G, by wire P to the primary of the induction coil, and thence by wire Y to binding post B, the two binding posts A B being the two battery terminals. The secondary wire E of the induction coil has its ends connected by wires X and W with the two binding posts C B, which are the line terminals, or one the line terminal and the other the ground connection. It will thus be seen that the primary current passes through the transmitter, and the secondary traverses the line. The most familiar forms of the telephone are those seen in [Figs. 61] and [62], but the ideal form is rigged in a cabinet or little room, which excludes all extraneous interfering sounds.