NEW TELEPHONE TRANSMITTER.
BY GEO. M. HOPKINS.
The microphone, with pendants, figured and described by the writer in the SCIENTIFIC AMERICAN of Nov. 16, 1878, was among the earliest of telephone transmitters, and although the device was crude in appearance and exceedingly simple in its construction, it contained the germ of a successful instrument, and was favorably noticed in the scientific papers of Europe.
The transmitter shown in the annexed engraving is based upon the same principle, and, so far as the devices for varying the currents go, it is even simpler than the original micro-phone. Fig. 1 shows the exterior of the instrument, Fig. 2 the interior, Fig. 3 a detail of the transmitter proper, Fig. 4 a sectional view of the receiver, and Fig. 5 is a diagram showing the battery and line connections. Everything, excepting the battery, bell, and receiver, is contained in the box. In the center of the cover is formed the mouthpiece, behind which is placed the diaphragm, consisting of ordinary Russia iron of the thickness commonly used in stove-pipe. It is 2 ¾ inches in diameter, and is held in position in a circular cast iron frame by two springs attached to the frame and pressing the diaphragm. The edge of the diaphragm is bound with soft rubber or felt. This arrangement, however, is not essential to the successful working of this instrument, as equally good results may be obtained when the diaphragm is clamped tightly at the edges between two rings fastened with screws to the front of the box.
To the center of the diaphragm a (see Fig. 3) is attached a metal clamp, b, which supports, in a horizontal position, a cylindrical pencil of hard electric-light carbon, % inch in diameter and 1 inch long. A disk, C, of battery carbon 1% inches in diameter and M. inch thick, is grooved around the edge and wound with fine copper wire, which terminates in a flexible spiral connected with the upper hinge of the box. The carbon disk is suspended by a silk thread from a spool formed on the inner end of a screw extending through the box cover, and capable of being turned so as to raise or lower the carbon disk, as may be required. The disk is slightly
inclined from the perpendicular, and the line of contact be¬tween it and the carbon pencil is a little above the center of gravity of the disk. This arrangement of the two carbons prevents any marked break in the local circuit, as the disk tends to rock on the carbon pencil rather than fly from it when the diaphragm is set in vibration. The carbon disk has been saturated with melted paraffine in some instances with beneficial results.
The clamp which holds the carbon pencil is electrically connected with the lower hinge of the box. From the hinges the connections may be more easily traced in Fig. 5 than in the perspective views.
This diagram shows all of the connections for one end of the line, both ends being alike. The connections are shown in condition to call or receive a call. When a call is received the current passes from the line through the switch, E, button 2, key, bottom or outer contact of the key, bell-magnet and ground wire, to the ground. When the key is depressed to call a distant station, the key touches the in¬ner or top contact, on the battery wire, sending the current through the button 2, switch, E, and line to the bell and ground of the distant station.
The current returns by the ground and wire A, to the battery. After calling, the switch, E, is moved to button 1, and the switch, F being connected with the switch, E, by an insulating connection, is at the same time moved to button 3 as shown in dotted lines. Now the line connection is through the switch, E, button 1, wire, G, secondary wire of the induction coil, and receiver to the ground. The switch, F, when turned as described. completes the local circuit, the current passing from one cell of the battery through the wire, D, switch, F, button 3, transmitter, primary of the induction coil, ground wire, A, and wire, C. The connections are now correct for talking. The diagram shows the connections adapted to the class of transmitter, employing but a single battery element, and to a line requiring several cells of battery to call. If a single cell of battery is sufficient to call, the posts of the wires, B D, will be connected together.
The button which moves the switch extends through the side of the box below the hook upon which the receiving instrument is hung. This arrangement insures the readjustment of the switch after talking, as the receiver cannot be hung up until the switch button is pushed in.
Three layers of No. 18 silk covered wire form the primary of the induction coil, and the secondary consists of some ten or twelve layers of No. 36 silk covered wire.
The receiver, shown in section in Fig. 4, has a diaphragm of the usual size mounted in a hard rubber case 2- inches in internal diameter and 1 inch deep. The bobbin of the usual style is placed on a soft iron core having a large convex head, and held in place by a screw extending through the bottom of the case. A soft rubber button is placed be¬tween the casing and the convex end of the core, and eight curved permanent magnets, one-eighth inch thick and one-quarter inch wide, touch the convex end of the bobbin core and are pressed upward into contact with the diaphragm by a rubber ring at the bottom of the case. The diaphragm at its points of con-tact with the magnets is freed from japan or oxide, and the ends of the magnets are let into notches cut in the case, so that when they press upon the diaphragm the latter is backed by the mouthpiece.
This receiver is very compact and light, and as to efficiency it is all that can be desired.
The transmitter works well, is perfectly simple, requires no particular care in its manufacture, and never gets out of adjustment.
Scientific American March 10, 1881