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ing the electrical connections between the leading-in wires and the magnet windings. In this respect the Acme double-pole, or bipolar, receiver shown in Fig. 5 differs, it being possible to examine all of the interior parts of this receiver by simply unscrewing the diaphragm cap and withdrawing the magnet from the body of the case. It will be noticed the receiver cord m is led through the case e, and the cord tips are secured to binding posts between the magnets. One of the binding posts is shown at c, and at s is one of the wires leading to the magnet coils a and v. Strain on the connections is prevented by the extra cord f, tied around the bend in the magnet bar d. Attention is also called to the small perforations b in the diaphragm cap, instead of an unobstructed opening, the object being to afford more ample protection to the diaphragm from pencils, etc., which are sometimes maliciously used to bend it out of shape.
The Head Receiver is a simple modification of the bipolar hand receiver. It is designed chiefly for switchboard operators and others who have almost constant use for a receiver or who desire the free use of both hands while listening. As seen from the head receiver shown in Fig. 6, it is very small and light, and is provided with a band b of spring steel covered with leather which fits over the head and holds the receiver against the ear. Fig. 7 shows the interior construction, which differs from other receivers only in that the body of the instrument, and therefore the magnet, are
shorter. The case is usually of brass or aluminum, and, being about the size of a watch, these instruments are often called “watch-case” receiv
Fig. 7.—A View of the Interior of a Watch-Case or Head
ers. Their entire weight is generally not over six ounces.
The Transmitter.—The magnet telephone as a transmitting instrument was a failure. Although so sensitive that but 3-100,000,000,000 watt (see Appendix) is sufficient to make it produce an audible sound as a receiver, it lacked the qualifications necessary for a good transmitter. After numerous experiments, transmitters employing carbon in one form or another were found most
Fig. 8.-Simple Telephone Circuit for Transmitting in
One Direction satisfactory, and to-day none but carbon transmitters are used for distances over a few hundred feet.
The Principle of the Carbon Transmitter is based upon the fact that finely ground or pulverized carbon or graphite varies in electrical resistance according to the mechanical pressure brought to bear upon it. Under high pressure the resistance of carbon is low, but under a low pressure its resistance is comparatively high. If, therefore, at the transmitting end of a line, carbon of one form or another, as shown at c in Fig. 8, be placed in series with a battery b and an induction coil ps, sound waves of a voice striking against the diaphragm d will cause varying pressures in the carbon and consequently a pulsating current in the circuit in which the battery and primary winding p of the induction coil are connected. The variations of the pulsating current will be in harmony with those of the voice, and will induce in the secondary winding s of the induction coil an alternating current which will have a frequency corresponding to that of the sound waves at the diaphragm of the transmitter. This alternating current will cause the distant receiver r to act as previously described and to reproduce the sounds of the voice at the transmitter. The form in which the carbon is used and the method of mounting it have given rise to many different kinds of carbon transmitters. Only a few of the more common forms will here be considered.
The Edison Transmitter employs the carbon in the form of a block as shown at s. Fig. 9. A metal casing c incloses the carbon, but is insulated from it except on the side r. The casing c is held within the metal chamber h by the bolt d. The circuit through the instrument from the binding post a, which is insulated from the chamber h by a hard-rubber bushing, runs by the conductor , to the right-hand side of the carbon block s, thence through the carbon to the uninsulated side and out through the bolt d, chamber h, and binding post v, which is in electrical contact with h.
Sound waves entering the mouthpiece 1 strike the diaphragm and cause it to vibrate. The vi
brations thus produced are transmitted to the carbon block s through an ivory button u and the side of the casing c, and cause a varying pressure on the carbon. This varying pressure, as previously explained, causes a varying resistance and therefore a varying current in the transmitter cir
Fig. 9.—The Edison Transmitter
cuit, the variations of the current corresponding to those of the sound waves. The Edison transmitter is not a very sensitive instrument, lacking in flexibility between the ivory button and the carbon plate.
The Blake Transmitter, which now is not generally in use, but is mentioned here to illustrate