Edison, His Life and Inventions [326]
therein, thus causing corresponding variations in the strength of the battery current. These variations, passing through the chalk cylinder produced more or less electrochemical decomposition, which in turn caused differences of adhesion between the pen and cylinder and hence gave rise to mechanical vibrations of the diaphragm by reason of which the speaker's words were reproduced. Telephones so operated repeated speaking and singing in very loud tones. In one instance, spoken words and the singing of songs originating at a distance were heard perfectly by an audience of over five thousand people.
The loud-speaking telephone is shown in section, diagrammatically, in the sketch (Fig. 3), in which A is the chalk cylinder mounted on a shaft, B. The palladium-faced pen or spring, C, is connected to diaphragm D. The instrument in its commercial form is shown in Fig. 4.
VI
THE TELEPHONE
ON April 27, 1877, Edison filed in the United States Patent Office an application for a patent on a telephone, and on May 3, 1892, more than fifteen years afterward, Patent No. 474,230 was granted thereon. Numerous other patents have been issued to him for improvements in telephones, but the one above specified may be considered as the most important of them, since it is the one that first discloses the principle of the carbon transmitter.
This patent embodies but two claims, which are as follows:
"1. In a speaking-telegraph transmitter, the combination of a metallic diaphragm and disk of plumbago or equivalent material, the contiguous faces of said disk and diaphragm being in contact, substantially as described.
"2. As a means for effecting a varying surface contact in the circuit of a speaking-telegraph transmitter, the combination of two electrodes, one of plumbago or similar material, and both having broad surfaces in vibratory contact with each other, substantially as described."
The advance that was brought about by Edison's carbon transmitter will be more apparent if we glance first at the state of the art of telephony prior to his invention.
Bell was undoubtedly the first inventor of the art of transmitting speech over an electric circuit, but, with his particular form of telephone, the field was circumscribed. Bell's telephone is shown in the diagrammatic sectional sketch (Fig. 1).
In the drawing M is a bar magnet contained in the rubber case, L. A bobbin, or coil of wire, B, surrounds one end of the magnet. A diaphragm of soft iron is shown at D, and E is the mouthpiece. The wire terminals of the coil, B, connect with the binding screws, C C.
The next illustration shows a pair of such telephones connected for use, the working parts only being designated by the above reference letters.
It will be noted that the wire terminals are here put to their proper uses, two being joined together to form a line of communication, and the other two being respectively connected to "ground."
Now, if we imagine a person at each one of the instruments (Fig. 2) we shall find that when one of them speaks the sound vibrations impinge upon the diaphragm and cause it to act as a vibrating armature. By reason of its vibrations, this diaphragm induces very weak electric impulses in the magnetic coil. These impulses, according to Bell's theory, correspond in form to the sound-waves, and, passing over the line, energize the magnet coil at the receiving end, thus giving rise to corresponding variations in magnetism by reason of which the receiving diaphragm is similarly vibrated so as to reproduce the sounds. A single apparatus at each end is therefore sufficient, performing the double function of transmitter and receiver. It will be noticed that in this arrangement no battery is used The strength of the impulses transmitted is therefore limited to that of the necessarily weak induction currents generated by the original sounds minus any loss arising by reason of resistance in the line.
Edison's carbon transmitter overcame this vital or limiting weakness by providing for independent power on the transmission circuit,
The loud-speaking telephone is shown in section, diagrammatically, in the sketch (Fig. 3), in which A is the chalk cylinder mounted on a shaft, B. The palladium-faced pen or spring, C, is connected to diaphragm D. The instrument in its commercial form is shown in Fig. 4.
VI
THE TELEPHONE
ON April 27, 1877, Edison filed in the United States Patent Office an application for a patent on a telephone, and on May 3, 1892, more than fifteen years afterward, Patent No. 474,230 was granted thereon. Numerous other patents have been issued to him for improvements in telephones, but the one above specified may be considered as the most important of them, since it is the one that first discloses the principle of the carbon transmitter.
This patent embodies but two claims, which are as follows:
"1. In a speaking-telegraph transmitter, the combination of a metallic diaphragm and disk of plumbago or equivalent material, the contiguous faces of said disk and diaphragm being in contact, substantially as described.
"2. As a means for effecting a varying surface contact in the circuit of a speaking-telegraph transmitter, the combination of two electrodes, one of plumbago or similar material, and both having broad surfaces in vibratory contact with each other, substantially as described."
The advance that was brought about by Edison's carbon transmitter will be more apparent if we glance first at the state of the art of telephony prior to his invention.
Bell was undoubtedly the first inventor of the art of transmitting speech over an electric circuit, but, with his particular form of telephone, the field was circumscribed. Bell's telephone is shown in the diagrammatic sectional sketch (Fig. 1).
In the drawing M is a bar magnet contained in the rubber case, L. A bobbin, or coil of wire, B, surrounds one end of the magnet. A diaphragm of soft iron is shown at D, and E is the mouthpiece. The wire terminals of the coil, B, connect with the binding screws, C C.
The next illustration shows a pair of such telephones connected for use, the working parts only being designated by the above reference letters.
It will be noted that the wire terminals are here put to their proper uses, two being joined together to form a line of communication, and the other two being respectively connected to "ground."
Now, if we imagine a person at each one of the instruments (Fig. 2) we shall find that when one of them speaks the sound vibrations impinge upon the diaphragm and cause it to act as a vibrating armature. By reason of its vibrations, this diaphragm induces very weak electric impulses in the magnetic coil. These impulses, according to Bell's theory, correspond in form to the sound-waves, and, passing over the line, energize the magnet coil at the receiving end, thus giving rise to corresponding variations in magnetism by reason of which the receiving diaphragm is similarly vibrated so as to reproduce the sounds. A single apparatus at each end is therefore sufficient, performing the double function of transmitter and receiver. It will be noticed that in this arrangement no battery is used The strength of the impulses transmitted is therefore limited to that of the necessarily weak induction currents generated by the original sounds minus any loss arising by reason of resistance in the line.
Edison's carbon transmitter overcame this vital or limiting weakness by providing for independent power on the transmission circuit,