Heroes of the Telegraph [83]
in the Bell apparatus, the transmitter and the receiver were alike, and hence Clerk Maxwell hinted that it would never be good for much until they became differentiated from each other. Consciously or unconsciously Edison accomplished the feat. With the hardihood of genius, he attempted to devise a telephone which would speak out loud enough to be heard in any corner of a large hall.
In the telephone of Bell, the voice of the speaker is the motive power which generates the current in the line. The vibrations of the sound may be said to transform themselves into electrical undulations. Hence the current is very weak, and the reproduction of the voice is relatively faint. Edison adopted the principle of making the vibrations of the voice control the intensity of a current which was independently supplied to the line by a voltaic battery. The plan of Bell, in short, may be compared to a man who employs his strength to pump a quantity of water into a pipe, and that of Edison to one who uses his to open a sluice, through which a stream of water flows from a capacious dam into the pipe. Edison was acquainted with two experimental facts on which to base the invention.
In 1873, or thereabout, he claimed to have observed, while constructing rheostats, or electrical resistances for making an artificial telegraph line, that powdered plumbago and carbon has the property of varying in its resistance to the passage of the current when under pressure. The variation seemed in a manner proportional to the pressure. As a matter of fact, powdered carbon and plumbago had been used in making small adjustable rheostats by M. Clerac, in France, and probably also in Germany, as early as 1865 or 1866. Clerac's device consisted of a small wooden tube containing the material, and fitted with contacts for the current, which appear to have adjusted the pressure. Moreover, the Count Du Moncel, as far back as 1856, had clearly discovered that when powdered carbon was subjected to pressure, its electrical resistance altered, and had made a number of experiments on the phenomenon. Edison may have independently observed the fact, but it is certain he was not the first, and his claim to priority has fallen to the ground.
Still he deserves the full credit of utilising it in ways which were highly ingenious and bold. The 'pressure-relay,' produced in 1877, was the first relay in which the strength of the local current working the local telegraph instrument was caused to vary in proportion to the variation; of the current in the main line. It consisted of an electro- magnet with double poles and an armature which pressed upon a disc or discs of plumbago, through which the local current Passed. The electro- magnet was excited by the main line current and the armature attracted to its poles at every signal, thus pressing on the plumbago, and by reducing its resistance varying the current in the local circuit. According as the main line current was strong or weak, the pressure on the plumbago was more or less, and the current in the local circuit strong or weak. Hence the signals of the local receiver were in accordance with the currents in the main line.
Edison found that the same property might be applied to regulate the strength of a current in conformity with the vibrations of the voice, and after a great number of experiments produced his 'carbon transmitter.' Plumbago in powder, in sticks, or rubbed on fibres and sheets of silk, were tried as the sensitive material, but finally abandoned in favour of a small cake or wafer of compressed lamp-black, obtained from the smoke of burning oil, such as benzolene or rigolene. This was the celebrated 'carbon button,' which on being placed between two platinum discs by way of contact, and traversed by the electric current, was found to vary in resistance under the pressure of the sound waves. The voice was concentrated upon it by means of a mouthpiece and a diaphragm.
The property on which the receiver was based had been observed and applied by him some time before. When a current is passed
In the telephone of Bell, the voice of the speaker is the motive power which generates the current in the line. The vibrations of the sound may be said to transform themselves into electrical undulations. Hence the current is very weak, and the reproduction of the voice is relatively faint. Edison adopted the principle of making the vibrations of the voice control the intensity of a current which was independently supplied to the line by a voltaic battery. The plan of Bell, in short, may be compared to a man who employs his strength to pump a quantity of water into a pipe, and that of Edison to one who uses his to open a sluice, through which a stream of water flows from a capacious dam into the pipe. Edison was acquainted with two experimental facts on which to base the invention.
In 1873, or thereabout, he claimed to have observed, while constructing rheostats, or electrical resistances for making an artificial telegraph line, that powdered plumbago and carbon has the property of varying in its resistance to the passage of the current when under pressure. The variation seemed in a manner proportional to the pressure. As a matter of fact, powdered carbon and plumbago had been used in making small adjustable rheostats by M. Clerac, in France, and probably also in Germany, as early as 1865 or 1866. Clerac's device consisted of a small wooden tube containing the material, and fitted with contacts for the current, which appear to have adjusted the pressure. Moreover, the Count Du Moncel, as far back as 1856, had clearly discovered that when powdered carbon was subjected to pressure, its electrical resistance altered, and had made a number of experiments on the phenomenon. Edison may have independently observed the fact, but it is certain he was not the first, and his claim to priority has fallen to the ground.
Still he deserves the full credit of utilising it in ways which were highly ingenious and bold. The 'pressure-relay,' produced in 1877, was the first relay in which the strength of the local current working the local telegraph instrument was caused to vary in proportion to the variation; of the current in the main line. It consisted of an electro- magnet with double poles and an armature which pressed upon a disc or discs of plumbago, through which the local current Passed. The electro- magnet was excited by the main line current and the armature attracted to its poles at every signal, thus pressing on the plumbago, and by reducing its resistance varying the current in the local circuit. According as the main line current was strong or weak, the pressure on the plumbago was more or less, and the current in the local circuit strong or weak. Hence the signals of the local receiver were in accordance with the currents in the main line.
Edison found that the same property might be applied to regulate the strength of a current in conformity with the vibrations of the voice, and after a great number of experiments produced his 'carbon transmitter.' Plumbago in powder, in sticks, or rubbed on fibres and sheets of silk, were tried as the sensitive material, but finally abandoned in favour of a small cake or wafer of compressed lamp-black, obtained from the smoke of burning oil, such as benzolene or rigolene. This was the celebrated 'carbon button,' which on being placed between two platinum discs by way of contact, and traversed by the electric current, was found to vary in resistance under the pressure of the sound waves. The voice was concentrated upon it by means of a mouthpiece and a diaphragm.
The property on which the receiver was based had been observed and applied by him some time before. When a current is passed