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 from a metal contact through certain chemical salts, a lubricating effect was noticeable. Thus if a metal stylus were rubbed or drawn over a prepared surface, the point of the stylus was found to slip or 'skid' every time a current passed between them, as though it had been oiled. If your pen were the stylus, and the paper on which you write the surface, each wave of electricity passing from the nib to the paper would make the pen start, and jerk your fingers with it. He applied the property to the recording of telegraph signals without the help of an electro-magnet, by causing the currents to alter the friction between the two rubbing surfaces, and so actuate a marker, which registered the message as in the Morse system.
This instrument was called the 'electromotograph,' and it occurred to Edison that in a similar way the undulatory currents from his carbon transmitter might, by varying the friction between a metal stylus and the prepared surface, put a tympanum in vibration, and reproduce the original sounds. Wonderful as it may appear, he succeeded in doing so by the aid of a piece of chalk, a brass pin, and a thin sheet or disc of mica. He attached the pin or stylus to the centre of the mica, and brought its point to bear on a cylindrical surface of prepared chalk.
The undulatory current from the line was passed through the stylus and the chalk, while the latter was moved by turning a handle; and at every pulse of the electricity the friction between the pin and chalk was diminished, so that the stylus slipped upon its surface. The consequence was a vibration of the mica diaphragm to which the stylus was attached. Thus the undulatory current was able to establish vibrations of the disc, which communicated themselves to the air and reproduced the original sounds. The replica was loud enough to be heard by a large audience, and by reducing the strength of the current it could be lowered to a feeble murmur. The combined transmitter and receiver took the form of a small case with a mouthpiece to speak into, an car-piece on a hinged bracket for listening to it, press-keys for manipulating the call-bell and battery, and a small handle by which to revolve the little chalk cylinder. This last feature was a practical drawback to the system, which was patented in 1877.
The Edison telephone, when at its best, could transmit all kinds of noises, gentle or harsh; it could lift up its voice and cry aloud, or sink it to a confidential whisper. There was a slight Punchinellian twang about its utterances, which, if it did not altogether disguise the individuality of the distant speaker, gave it the comicality of a clever parody, and to hear it singing a song, and quavering jauntily on the high notes, was irresistibly funny. Instrumental notes were given in all their purity, and, after the phonograph, there was nothing more magical in the whole range of science than to hear that fragment of common chalk distilling to the air the liquid melody of sweet bells jingling in tune. It brought to mind that wonderful stone of Memnon, which responded to the rays of sunrise. It seemed to the listener that if the age of miracles was past that of marvels had arrived, and considering the simplicity of the materials, and the obscurity of its action, the loud-speaking telephone was one of the most astonishing of recent inventions.
After Professor Hughes had published his discovery of the microphone, Edison, recognising, perhaps, that it and the carbon transmitter were based on the same principle, and having learnt his knowledge of the world in the hard school of adversity, hastily claimed the microphone as a variety of his invention, but imprudently charged Professor Hughes and his friend, Mr. W. H. Preece, who had visited Edison at Menlo Park, with having 'stolen his thunder.' The imputation was indignantly denied, and it was obvious to all impartial electricians that Professor Hughes had arrived at his results by a path quite independent of the carbon transmitter, and discovered a great deal more than Edison had done. For one thing, Edison believed the action of his transmitter as due to a property of certain poor or 'semi-conductors,' whereby their electric resistance varied under pressure. Hughes taught us to understand that it was owing to a property of loose electrical contact between any two conductors.
