a property of loose electrical contact between any two conductors.
The soft and springy button of lamp-black became no longer necessary, since it was not so much the resistance of the material which varied as the resistance at the contacts of its parts and the platinum electrodes. Two metals, or two pieces of hard carbon, or a piece of metal and a piece of hard carbon, were found to regulate the current in accordance with the vibrations of the voice. Edison therefore discarded the soft and fragile button, replacing it by contacts of hard carbon and metal, in short, by a form of microphone. The carbon, or microphone transmitter, was found superior to the magneto-electric transmitter of Bell; but the latter was preferable as a receiver to the louder but less convenient chemical receiver of Edison, and the most successful telephonic system of the day is a combination of the microphone, or new carbon transmitter, with the Bell receiver.
The 'micro-tasimeter,' a delicate thermoscope, was constructed in 1878, and is the outcome of Edison's experiments with the carbon button. Knowing the latter to be extremely sensitive to minute changes of pressure, for example, those of sonorous vibrations, he conceived the idea of measuring radiant heat by causing it to elongate a thin bar or strip of metal or vulcanite, bearing at one end on the button. To indicate the effect, he included a galvanometer in the circuit of the battery and the button. The apparatus consisted of a telephone button placed between two discs of platinum and connected in circuit with the battery and a sensitive galvanometer. The strip was supported so that one end bore upon the button with a pressure which could be regulated by an adjustable screw at the other. The strip expanded or contracted when exposed to heat or cold, and thrust itself upon
