Both Bell and Gray reached the conclusion that the transmission of articulate speech was impossible unless they could produce electrical undulations corresponding exactly with the vibrations of the air or sound waves. They brought this similarity about by introducing a variable resistance into the electric current by means of an interposing liquid, just as Drawbaugh had done years before with his tumbler of powdered charcoal. Bell exhibited his instrument with comparative success at the Centennial Exhibition in 1876 in Philadelphia; but much had yet to be done to perfect a telephone of real commercial value. The years 1877–78 were years of great activity among electricians, whose prime object was to perfect a telephone transmitter and receiver, by means of whose mutual operations at opposite ends of a circuit all the modulations of speech could be preserved and passed. To this end Berliner introduced into a transmitter or sender the then well-known principle of the microphone (Gr. _mikros_, small, _phone_, sound), which magnified the faint sounds by the variation in electrical resistance, caused by variation of pressure at loose contact between two metal points or electrodes. Edison quickly followed with a similar transmitter or sender, in which one of the electrodes was of soft carbon, the other of metal. Then came (1878) Hughes and Blake with senders, in which both of the electrodes were of hard carbon. Subsequently came other and rapid modifications of the sender, both in the United States and Europe, till the form of telephone now in popular use was arrived at, and which, strange to say, is, in its method of securing the necessary variable resistance in the circuit, quite like that employed by Mr. Drawbaugh; to wit, the introduction of fine carbon granules into a small metal cup just behind the vibrating diaphragm or disk of the sender. The circuit goes into the diaphragm in front, passing through the carbon granules and out through the back of the instrument. The action of talking into the sender causes the granules to be agitated, thus opening and closing the circuit and producing the conditions necessary to the transmission of articulate speech. The diaphragm or disk is the very thin covering of the cup containing the granules. It is sometimes made of carbon, but generally of hard metal, as steel. On being struck by the sound waves of the voice, it vibrates to correspond. The same vibrations are reproduced in the receiver at the opposite end of the circuit, and thus one listens to the phenomenon of transmitted human speech. The current for telephonic purposes is furnished by one or more batteries or cells, whose effect is heightened by the presence of an induction coil. The tendency now is to make “bipolars”—two contacts at the diaphragm—in place of a single contact. This style is becoming more in vogue in order to meet the demands of long-distance work. To each telephone is attached a