The first of Edison's greater inventions in Menlo Park was the 'loud-speaking telephone.' Professor Graham Bell had introduced his magneto-electric telephone, but its effect was feeble. It is, we believe, a maxim in biology that a similarity between the extremities of a creature is an infallible sign of its inferiority, and that in proportion as it rises in the scale of being, its head is found to differ from its tail. Now, 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.