This apparatus was completed on June 2, 1875, and the same day he
succeeded in transmitting SOUNDS and audible signals by magneto-electric
currents and without the aid of a battery. On July 1, 1875, he
instructed his assistant to make a second membrane-receiver which could
be used with the first, and a few days later they were tried together,
one at each end of the line, which ran from a room in the inventor's
house at Boston to the cellar underneath. Bell, in the room, held one
instrument in his hands, while Watson in the cellar listened at the
other. The inventor spoke into his instrument, 'Do you understand what
I say?' and we can imagine his delight when Mr. Watson rushed into the
room, under the influence of his excitement, and answered,'Yes.'
A finished instrument was then made, having a transmitter formed of a
double electro-magnet, in front of which a membrane, stretched on a
ring, carried an oblong piece of soft iron cemented to its middle. A
mouthpiece before the diaphragm directed the sounds upon it, and as it
vibrated with them, the soft iron 'armature' induced corresponding
currents in the cells of the electro-magnet. These currents after
traversing the line were passed through the receiver, which consisted of
a tubular electro-magnet, having one end partially closed by a thin
circular disc of soft iron fixed at one point to the end of the tube.
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