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Source: Library of Congress- boarding house, Lab, Detroit News, Lab Notebook

On this day 140 years ago, the 29 year-old Scot Alexander Graham Bell (1847-1922) and his assistant Thomas A. Watson (1854-1934) had, through experiments conducted since 1875, improved the technique of sound transmission via a wire to a point that what was said at one end of the wire could be understood at the other end.

Here is what he wrote in his lab notebook on March 10, 1876:

1.The improved instrument shown in Fig.I was constructed this morning
and tried this evening. P is a brass pipe and W the platinum wire M 
the mouth piece and S the armature of the Receiving Instrument.
Mr Watson was stationed in one room with the Receiving Instrument. 
He pressed one ear closely against S and closed his other ear with 
his hand. The Transmitting Instrument was placed in another room and
the door of both rooms were closed. 
I then shouted into M the following sentence: “Mr. Watson – come here
– I want to see you”. To my delight he came and declared that he had
heard and understood what I said. I asked him to repeat the words. He
answered “You said ‘Mr. Watson – come here – I want to see you’.” We 
then changed places and I listened at S while Mr. Watson read a few 
passages from a book into the mouth piece M. It was certainly the case
that articulate sounds proceeded from S. The effect was loud but 
indistinct and muffled. If I had read beforehand the passage given 
by Mr. Watson I should have recognized every word. As it was I could
not make out the sense – but an occasional word here and there was 
quite distinct. I made out “to” and “out” and “further”, and finally 
the sentence “Mr. Bell do you understand what I say? DO – YOU – un-
der - stand – what – I – say” came quite clearly and intelligibly. No
sound was audible when the armature S was removed.

More about this day at the Library of Congress’s Treasure Exhibit page. See also Janeiro 25. The Library of Congress has a rich offer of digitalised material concerning Alexander Graham Bell.