‘s’

The first radio transmission across the Atlantic was received by Guglielmo Marconi on this date in 1901 (Cornwall, England, to Newfoundland). The message consisted of Morse code for the letter “s”.

That would be dot-dot-dot.

Which is better than a lot of stuff on the airwaves 110 years later.

There is some uncertainty about Marconi’s claim. There were no disinterested observers. The distance was approximately 2,200 miles. The antenna in Newfoundland was a large kite.