The transmission site in Cornwall was chosen for its remoteness to keep the project out of the public eye and out of the newspapers. In 1900, Marconi decided to work in secret without the press hounding him or speculating on the outcome of his endeavor.
Originally Marconi had favored a site on Cape Cod. But including inclement weather forced him to move the receiving station to St. John's Newfoundland. As an added bonus, this was also 600 miles closer to Cornwall. Marconi send a message from Newfoundland to England, via the underwater telegraph cable, to tell the Poldhu transmitter to send radio signals between 12 noon and 3pm local time. more here.
On December 11th 1901 Marconi made the first attempt at a transatlantic transmission from Poldhu. He failed. He did not have the money for a full receiving array. Instead he was using a single wire aerial 550 feel long and 4 inches thick. That first day the wind was so strong that the balloon holding his aerial aloft was swept away. The following day a second was launched with the aerial attached and Marconi was finally able to hear the signal. It had traveled 1,800 miles.
He had sent and received radio signals before, even some over large distances. the question here was whether radio waves would be stopped by the curvature of the Earth. At 12:30 that day, Marconi heard "dot dot dot" [the Morse Code letter "S"]And got his answer. This was the start of a career that would win him the Nobel Prize in Physics in 1909.
Each year, hundreds of radio enthusiasts from all over the world make a pilgrimage to Poldhu to pay homage to Marconi. The ruins of the original wireless building that Marconi would have used are still on the site. It was dismantled in 1933, four years before Marconi's death.