Classic TV - Tales Of Tomorrow - "Ahead of His Time" (1952)

The world in 2052 is a pretty wonderful place - no wars, no poverty, no famines - but all humankind is about to be destroyed because of a small miscalculation in a scientific experiment a century earlier set off a chain reaction that will contaminate the Earth with a lethal amount of radiation. With only hours before the end of the world, a 21st century scientist attempts to send a message to colleagues a hundred years in the past to prevent the mistake using a 20th century man's new time machine.
https://www.imdb.com/title/tt0717016/plotsummary?ref_=tt_ov_pl

This video is in the Public Domain.