The G-Bomb — John Russell Fearn

Despite being the cleverest man on Earth, Jonas Glebe becomes the unwitting tool of a baleful intelligence. He hopes that his new invention, the G-Bomb, will bring riches to himself and his daughter Margaret--but instead, it brings death, and a deadly threat to mankind. Val Turner knows the danger, but he is imprisoned, having been framed for Margaret's murder. When Val's eventually released, he's too late to prevent the cataclysm that has engulfed the world. But fate intervenes: he saves a strange little man from drowning, and thereby changes the destiny of the world....