The Magnetic Monster (1953)

Starring Richard Carlson, King Donovan, Jean Byron, Harry Ellerbe, Leo Britt
Directed by Curt Siodmak
(actor & director credits courtesy IMDB.com)

The Office of Scientific Investigation finds itself the hope of the world when a dangerous new element starts doubling in size every 12 hours, drawing in the energy it needs with magnetism. 

The first of a series of films by producer Tors and writer/director Siodmak, focusing on science-based threats to Earth, is in my estimation the best of them, with Carlson providing a strong lead, as the story follows his investigation in a Dragnet-like style.  There's a lot of technical concepts and terminology but I'd have to say overall the filmmakers succeed in conveying the threat of the element without confusing the audience.  It was a big risk on their part to make the "monster" of the film a largely intangible menace, but with the real-life atomic threats of the time, people did have such menaces already in their mind, and they were able to tap into that paranoia effectively.  Without many special effects or likely the budget for them, the film has to depend on cribbing footage from the impressively staged 1934 film Gold for its climax, but it works well enough for a satisfying viewing experience.

Invisible Invaders (1959)

Starring John Agar, Jean Byron, Philip Tonge, Robert Hutton, John Carradine
Directed by Edward L. Cahn
(actor & director credits courtesy IMDB.com)

Invisible aliens from the moon with the power to possess the bodies of the dead launch an invasion of Earth, while a nuclear scientist labors to find a defense against their attacks.

Although the story borders on the ludicrous, and the alien attacks are primarily composed of a generous sampling of stock footage, this film has been a long-time favorite of mine, with distinguished actors Tonge and Carradine lending it a certain sense of credibility despite the low-budget trappings.  Although the special effects are low budget as well, they're unique and have a certain charm, as when the invisible creatures are represented by mounds of earth moving on their own accompanied by fearsome growls on the soundtrack.  As long as you don't take the film too seriously, you'll have a good time.