Churchill launches the largest military evacuation in military history. But there's a secret killer in the waters.
Hilter’s Secret Weapon
Published March 9, 2018
Onscreen: Dunkirk beach, France
Winston Churchill: We shall prove ourselves, once more, able to defend our island’s home. To ride out the storm of war, and to outlive the menace of tyranny. If necessary for years. If necessary, alone.
Onscreen: Early in WWII—Nazis surround 400,000 British soldiers. If defeated, Hitler will likely conquer all of Europe.
Churchill: The situation of the British and French armies now engaged in a most severe battle, and beset on three sides, and from the air, is evidently extremely grave.
Onscreen: Churchill launches the largest evacuation in military history, sending every available ship to rescue the troops.
But there is a secret killer in the waters. Hitler placed undetectable sea mines that only need a ship’s proximity to detonate, sinking hundreds of thousands of allied ships. The only way to rescue the soldiers is to figure out how Hitler’s secret weapon works.
Simon Foster: Did they have any idea of what’s inside?
Nigel Froude: Absolutely not, so they knew nothing about this, so they were going completely blind. These guys were trained in bomb disposal, but this was completely new to them. So a lot of it was just trial and error. The bravery of these guys is phenomenal.
Foster: So, is this the key to it all? Inside here, this mine is going to reveal its secrets.
Froude: Yeah. Grand reveal.
Foster: This would’ve been the moment of truth.
Froude: And there we have it. This is the trigger that’s going to make the whole mine go bang. If we now remove this place here, we can see the trigger just inside there. If I move it with this screwdriver, you can see the movement of the switch there.
Foster: And that’s just like a seesaw.
Froude: Exactly, just like a seesaw. It moves, makes the circuit, and the mine would go bang.
Foster: Now, what’s making that seesaw move?
Froude: Magnetism, that was the big secret.
Foster: The seesaw inside the German, is a dip compass and we’ve got our own version here. Now, unlike a normal compass that moves left or right to indicate the magnetic field, this actually moves up and down.
The Earth’s magnetic field, which is all around us, it finds it easier to pass through this steel plate than it does in the surrounding area. And this plate here is actually concentrating the magnetic field lines. The magnetic field passing through this is going to be more concentrated here than it is out here. This, this is almost like a lens for magnetism. And as you can see, as it passes over the compass, it’s going to trigger the mine.
Narrator: This concentrating effect turns a steel ship into a gigantic magnet with its north pole under the ship. It is this strong north pole which triggers the mine. If they can create an artificial magnetic field that generates a south pole under the ship instead, the mine will not go off.
Foster: The first method they came up with was called Degaussing. If you have a line of cabling like this and run some current through it, it’s going to create a small magnetic field in the opposite direction to the Earth’s magnetic field. Now, if I take my bit of steel and place it over the mine as we did previously, hopefully the mine will no longer be triggered. And this is how they solved the problem. They wrapped a long line of cabling around the outside of the ships, and run huge currents through them. Creating a magnetic field that actually countered the concentrating, the lensing effect of the ship’s hull and prevented the magnetic mine being set off.
- Digital Producer
- Arlo Perez
- Editorial Review
- Julia Cort, Ari Daniel
- © WGBH Educational Foundation 2018