Monroe effect
The Monroe effect refers to the characteristics of the detonation of a volume of explosive around a hollow or void, a shaped charge. Explosive energy is released directly away from (normal to) the surface of an explosive, so shaping the explosive will concentrate the explosive energy in the void. If the void is properly shaped (usually conically), a high-velocity jet of plasma will form.
It was discovered accidentally in the 1890s but not applied to military purposes until the Second World War.
In military applications, a Monroe-effect shaped-charge warhead can be expected to penetrate solid steel armor equal to 150-250% of the warhead diameter, though it will tend to be somewhat less effective against modern composite armors.
In peaceful engineering applications, shaped charges are very useful for cutting steel girders, perhaps in order to demolish an old building.
Compare to the Misznay-Schardin effect.
