At this time, the predominant ammunition used against tanks was the armor-piercing kinetic energy shell that defeated armor by direct pressure, spiking or punching through it. During the late 1930s shaped charge ammunition was experimented with that used chemical energy for armor penetration. The shaped charge concept is officially known as the "Munroe Effect" and was discovered by accident decades earlier by Professor Charles E. Munroe at the U.S. Torpedo Station, Providence, RI. Professor Munroe was detonating different manufactured blocks of explosives on a sheet of armor plating and observed the blocks having the manufacturing letters recessed (vs. raised) cut an imprint of the manufacturing letters into the armor plate—the birth of the shaped-charged explosive which focuses the blast energy caused by an indentation on the surface area of an explosive. Although shaped charges are somewhat more difficult to manufacture, the advantage is that the projectile does not require as high velocity as typical kinetic energy shells, yet on impact it creates a high-velocity jet of metal flowing like a liquid due to the immense pressure (though x-ray diffraction has shown the metal stays solid) which hydrodynamically penetrates the armor and kills occupants inside. The depth of the penetration, though proportional to the length of the jet and the square root of its density, is also dependent on the strength of the armor. With the development of this new ammunition begun more advanced research into steel manufacturing, and development of spaced armor that caused "jet waver" by detonating prematurely or at the wrong angle to the surface of the main armor.