The test conducted by the United States Army at 5:30 a.m. on July 16, 1945, as part of the Manhattan Project. The date of the Trinity test is usually considered to be the beginning of the Atomic Age.
The code name was assigned by J. Robert Oppenheimer, the director of the Los Alamos Laboratory. The test was of an implosion-design plutonium device, of the same design as the Fat Man bomb later (three weeks) detonated over Nagasaki, Japan, on August 9, 1945.
The term “Gadget” was a laboratory euphemism for the bomb, from which the laboratory’s weapon physics division, “G Division”, took its name in August 1944.
The Gadget was an implosion device, which means the plutonium core is surrounded by many small explosives, these compress the plutonium and bring it closer to the point of causing it to go super critical. All those wires are attached to different explosives which burn at different frequencies. The trick of the 20 explosions is that they push the pieces of uranium (or plutonium) together to a ball with an over-critical mass, which explodes.
Assembly of the nuclear capsule began on 13 July 1945 at the McDonald Ranch House, where the master bedroom had been turned into a clean room. The bomb was placed atop a 30 meter steel tower that was designated Zero. Ground Zero was at the foot of the tower. Equipment, instruments, and observation points were established at varying distances. The wooden observation shelters were protected by concrete and earthen barricades, and the nearest observation point was 10 km from Ground Zero.
To most observers—watching through dark glasses—the brilliance of the light from the explosion overshadowed the shock wave and sound that arrived some seconds later. A multi-colored cloud surged 38,000 feet into the air within seven minutes. Although no information on the test was released until after the atomic bomb had been used as a weapon, the flash of light and shock wave made a vivid impression over an area with a radius of at least 250 km.