These arming plugs for Little Boy are displayed at the Udvar-Hazy Center of the National Air and Space Museum. They were found in the navigator’s compartment during the restoration of the Enola Gay. It’s not known whether these plugs were from the assembled Little Boy atomic bomb (L-11), or one of the pre-assemblies (without uranium projectiles) used for testing or practice drops (L-1, L-2, L-5, and L-6).

Shortly after takeoff from Tinian island, Deak Parsons and Morris Jeppson crawled into the bomb bay of Enola Gay to follow the eleven steps¹ to arm the atomic bomb. Step 1 was to check that the three wood-handled green plugs that blocked the firing signal between the fuse and the bomb were installed.

About an hour and a half before the bomb run, Jeppson crawled back into the bomb bay with three wood-handled, five-pin red plugs. He carefully removed each green plug, one-by-one, and replaced them with the red plugs, closing the firing circuits.

At 8:15 a.m. (Hiroshima time), the bomb was released, dislodging the instrument cords that connected it to the airplane and instrument monitoring panels. This immediately transferred power to the 24-volt battery, beginning a 44-second detonation cycle.

First, a timer of eight spring-wound clocks safeguarded that the bomb would not explode until at least 15 seconds after release, about one quarter of the predicted fall time, to ensure the safety of the aircraft.

Second, the firing signal transferred to the barometric pressure switch, designed to close at 7,000 feet. With 9 seconds to detonation, the switch closed and activated the Archie radars, with the Japanese-invented Yagi antennas which began bouncing their signals off the approaching ground.

At close to 1900 feet above the ground, the last relay switch closed. The firing signal jumped across the three red arming plugs, sending the signal to the breech primers and detonating the cordite charges, propelling the uranium 235 projectile down the six-foot barrel toward its uranium target, creating an uncontrolled chain reaction and changing history.

⁽¹⁾ Walker, S. (2005). Shockwave: Countdown to Hiroshima. HarperCollins. p. 192.

The Manhattan Project needed lots of computers for such things as design, explosive yield, the physics of implosion, and more. At the time “computers” usually meant a woman whose job was to perform calculations by hand or with a mechanical calculator, the Marchant. Women with degrees in math and science often took jobs as computers because of discrimination in their own fields. As such, many of the women who became computers for the Manhattan Project were grossly overqualified for these jobs.

By 1943, about 20 computers worked in the T-5 Computation group at Los Alamos, under the supervision of Mary Frankel, wife of Stanley Frankel, who, with Eldred Nelson, organized the computing program. The wartime mechanical calculators were integral to the project, but lacked mechanical reliability and required routine repairs. Richard Feynman and Nicholas Metropolis started repairing the Marchant machines as an extracurricular activity and grew more adept at maintaining them, enabling the scientific staff to model complex experiments. Metropolis would later build the MANIAC computer at Los Alamos from a design by John von Neumann.

The Marchant calculator on display at the Los Alamos History Museum is a Figurematic from the 1950s. The women computers at Los Alamos would have worked on Marchant Silent Speed calculators, first developed in 1932, and continuously improved until the Figurematic line which was produced until the business closed in the early 1970s.