Final Assembly
A view showing the command lines running from the ground up into the tower. 
The tower holding the Gadget was kept above the ground in order to mute the explosion and amount of radiation that would be kicked up in the blast. 
In this famous photo, SGT Herbert Lehr is holding the plutonium core in the doorway at the McDonald Ranch House. 
The core was loaded into a Plymouth sedan and driven to the Test Site. 
The entire assembly was hoisted up into the tower. 
The final preparations were made as the scientists and engineers hooked up and tested cables, connections, and the command wire. 
When the personnel working were satisfied with their final preparations, all that was left to do was wait until the weather provided favorable conditions to best record the explosion.
On 12 July 1945, two hemispheres of plutonium left Los Alamos for their final assembly at the Schmidt-McDonald ranch house. They traveled in a Plymouth sedan with a Women’s Army Corps (WAC) driver, an armed escort in a lead car, and a car with scientists behind. When they arrived at the ranch house, Robert Bacher picked up the special carrier box by its handle and carried it to the porch. In a famous picture, SGT Bert Lehr picked it up and carried it inside.
Major General Groves’ deputy, Brigadier General Thomas Farrell was asked to sign a hand receipt for the plutonium. He said, “When I took this heavy ball in my hand, I felt it growing warm. I got a certain sense of its hidden power.” On 13 July 1945, the plutonium core was assembled inside the northeast room of the ranch house by a team consisting of American physicists Robert Bacher, Marshall Holloway, Phillip Morrison, Boyce McDaniel, the Canadian physicist and chemist Louis Slotin, and British metallurgist Cyril Smith.
This first atomic bomb had a plutonium core surrounded by carefully cut and shaped explosives. The Trinity Test was conducted to ensure that the high explosives could be detonated at the same instant to compress the plutonium ball into the critical mass needed to start the chain reaction that would produce a nuclear explosion. The core consisted of the polonium and beryllium neutron initiator, called the Urchin. Outside of that were the baseball sized hemispheres of plutonium, surrounded by a larger quantity of uranium that all formed a plug to be inserted into the bomb.
The uranium slug was cut in half with a spherical cup in the bottom half to hold the plutonium core and initiator. The two halves were then put together and placed in a box with holes in it for cooling. It sat in the assembly room at the ranch house until the next day when it was taken to ground zero to be inserted into the high explosive components that had been delivered from Los Alamos.
At 0100 on 13 July, the pre-assembled high explosive components left Los Alamos for Trinity in a truck between Army Intelligence cars with chemist and explosive expert George Kistiakowsky in front.
After the explosives arrived at the tower on 13 July, the assembled core, which weighed over 100 pounds, was taken in its box for insertion into the Gadget as it sat inside a tent under the tower. In a famous photo, Harry Daghlian, the physicist who would accidentally fatally irradiate himself in August, and Herb Lehr carried the assembled core to the 1942 Plymouth sedan that would take it to the tower. The 100-foot tower would simulate an aerial blast. The cushion of air also limited the depth of the crater and the radioactive fallout.
The morning of 13 July, the bomb was opened and the explosive lenses taken out to allow insertion of the slug by using a crane to lower it. It fit at Los Alamos, but scientists were puzzled as to it didn’t fit at Trinity. While pondering the problem for several minutes, the slug, still sitting above the bomb, went in a quarter inch. A few minutes later it went in a bit further. It was a thermal problem. The bomb at the base of the tower cooled off during the night, but the plutonium remained warm. As the temperatures between the two equalized, it slid in.
By late afternoon, the portions from the ranch house and the portion under the tower were assembled and the Gadget was hoisted to the top and held under a steel shed where the detonators and final wiring would be installed. One side became partially unhinged. In case it fell, technicians covered the ground below the device with all the mattresses they could find. That night, the detonator crew climbed the tower and installed the detonators. The bomb was essentially ready.
Once they cleared the area and secured their guard posts, the MPs withdrew to their bunkers and foxholes on site and in slit trenches dug in at Mockingbird Gap. Twenty military intelligence officers who carried instruments to measure blast and earth shock were stationed in nearby towns. An evacuation detachment of 160 personnel stood by at a remote location in the event a larger evacuation of nearby ranches was needed.
At about 2100 hours, with lightening all around, Donald Horning came out, climbed to the top of the tower, moved the detonating circuit from the dummy circuit to the gadget, and returned to the South 10,000 bunker. George Kistiakowsky climbed thirty feet up the tower and adjusted a light at the radioed request of a cameraman, then returned to the car to sleep. Periodically, Captain Bush or the guard turned their headlights on the tower to check cabling.
At 2300 on 15 July in the rain, the arming crew – Bainbridge, Kistiakowsky, Joe McKibben, Captain Bush, 2 Army weathermen, and a guard – assembled at base camp for the final trip to the tower. Joe McKibben, responsible for supplying the timing and remote operating signals, had the 47-task checklist.
At 0100 on 16 July, with the tasks complete, a few of them got a couple of hours sleep. Donald Horning from the firing unit spent the night with the bomb reading a book, hoping errant lightning didn’t strike the tower and send him to kingdom come.
At 0200, VIPs were positioned on Campania Hill, about 20 miles northwest of ground zero, close to US 380. Engineers and MPs not on guard duty were told to lay down behind a berm near the earthen stock tanks north of base camp with their feet towards ground zero when the bomb went off. They were allowed to turn around to watch the mushroom cloud through pieces of smoked glass.
There were 99 people in 3 bunkers: 29 in the north bunker, 37 in the west bunker, and 33 in the south bunker. Key personnel were sent to different bunkers as a precaution. Lead scientist Dr. J. Robert Oppenheimer, chemist and explosives expert George Kistiakowsky, meteorologist Jack Hubbard, Test Director Kenneth Bainbridge, and Deputy Project head Brigadier General Thomas Farrell were in the south bunker. Other key personnel, such as Enrico Fermi, watched from base camp. Major General Groves, who had been at South 10,000, left for base camp as the countdown started.
Unresolved equipment problems still existed. From the beginning, the shortwave ground radio frequency issued to the site was shared with a stockyard in San Antonio, Texas. The FM ground-to-air shortwave radio frequency to communicate with the airborne B-29s conflicted with the Voice of America radio broadcast in California. Two B-29s flying overhead for observation were held back from the site due to the weather but still managed to observe the test through overcast skies.
Hubbard and his assistants kept measuring the weather. At 0400, the rain stopped. At 0445, the final weather report came in. After final consultations with no dissenting opinions, the decision was made: the detonation would be 0530. Immediately, the arming party sprang into action. Circuits were opened. Connections were checked. Leads were connected. Lights were switched on.
Those at Los Alamos who knew about the detonation 200 miles away waited outside. President Truman was at the Potsdam Conference in Germany waiting to hear the outcome of the test while preparing to meet with Churchill and Stalin to determine the future of Europe and discuss the ongoing war with Japan.