Triggering the bomb

The only protection is to keep the mass of U-235 below a certain "critical" size. Scientists had to calculate this size—they could not experiment in a laboratory to find it out. The bomb had to be made in two parts. Each part would be below the critical size, but the two parts when brought together would be above the critical size. So the "trigger" would be some mechanism to unite the two parts.

The reaction, once it starts, runs through the entire load in about one millionth of a second. It explodes long before any but a small fraction of the material (about three-tenths of one per cent) is fission. Before the test bomb gave the answer, there was some question whether the two parts could be brought together fast enough to produce any more than a small "pop" before the first atoms blew the container to pieces. For long after the war, most details of the atomic bomb have been kept secret. But the United States government has disclosed enough to show that the newest weapons have almost unlimited powers of destruction, and that their best use should be to discourage all nations from making war.