It was only useful for sending objects into the future, as sending an object even a few minutes into the past caused a buildup of flux energy. If an object was sent any further back than that, the arrival of the object would overheat the capacitor and start an electrical fire.
On October 24, 1962, Doc told the United States military about the device, as he needed funding for his inventions. He later had worries that he could be ushering in an arms race through time, coupled with his regrets about ushering in the nuclear arms race of the Cold War through his participation in the Manhattan Project.
The next day, he wrote himself a letter about his plans, and purposefully sent the letter back several months in the past, to August 1, when he knew that he and his dog, Copernicus, would be safe asleep in his fireproof garage. The Brown family mansion burned down, and the military personnel came and left in the new timeline without ever talking to Doc. Doc used the insurance money to fund the rebuilding of the temporal field capacitor, as well as his future inventions.
The invention of the temporal field capacitor was a necessary step towards the invention of the flux capacitor, which was the device that made time travel possible, as it solved the problem of the excess flux energy when traveling to the past.
Parts of its design were later incorporated into its successor, the temporal field generator Mk II - namely, the panel with dials on it is present in both, sitting above the main chamber.
Behind the scenes
- In the fourth draft screenplay of Back to the Future, the temporal field capacitor was the name for the flux capacitor.