Richard Feynman at the Museum of Hoaxes website:
There were quite a few wires running from the engine down to where Mr. Papp and the spectators were standing, into a set of instruments used for measurement; these included a variac, a variable transformer with a dial which could put out different voltages. The instruments were, in turn, connected by a cord to an electrical outlet in the side of the building. So it was pretty obvious where the power supply was.
The engine started to go around, and there was a bit of disappointment: the propeller of the fan went around quietly without the noise of an ordinary engine with powerful explosions in the cylinders, and everything- it looked very much like an electric motor.
Mr. Papp pulled the plug from the wall, and the fan propeller continued to turn. 'You see, this cord has nothing to do with the engine; it's only supplying power to the instruments,' he said. Well, that was easy. He's got a storage battery inside the engine. 'Do you mind if I hold the plug?' I asked? 'Not at all,' replied Mr. Papp, and he handed it to me.
It wasn't very long before he asked me to give me back the plug. 'I'd like to hold it a little longer,' I said, figuring that if I stalled around enough, the damn thing would stop.
Pretty soon Mr. Papp was frantic, so I (Richard Feynman) gave him back the plug and he plugged it back into the wall. A few moments later there was a big explosion:
A cone of silvery uniform stuff shot out and turned to smoke. The ruined engine fell over on its side. The man standing next to me said, 'I've been hit!' I looked at him, the whole side of his arm was torn open, you could see all the muscle fibers, tendons-everything.