A record of the start-up of Chicago Pile 1 (CP-1). The traces represent the motion of the needle of a Lindeman electrometer for a fixed time interval. The slope of the line measures the current resulting from the ionization of the gas in a ion chamber by gamma rays emanating from graphite reactor at the time of start-up. The recording unit was located on the balcony a short distance from the strip chart which recorded the neutron activity and around which several people were gathered at the time of that famous event. Courtesy: Oak Ridge National Laboratory, AlP Emilio Segrè Visual Archives.