Meanwhile, innovations continued on another significant front. In 1900, as part of the research he was conducting on the photophone, the German physicist Ernst Ruhmer recorded the fluctuations of the transmitting arc-light as varying shades of light and dark bands onto a continuous roll of photographic film. He then determined that he could reverse the process and reproduce the recorded sound from this photographic strip by shining a bright light through the running filmstrip, with the resulting varying light illuminating a selenium cell. The changes in brightness caused a corresponding change to the selenium's resistance to electrical currents, which was used to modulate the sound produced in a telephone receiver. He called this invention the photographophone, which he summarized as: "It is truly a wonderful process: sound becomes electricity, becomes light, causes chemical actions, becomes light and electricity again, and finally sound."