Photoelectric observations – however not from surfaces, as in Berlin, but rather in bulk insulators – were started by Pohl together with his assistant Bernhard Gudden in 1919. (The research work from this period was described in detail in (first part).) They discovered that diamond crystals become electrically conducting after irradiation with light. Later, they observed the same effect in the alkali halide sodium chloride; however only after it had been colored as a result of X-ray irradiation. Systematic observations of this coloration effect using artificially-prepared crystals led to the discovery of color centers, which were investigated in detail in the following years. By inserting three electrodes into a potassium bromide crystal, Pohl and Rudolf Hilsch were able to demonstrate the first model of a transistor based on color centers in 1938.