 Franck enjoyed talking about his problems , not so much to explain them to others as to satisfy his own mind . Once a problem had aroused his interest he was completely captivated , indeed obsessed by it . Common sense and straight logic were his main tools , together with simple apparatus . His research followed an almost straight line , from his early studies of ion <unk> to his last work on photosynthesis ; it was always the energy exchange between atoms or molecules that fascinated him . 
