While still a student he conceived an "electric telescope", mainly known for the idea of using a spiral-perforated disk (Nipkow disk), to divide a picture into a linear sequence of points. Accounts of its invention state that the idea came to him while sitting alone at home with an oil lamp on Christmas Eve, 1883. Alexander Bain had transmitted images telegraphically in the 1840s but the Nipkow disk improved the encoding process.