In 1927, George Paget Thomson, professor of Natural Philosophy at the University of Aberdeen, obtained the first photographs of the diffraction of electrons, thus visually demonstrating for the first time de Broglie’s principle on the wave-particle duality. His findings were a by-product of a long research program that he had started after graduation in Cambridge, in 1913, following after his father’s steps. As a matter of fact that project was not interested in the behaviour of electrons but on positive rays and their uses for chemical analysis. The discovery of electron diffraction by two different techniques and two independent research teams (G.P. Thomson in Aberdeen and C. Davisson and L. Germer at the Bell laboratory) is a very nice example of the at times apparent disconnection between so-called pure and applied science. Neither team was, in the traditional sense of the word, carrying on theoretically-oriented research. Even in the case of G.P. Thomson, who was trained in both the Mathematical and Natural Sciences triposes in Cambridge, he hardly tried to be a theoretical (or mathematical) physicist, not even when his involvement in the de Broglie principle turned him into a spokesman and populariser of one of the most esoteric principles of the new quantum mechanics. In this paper I explore the career of G.P. Thomson after his experimental proof of electron diffraction, when he moved to Imperial College. There, he formed a team to develop what was known as electron diffraction cameras as a way to analyse microscopic and crystalline structures. Particular attention will be paid to the continuity between his pre-1926 positive-ray techniques and his post-1929 electron diffraction cameras, as well as to the mathematical technologies used in both programs. The latter will certainly help us better understand the extent to which Thomson’s cameras should be seen only in the light of so-called applied science, especially at a time when theoretical physics was almost exclusively synonymous with foundational quantum and relativistic physics.